From 4b0a34e222e5d087a623651a3f2df7c9bfec8e6c Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Tue, 15 Jun 2021 18:23:53 -0700 Subject: mm/thp: try_to_unmap() use TTU_SYNC for safe splitting [ Upstream commit 732ed55823fc3ad998d43b86bf771887bcc5ec67 ] Stressing huge tmpfs often crashed on unmap_page()'s VM_BUG_ON_PAGE (!unmap_success): with dump_page() showing mapcount:1, but then its raw struct page output showing _mapcount ffffffff i.e. mapcount 0. And even if that particular VM_BUG_ON_PAGE(!unmap_success) is removed, it is immediately followed by a VM_BUG_ON_PAGE(compound_mapcount(head)), and further down an IS_ENABLED(CONFIG_DEBUG_VM) total_mapcount BUG(): all indicative of some mapcount difficulty in development here perhaps. But the !CONFIG_DEBUG_VM path handles the failures correctly and silently. I believe the problem is that once a racing unmap has cleared pte or pmd, try_to_unmap_one() may skip taking the page table lock, and emerge from try_to_unmap() before the racing task has reached decrementing mapcount. Instead of abandoning the unsafe VM_BUG_ON_PAGE(), and the ones that follow, use PVMW_SYNC in try_to_unmap_one() in this case: adding TTU_SYNC to the options, and passing that from unmap_page(). When CONFIG_DEBUG_VM, or for non-debug too? Consensus is to do the same for both: the slight overhead added should rarely matter, except perhaps if splitting sparsely-populated multiply-mapped shmem. Once confident that bugs are fixed, TTU_SYNC here can be removed, and the race tolerated. Link: https://lkml.kernel.org/r/c1e95853-8bcd-d8fd-55fa-e7f2488e78f@google.com Fixes: fec89c109f3a ("thp: rewrite freeze_page()/unfreeze_page() with generic rmap walkers") Signed-off-by: Hugh Dickins Cc: Alistair Popple Cc: Jan Kara Cc: Jue Wang Cc: Kirill A. Shutemov Cc: "Matthew Wilcox (Oracle)" Cc: Miaohe Lin Cc: Minchan Kim Cc: Naoya Horiguchi Cc: Oscar Salvador Cc: Peter Xu Cc: Ralph Campbell Cc: Shakeel Butt Cc: Wang Yugui Cc: Yang Shi Cc: Zi Yan Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Note on stable backport: upstream TTU_SYNC 0x10 takes the value which 5.11 commit 013339df116c ("mm/rmap: always do TTU_IGNORE_ACCESS") freed. It is very tempting to backport that commit (as 5.10 already did) and make no change here; but on reflection, good as that commit is, I'm reluctant to include any possible side-effect of it in this series. Signed-off-by: Hugh Dickins Signed-off-by: Greg Kroah-Hartman --- include/linux/rmap.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'include') diff --git a/include/linux/rmap.h b/include/linux/rmap.h index d7d6d4eb1794..91ccae946716 100644 --- a/include/linux/rmap.h +++ b/include/linux/rmap.h @@ -98,7 +98,8 @@ enum ttu_flags { * do a final flush if necessary */ TTU_RMAP_LOCKED = 0x80, /* do not grab rmap lock: * caller holds it */ - TTU_SPLIT_FREEZE = 0x100, /* freeze pte under splitting thp */ + TTU_SPLIT_FREEZE = 0x100, /* freeze pte under splitting thp */ + TTU_SYNC = 0x200, /* avoid racy checks with PVMW_SYNC */ }; #ifdef CONFIG_MMU -- cgit v1.2.3