From 7496fea9a6bf644afe360af795b121a77635b37d Mon Sep 17 00:00:00 2001 From: Zhou Chengming Date: Thu, 12 May 2016 15:42:21 -0700 Subject: ksm: fix conflict between mmput and scan_get_next_rmap_item A concurrency issue about KSM in the function scan_get_next_rmap_item. task A (ksmd): |task B (the mm's task): | mm = slot->mm; | down_read(&mm->mmap_sem); | | ... | | spin_lock(&ksm_mmlist_lock); | | ksm_scan.mm_slot go to the next slot; | | spin_unlock(&ksm_mmlist_lock); | |mmput() -> | ksm_exit(): | |spin_lock(&ksm_mmlist_lock); |if (mm_slot && ksm_scan.mm_slot != mm_slot) { | if (!mm_slot->rmap_list) { | easy_to_free = 1; | ... | |if (easy_to_free) { | mmdrop(mm); | ... | |So this mm_struct may be freed in the mmput(). | up_read(&mm->mmap_sem); | As we can see above, the ksmd thread may access a mm_struct that already been freed to the kmem_cache. Suppose a fork will get this mm_struct from the kmem_cache, the ksmd thread then call up_read(&mm->mmap_sem), will cause mmap_sem.count to become -1. As suggested by Andrea Arcangeli, unmerge_and_remove_all_rmap_items has the same SMP race condition, so fix it too. My prev fix in function scan_get_next_rmap_item will introduce a different SMP race condition, so just invert the up_read/spin_unlock order as Andrea Arcangeli said. Link: http://lkml.kernel.org/r/1462708815-31301-1-git-send-email-zhouchengming1@huawei.com Signed-off-by: Zhou Chengming Suggested-by: Andrea Arcangeli Reviewed-by: Andrea Arcangeli Cc: Hugh Dickins Cc: Kirill A. Shutemov Cc: Vlastimil Babka Cc: Geliang Tang Cc: Minchan Kim Cc: Hanjun Guo Cc: Ding Tianhong Cc: Li Bin Cc: Zhen Lei Cc: Xishi Qiu Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/ksm.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-) (limited to 'mm') diff --git a/mm/ksm.c b/mm/ksm.c index b99e828172f6..4786b4150f62 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -783,6 +783,7 @@ static int unmerge_and_remove_all_rmap_items(void) } remove_trailing_rmap_items(mm_slot, &mm_slot->rmap_list); + up_read(&mm->mmap_sem); spin_lock(&ksm_mmlist_lock); ksm_scan.mm_slot = list_entry(mm_slot->mm_list.next, @@ -794,12 +795,9 @@ static int unmerge_and_remove_all_rmap_items(void) free_mm_slot(mm_slot); clear_bit(MMF_VM_MERGEABLE, &mm->flags); - up_read(&mm->mmap_sem); mmdrop(mm); - } else { + } else spin_unlock(&ksm_mmlist_lock); - up_read(&mm->mmap_sem); - } } /* Clean up stable nodes, but don't worry if some are still busy */ @@ -1663,8 +1661,15 @@ next_mm: up_read(&mm->mmap_sem); mmdrop(mm); } else { - spin_unlock(&ksm_mmlist_lock); up_read(&mm->mmap_sem); + /* + * up_read(&mm->mmap_sem) first because after + * spin_unlock(&ksm_mmlist_lock) run, the "mm" may + * already have been freed under us by __ksm_exit() + * because the "mm_slot" is still hashed and + * ksm_scan.mm_slot doesn't point to it anymore. + */ + spin_unlock(&ksm_mmlist_lock); } /* Repeat until we've completed scanning the whole list */ -- cgit v1.2.3 From 6d0a07edd17cfc12fdc1f36de8072fa17cc3666f Mon Sep 17 00:00:00 2001 From: Andrea Arcangeli Date: Thu, 12 May 2016 15:42:25 -0700 Subject: mm: thp: calculate the mapcount correctly for THP pages during WP faults This will provide fully accuracy to the mapcount calculation in the write protect faults, so page pinning will not get broken by false positive copy-on-writes. total_mapcount() isn't the right calculation needed in reuse_swap_page(), so this introduces a page_trans_huge_mapcount() that is effectively the full accurate return value for page_mapcount() if dealing with Transparent Hugepages, however we only use the page_trans_huge_mapcount() during COW faults where it strictly needed, due to its higher runtime cost. This also provide at practical zero cost the total_mapcount information which is needed to know if we can still relocate the page anon_vma to the local vma. If page_trans_huge_mapcount() returns 1 we can reuse the page no matter if it's a pte or a pmd_trans_huge triggering the fault, but we can only relocate the page anon_vma to the local vma->anon_vma if we're sure it's only this "vma" mapping the whole THP physical range. Kirill A. Shutemov discovered the problem with moving the page anon_vma to the local vma->anon_vma in a previous version of this patch and another problem in the way page_move_anon_rmap() was called. Andrew Morton discovered that CONFIG_SWAP=n wouldn't build in a previous version, because reuse_swap_page must be a macro to call page_trans_huge_mapcount from swap.h, so this uses a macro again instead of an inline function. With this change at least it's a less dangerous usage than it was before, because "page" is used only once now, while with the previous code reuse_swap_page(page++) would have called page_mapcount on page+1 and it would have increased page twice instead of just once. Dean Luick noticed an uninitialized variable that could result in a rmap inefficiency for the non-THP case in a previous version. Mike Marciniszyn said: : Our RDMA tests are seeing an issue with memory locking that bisects to : commit 61f5d698cc97 ("mm: re-enable THP") : : The test program registers two rather large MRs (512M) and RDMA : writes data to a passive peer using the first and RDMA reads it back : into the second MR and compares that data. The sizes are chosen randomly : between 0 and 1024 bytes. : : The test will get through a few (<= 4 iterations) and then gets a : compare error. : : Tracing indicates the kernel logical addresses associated with the individual : pages at registration ARE correct , the data in the "RDMA read response only" : packets ARE correct. : : The "corruption" occurs when the packet crosse two pages that are not physically : contiguous. The second page reads back as zero in the program. : : It looks like the user VA at the point of the compare error no longer points to : the same physical address as was registered. : : This patch totally resolves the issue! Link: http://lkml.kernel.org/r/1462547040-1737-2-git-send-email-aarcange@redhat.com Signed-off-by: Andrea Arcangeli Reviewed-by: "Kirill A. Shutemov" Reviewed-by: Dean Luick Tested-by: Alex Williamson Tested-by: Mike Marciniszyn Tested-by: Josh Collier Cc: Marc Haber Cc: [4.5] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/huge_memory.c | 71 +++++++++++++++++++++++++++++++++++++++++++++++++------- mm/memory.c | 22 +++++++++++------- mm/swapfile.c | 13 ++++++----- 3 files changed, 83 insertions(+), 23 deletions(-) (limited to 'mm') diff --git a/mm/huge_memory.c b/mm/huge_memory.c index f7daa7de8f48..b49ee126d4d1 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1298,15 +1298,9 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma, VM_BUG_ON_PAGE(!PageCompound(page) || !PageHead(page), page); /* * We can only reuse the page if nobody else maps the huge page or it's - * part. We can do it by checking page_mapcount() on each sub-page, but - * it's expensive. - * The cheaper way is to check page_count() to be equal 1: every - * mapcount takes page reference reference, so this way we can - * guarantee, that the PMD is the only mapping. - * This can give false negative if somebody pinned the page, but that's - * fine. + * part. */ - if (page_mapcount(page) == 1 && page_count(page) == 1) { + if (page_trans_huge_mapcount(page, NULL) == 1) { pmd_t entry; entry = pmd_mkyoung(orig_pmd); entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma); @@ -2079,7 +2073,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma, if (pte_write(pteval)) { writable = true; } else { - if (PageSwapCache(page) && !reuse_swap_page(page)) { + if (PageSwapCache(page) && + !reuse_swap_page(page, NULL)) { unlock_page(page); result = SCAN_SWAP_CACHE_PAGE; goto out; @@ -3222,6 +3217,64 @@ int total_mapcount(struct page *page) return ret; } +/* + * This calculates accurately how many mappings a transparent hugepage + * has (unlike page_mapcount() which isn't fully accurate). This full + * accuracy is primarily needed to know if copy-on-write faults can + * reuse the page and change the mapping to read-write instead of + * copying them. At the same time this returns the total_mapcount too. + * + * The function returns the highest mapcount any one of the subpages + * has. If the return value is one, even if different processes are + * mapping different subpages of the transparent hugepage, they can + * all reuse it, because each process is reusing a different subpage. + * + * The total_mapcount is instead counting all virtual mappings of the + * subpages. If the total_mapcount is equal to "one", it tells the + * caller all mappings belong to the same "mm" and in turn the + * anon_vma of the transparent hugepage can become the vma->anon_vma + * local one as no other process may be mapping any of the subpages. + * + * It would be more accurate to replace page_mapcount() with + * page_trans_huge_mapcount(), however we only use + * page_trans_huge_mapcount() in the copy-on-write faults where we + * need full accuracy to avoid breaking page pinning, because + * page_trans_huge_mapcount() is slower than page_mapcount(). + */ +int page_trans_huge_mapcount(struct page *page, int *total_mapcount) +{ + int i, ret, _total_mapcount, mapcount; + + /* hugetlbfs shouldn't call it */ + VM_BUG_ON_PAGE(PageHuge(page), page); + + if (likely(!PageTransCompound(page))) { + mapcount = atomic_read(&page->_mapcount) + 1; + if (total_mapcount) + *total_mapcount = mapcount; + return mapcount; + } + + page = compound_head(page); + + _total_mapcount = ret = 0; + for (i = 0; i < HPAGE_PMD_NR; i++) { + mapcount = atomic_read(&page[i]._mapcount) + 1; + ret = max(ret, mapcount); + _total_mapcount += mapcount; + } + if (PageDoubleMap(page)) { + ret -= 1; + _total_mapcount -= HPAGE_PMD_NR; + } + mapcount = compound_mapcount(page); + ret += mapcount; + _total_mapcount += mapcount; + if (total_mapcount) + *total_mapcount = _total_mapcount; + return ret; +} + /* * This function splits huge page into normal pages. @page can point to any * subpage of huge page to split. Split doesn't change the position of @page. diff --git a/mm/memory.c b/mm/memory.c index 52c218e2b724..07493e34ab7e 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2373,6 +2373,7 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma, * not dirty accountable. */ if (PageAnon(old_page) && !PageKsm(old_page)) { + int total_mapcount; if (!trylock_page(old_page)) { get_page(old_page); pte_unmap_unlock(page_table, ptl); @@ -2387,13 +2388,18 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma, } put_page(old_page); } - if (reuse_swap_page(old_page)) { - /* - * The page is all ours. Move it to our anon_vma so - * the rmap code will not search our parent or siblings. - * Protected against the rmap code by the page lock. - */ - page_move_anon_rmap(old_page, vma, address); + if (reuse_swap_page(old_page, &total_mapcount)) { + if (total_mapcount == 1) { + /* + * The page is all ours. Move it to + * our anon_vma so the rmap code will + * not search our parent or siblings. + * Protected against the rmap code by + * the page lock. + */ + page_move_anon_rmap(compound_head(old_page), + vma, address); + } unlock_page(old_page); return wp_page_reuse(mm, vma, address, page_table, ptl, orig_pte, old_page, 0, 0); @@ -2617,7 +2623,7 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma, inc_mm_counter_fast(mm, MM_ANONPAGES); dec_mm_counter_fast(mm, MM_SWAPENTS); pte = mk_pte(page, vma->vm_page_prot); - if ((flags & FAULT_FLAG_WRITE) && reuse_swap_page(page)) { + if ((flags & FAULT_FLAG_WRITE) && reuse_swap_page(page, NULL)) { pte = maybe_mkwrite(pte_mkdirty(pte), vma); flags &= ~FAULT_FLAG_WRITE; ret |= VM_FAULT_WRITE; diff --git a/mm/swapfile.c b/mm/swapfile.c index 83874eced5bf..031713ab40ce 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -922,18 +922,19 @@ out: * to it. And as a side-effect, free up its swap: because the old content * on disk will never be read, and seeking back there to write new content * later would only waste time away from clustering. + * + * NOTE: total_mapcount should not be relied upon by the caller if + * reuse_swap_page() returns false, but it may be always overwritten + * (see the other implementation for CONFIG_SWAP=n). */ -int reuse_swap_page(struct page *page) +bool reuse_swap_page(struct page *page, int *total_mapcount) { int count; VM_BUG_ON_PAGE(!PageLocked(page), page); if (unlikely(PageKsm(page))) - return 0; - /* The page is part of THP and cannot be reused */ - if (PageTransCompound(page)) - return 0; - count = page_mapcount(page); + return false; + count = page_trans_huge_mapcount(page, total_mapcount); if (count <= 1 && PageSwapCache(page)) { count += page_swapcount(page); if (count == 1 && !PageWriteback(page)) { -- cgit v1.2.3