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As Dave Chinner pointed out at the 2013 LSF/MM workshop, it's
important that metadata I/O requests are marked as such to avoid
priority inversions caused by I/O bandwidth throttling.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Currently in ENOSPC condition when writing into unwritten space, or
punching a hole, we might need to split the extent and grow extent tree.
However since we can not allocate any new metadata blocks we'll have to
zero out unwritten part of extent or punched out part of extent, or in
the worst case return ENOSPC even though use actually does not allocate
any space.
Also in delalloc path we do reserve metadata and data blocks for the
time we're going to write out, however metadata block reservation is
very tricky especially since we expect that logical connectivity implies
physical connectivity, however that might not be the case and hence we
might end up allocating more metadata blocks than previously reserved.
So in future, metadata reservation checks should be removed since we can
not assure that we do not under reserve.
And this is where reserved space comes into the picture. When mounting
the file system we slice off a little bit of the file system space (2%
or 4096 clusters, whichever is smaller) which can be then used for the
cases mentioned above to prevent costly zeroout, or unexpected ENOSPC.
The number of reserved clusters can be set via sysfs, however it can
never be bigger than number of free clusters in the file system.
Note that this patch fixes the failure of xfstest 274 as expected.
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
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Currently on many places in ext4 we're using
ext4_get_group_no_and_offset() even though we're only interested in
knowing the block group of the particular block, not the offset within
the block group so we can use more efficient way to compute block
group.
This patch introduces ext4_get_group_number() which computes block
group for a given block much more efficiently. Use this function
instead of ext4_get_group_no_and_offset() everywhere where we're only
interested in knowing the block group.
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Currently in when getting the block group number for a particular
block in ext4_block_in_group() we're using
ext4_get_group_no_and_offset() which uses do_div() to get the block
group and the remainer which is offset within the group.
We don't need all of that in ext4_block_in_group() as we only need to
figure out the group number.
This commit changes ext4_block_in_group() to calculate group number
directly. This shows as a big improvement with regards to cpu
utilization. Measuring fallocate -l 15T on fresh file system with perf
showed that 23% of cpu time was spend in the
ext4_get_group_no_and_offset(). With this change it completely
disappears from the list only bumping the occurrence of
ext4_init_block_bitmap() which is the biggest user of
ext4_block_in_group() by 4%. As the result of this change on my system
the fallocate call was approx. 10% faster.
However since there is '-g' option in mkfs which allow us setting
different groups size (mostly for developers) I've introduced new per
file system flag whether we have a standard block group size or
not. The flag is used to determine whether we can use the bit shift
optimization or not.
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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We're using macro EXT4_B2C() to convert number of blocks to number of
clusters for bigalloc file systems. However, we should be using
EXT4_NUM_B2C().
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org
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ext4_has_free_clusters() should tell us whether there is enough free
clusters to allocate, however number of free clusters in the file system
is converted to blocks using EXT4_C2B() which is not only wrong use of
the macro (we should have used EXT4_NUM_B2C) but it's also completely
wrong concept since everything else is in cluster units.
Moreover when calculating number of root clusters we should be using
macro EXT4_NUM_B2C() instead of EXT4_B2C() otherwise the result might be
off by one. However r_blocks_count should always be a multiple of the
cluster ratio so doing a plain bit shift should be enough here. We
avoid using EXT4_B2C() because it's confusing.
As a result of the first problem number of free clusters is much bigger
than it should have been and ext4_has_free_clusters() would return 1 even
if there is really not enough free clusters available.
Fix this by removing the EXT4_C2B() conversion of free clusters and
using bit shift when calculating number of root clusters. This bug
affects number of xfstests tests covering file system ENOSPC situation
handling. With this patch most of the ENOSPC problems with bigalloc file
system disappear, especially the errors caused by delayed allocation not
having enough space when the actual allocation is finally requested.
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org
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Validate the bh pointer before using it, since
ext4_read_block_bitmap_nowait() might return NULL.
I've seen this in fsfuzz testing.
EXT4-fs error (device loop0): ext4_read_block_bitmap_nowait:385: comm touch: Cannot get buffer for block bitmap - block_group = 0, block_bitmap = 3925999616
BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffff8121de25>] ext4_wait_block_bitmap+0x25/0xe0
...
Call Trace:
[<ffffffff8121e1e5>] ext4_read_block_bitmap+0x35/0x60
[<ffffffff8125e9c6>] ext4_free_blocks+0x236/0xb80
[<ffffffff811d0d36>] ? __getblk+0x36/0x70
[<ffffffff811d0a5f>] ? __find_get_block+0x8f/0x210
[<ffffffff81191ef3>] ? kmem_cache_free+0x33/0x140
[<ffffffff812678e5>] ext4_xattr_release_block+0x1b5/0x1d0
[<ffffffff812679be>] ext4_xattr_delete_inode+0xbe/0x100
[<ffffffff81222a7c>] ext4_free_inode+0x7c/0x4d0
[<ffffffff812277b8>] ? ext4_mark_inode_dirty+0x88/0x230
[<ffffffff8122993c>] ext4_evict_inode+0x32c/0x490
[<ffffffff811b8cd7>] evict+0xa7/0x1c0
[<ffffffff811b8ed3>] iput_final+0xe3/0x170
[<ffffffff811b8f9e>] iput+0x3e/0x50
[<ffffffff812316fd>] ext4_add_nondir+0x4d/0x90
[<ffffffff81231d0b>] ext4_create+0xeb/0x170
[<ffffffff811aae9c>] vfs_create+0xac/0xd0
[<ffffffff811ac845>] lookup_open+0x185/0x1c0
[<ffffffff8129e3b9>] ? selinux_inode_permission+0xa9/0x170
[<ffffffff811acb54>] do_last+0x2d4/0x7a0
[<ffffffff811af743>] path_openat+0xb3/0x480
[<ffffffff8116a8a1>] ? handle_mm_fault+0x251/0x3b0
[<ffffffff811afc49>] do_filp_open+0x49/0xa0
[<ffffffff811bbaad>] ? __alloc_fd+0xdd/0x150
[<ffffffff8119da28>] do_sys_open+0x108/0x1f0
[<ffffffff8119db51>] sys_open+0x21/0x30
[<ffffffff81618959>] system_call_fastpath+0x16/0x1b
Also fix comment for ext4_read_block_bitmap_nowait()
Signed-off-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org
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In mke2fs, we only checksum the whole bitmap block and it is right.
While in the kernel, we use EXT4_BLOCKS_PER_GROUP to indicate the
size of the checksumed bitmap which is wrong when we enable bigalloc.
The right size should be EXT4_CLUSTERS_PER_GROUP and this patch fixes
it.
Also as every caller of ext4_block_bitmap_csum_set and
ext4_block_bitmap_csum_verify pass in EXT4_BLOCKS_PER_GROUP(sb)/8,
we'd better removes this parameter and sets it in the function itself.
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Cc: stable@vger.kernel.org
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While in ext4_validate_block_bitmap(), if an block allocation bitmap
is found to be invalid, we call ext4_error() while the block group is
still locked. This causes ext4_commit_super() to call a function
which might sleep while in an atomic context.
There's no need to keep the block group locked at this point, so hoist
the ext4_error() call up to ext4_validate_block_bitmap() and release
the block group spinlock before calling ext4_error().
The reported stack trace can be found at:
http://article.gmane.org/gmane.comp.file-systems.ext4/33731
Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org
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Make it possible for ext4_count_free to operate on buffers and not
just data in buffer_heads.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org
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Ext3 filesystems that are converted to use as many ext4 file system
features as possible will enable uninit_bg to speed up e2fsck times.
These file systems will have a native ext3 layout of inode tables and
block allocation bitmaps (as opposed to ext4's flex_bg layout).
Unfortunately, in these cases, when first allocating a block in an
uninitialized block group, ext4 would incorrectly calculate the number
of free blocks in that block group, and then errorneously report that
the file system was corrupt:
EXT4-fs error (device vdd): ext4_mb_generate_buddy:741: group 30, 32254 clusters in bitmap, 32258 in gd
This problem can be reproduced via:
mke2fs -q -t ext4 -O ^flex_bg /dev/vdd 5g
mount -t ext4 /dev/vdd /mnt
fallocate -l 4600m /mnt/test
The problem was caused by a bone headed mistake in the check to see if a
particular metadata block was part of the block group.
Many thanks to Kees Cook for finding and bisecting the buggy commit
which introduced this bug (commit fd034a84e1, present since v3.2).
Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Reported-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Tested-by: Kees Cook <keescook@chromium.org>
Cc: stable@kernel.org
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git://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4
Pull Ext4 updates from Theodore Ts'o:
"The major new feature added in this update is Darrick J Wong's
metadata checksum feature, which adds crc32 checksums to ext4's
metadata fields.
There is also the usual set of cleanups and bug fixes."
* tag 'ext4_for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4: (44 commits)
ext4: hole-punch use truncate_pagecache_range
jbd2: use kmem_cache_zalloc wrapper instead of flag
ext4: remove mb_groups before tearing down the buddy_cache
ext4: add ext4_mb_unload_buddy in the error path
ext4: don't trash state flags in EXT4_IOC_SETFLAGS
ext4: let getattr report the right blocks in delalloc+bigalloc
ext4: add missing save_error_info() to ext4_error()
ext4: add debugging trigger for ext4_error()
ext4: protect group inode free counting with group lock
ext4: use consistent ssize_t type in ext4_file_write()
ext4: fix format flag in ext4_ext_binsearch_idx()
ext4: cleanup in ext4_discard_allocated_blocks()
ext4: return ENOMEM when mounts fail due to lack of memory
ext4: remove redundundant "(char *) bh->b_data" casts
ext4: disallow hard-linked directory in ext4_lookup
ext4: fix potential integer overflow in alloc_flex_gd()
ext4: remove needs_recovery in ext4_mb_init()
ext4: force ro mount if ext4_setup_super() fails
ext4: fix potential NULL dereference in ext4_free_inodes_counts()
ext4/jbd2: add metadata checksumming to the list of supported features
...
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Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
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metadata_csum supersedes uninit_bg. Convert the ROCOMPAT uninit_bg
flag check to a helper function that covers both, and make the
checksum calculation algorithm use either crc16 or the metadata_csum
chosen algorithm depending on which flag is set. Print a warning if
we try to mount a filesystem with both feature flags set.
Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Compute and verify the checksum of the block bitmap; this checksum is
stored in the block group descriptor.
Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Get rid of this one:
fs/ext4/balloc.c: In function 'ext4_wait_block_bitmap':
fs/ext4/balloc.c:405:3: warning: format '%llu' expects argument of
type 'long long unsigned int', but argument 6 has type 'sector_t' [-Wformat]
Happens because sector_t is u64 (unsigned long long) or unsigned long
dependent on CONFIG_64BIT.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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In ext4_read_{inode,block}_bitmap() we were setting bitmap_uptodate()
before submitting the buffer for read. The is bad, since we check
bitmap_uptodate() without locking the buffer, and so if another
process is racing with us, it's possible that they will think the
bitmap is uptodate even though the read has not completed yet,
resulting in inodes and blocks potentially getting allocated more than
once if we get really unlucky.
Addresses-Google-Bug: 2828254
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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A couple more functions can reasonably be made static if desired.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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sbi is not defined, so let ext4_free_blocks use EXT4_SB(sb) instead
when EXT4FS_DEBUG is defined.
Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
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Rename the function so it is more clear what is going on. Also rename
the various variables so it's clearer what's happening.
Also fix a missing blocks to cluster conversion when reading the
number of reserved blocks for root.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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This function really claims a number of free clusters, not blocks, so
rename it so it's clearer what's going on.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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This function really returns the number of clusters after initializing
an uninitalized block bitmap has been initialized.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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This function really counts the free clusters reported in the block
group descriptors, so rename it to reduce confusion.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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The field bg_free_blocks_count_{lo,high} in the block group
descriptor has been repurposed to hold the number of free clusters for
bigalloc functions. So rename the functions so it makes it easier to
read and audit the block allocation and block freeing code.
Note: at this point in bigalloc development we doesn't support
online resize, so this also makes it really obvious all of the places
we need to fix up to add support for online resize.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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With bigalloc changes, the i_blocks value was not correctly set (it was still
set to number of blocks being used, but in case of bigalloc, we want i_blocks
to represent the number of clusters being used). Since the quota subsystem sets
the i_blocks value, this patch fixes the quota accounting and makes sure that
the i_blocks value is set correctly.
Signed-off-by: Aditya Kali <adityakali@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Convert the percpu counters s_dirtyblocks_counter and
s_freeblocks_counter in struct ext4_super_info to be
s_dirtyclusters_counter and s_freeclusters_counter.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Certain parts of the ext4 code base, primarily in mballoc.c, use a
block group number and offset from the beginning of the block group.
This offset is invariably used to index into the allocation bitmap, so
change the offset to be denominated in units of clusters.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Add bigalloc support to ext4_init_block_bitmap() and
ext4_free_blocks_after_init().
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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The function ext4_free_blocks_after_init() used to be a #define of
ext4_init_block_bitmap(). This actually made it difficult to
understand how the function worked, and made it hard make changes to
support clusters. So as an initial cleanup, I've separated out the
functionality of initializing block bitmap from calculating the number
of free blocks in the new block group.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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This makes it easier to understand how ext4_init_block_bitmap() works,
and it will assist when we split out ext4_free_blocks_after_init() in
the next commit.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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I found that ext4_ext_find_goal() and ext4_find_near()
share the same code for returning a coloured start block
based on i_block_group.
We can refactor this into a common function so that they
don't diverge in the future.
Thanks to adilger for suggesting the new function name.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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This patch adds an allocation request flag to the ext4_has_free_blocks
function which enables the use of reserved blocks. This will allow a
punch hole to proceed even if the disk is full. Punching a hole may
require additional blocks to first split the extents.
Because ext4_has_free_blocks is a low level function, the flag needs
to be passed down through several functions listed below:
ext4_ext_insert_extent
ext4_ext_create_new_leaf
ext4_ext_grow_indepth
ext4_ext_split
ext4_ext_new_meta_block
ext4_mb_new_blocks
ext4_claim_free_blocks
ext4_has_free_blocks
[ext4 punch hole patch series 1/5 v7]
Signed-off-by: Allison Henderson <achender@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Mingming Cao <cmm@us.ibm.com>
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In preparation for the next patch, the function ext4_add_groupblocks()
is moved to mballoc.c, where it could use some static functions.
Signed-off-by: Amir Goldstein <amir73il@users.sf.net>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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percpu_counter_sum_positive() never returns a negative value.
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Fixes generated by 'codespell' and manually reviewed.
Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
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- Add more ext4 tracepoints.
- Change ext4 tracepoints to use dev_t field with MAJOR/MINOR macros
so that we can save 4 bytes in the ring buffer on some platforms.
- Add sync_mode to ext4_da_writepages, ext4_da_write_pages, and
ext4_da_writepages_result tracepoints. Also remove for_reclaim
field from ext4_da_writepages since it is usually not very useful.
Signed-off-by: Jiaying Zhang <jiayingz@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Remove the short element i_delalloc_reserved_flag from the
ext4_inode_info structure and replace it a new bit in i_state_flags.
Since we have an ext4_inode_info for every ext4 inode cached in the
inode cache, any savings we can produce here is a very good thing from
a memory utilization perspective.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Fix a namespace leak from fs/ext4
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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These functions have no need to be exported beyond file context.
No functions needed to be moved for this commit; just some function
declarations changed to be static and removed from header files.
(A similar patch was submitted by Eric Sandeen, but I wanted to handle
code movement in separate patches to make sure code changes didn't
accidentally get dropped.)
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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We don't need to set s_dirt in most of the ext4 code when journaling
is enabled. In ext3/4 some of the summary statistics for # of free
inodes, blocks, and directories are calculated from the per-block
group statistics when the file system is mounted or unmounted. As a
result the superblock doesn't have to be updated, either via the
journal or by setting s_dirt. There are a few exceptions, most
notably when resizing the file system, where the superblock needs to
be modified --- and in that case it should be done as a journalled
operation if possible, and s_dirt set only in no-journal mode.
This patch will optimize out some unneeded disk writes when using ext4
with a journal.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Because we can badly over-reserve metadata when we
calculate worst-case, it complicates things for quota, since
we must reserve and then claim later, retry on EDQUOT, etc.
Quota is also a generally smaller pool than fs free blocks,
so this over-reservation hurts more, and more often.
I'm of the opinion that it's not the worst thing to allow
metadata to push a user slightly over quota. This simplifies
the code and avoids the false quota rejections that result
from worst-case speculation.
This patch stops the speculative quota-charging for
worst-case metadata requirements, and just charges quota
when the blocks are allocated at writeout. It also is
able to remove the try-again loop on EDQUOT.
This patch has been tested indirectly by running the xfstests
suite with a hack to mount & enable quota prior to the test.
I also did a more specific test of fragmenting freespace
and then doing a large delalloc write under quota; quota
stopped me at the right amount of file IO, and then the
writeout generated enough metadata (due to the fragmentation)
that it put me slightly over quota, as expected.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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There are duplicate macro definitions of in_range() in mballoc.h and
balloc.c. This consolidates these two definitions into ext4.h, and
changes extents.c to use in_range() as well.
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andreas Dilger <adilger@sun.com>
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This is a cleanup and simplification patch which takes some open-coded
calculations to calculate the first block number of a group and
converts them to use the (already defined) ext4_group_first_block_no()
function.
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andreas Dilger <adilger@sun.com>
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Just a pet peeve of mine; we had a mishash of calls with either __func__
or "function_name" and the latter tends to get out of sync.
I think it's easier to just hide the __func__ in a macro, and it'll
be consistent from then on.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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ext4_mb_free_blocks() is only called by ext4_free_blocks(), and the
latter function doesn't really do much. So merge the two functions
together, such that ext4_free_blocks() is now found in
fs/ext4/mballoc.c. This saves about 200 bytes of compiled text space.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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The number of old-style block group descriptor blocks is
s_meta_first_bg when the meta_bg feature flag is set.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org
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ext4_mb_update_group_info is only called in one place, and it's
extremely simple. There's no reason to have it in a separate function
in a separate file as far as I can tell, it just obfuscates what's
really going on.
Perhaps it was intended to keep the grp->bb_* manipulation local to
mballoc.c but we're already accessing other grp-> fields in balloc.c
directly so this seems ok.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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We have sb_bgl_lock() and ext4_group_info.bb_state
bit spinlock to protech group information. The later is only
used within mballoc code. Consolidate them to use sb_bgl_lock().
This makes the mballoc.c code much simpler and also avoid
confusion with two locks protecting same info.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Move the function prototypes in group.h into ext4.h so they are all
defined in one place.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Ext4's on-line resizing adds a new block group and then, only at the
last step adjusts s_groups_count. However, it's possible on SMP
systems that another CPU could see the updated the s_group_count and
not see the newly initialized data structures for the just-added block
group. For this reason, it's important to insert a SMP read barrier
after reading s_groups_count and before reading any (for example) the
new block group descriptors allowed by the increased value of
s_groups_count.
Unfortunately, we rather blatently violate this locking protocol
documented in fs/ext4/resize.c. Fortunately, (1) on-line resizes
happen relatively rarely, and (2) it seems rare that the filesystem
code will immediately try to use just-added block group before any
memory ordering issues resolve themselves. So apparently problems
here are relatively hard to hit, since ext3 has been vulnerable to the
same issue for years with no one apparently complaining.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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