From: Hugh Dickins hughd@google.com
[ 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 hughd@google.com Cc: Alistair Popple apopple@nvidia.com Cc: Jan Kara jack@suse.cz Cc: Jue Wang juew@google.com Cc: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: "Matthew Wilcox (Oracle)" willy@infradead.org Cc: Miaohe Lin linmiaohe@huawei.com Cc: Minchan Kim minchan@kernel.org Cc: Naoya Horiguchi naoya.horiguchi@nec.com Cc: Oscar Salvador osalvador@suse.de Cc: Peter Xu peterx@redhat.com Cc: Ralph Campbell rcampbell@nvidia.com Cc: Shakeel Butt shakeelb@google.com Cc: Wang Yugui wangyugui@e16-tech.com Cc: Yang Shi shy828301@gmail.com Cc: Zi Yan ziy@nvidia.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org
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 hughd@google.com Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/rmap.h | 3 ++- mm/huge_memory.c | 2 +- mm/page_vma_mapped.c | 11 +++++++++++ mm/rmap.c | 17 ++++++++++++++++- 4 files changed, 30 insertions(+), 3 deletions(-)
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 diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 82ed62775c00..78c1ad5f8109 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2430,7 +2430,7 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma, static void unmap_page(struct page *page) { enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS | - TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD; + TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD | TTU_SYNC; bool unmap_success;
VM_BUG_ON_PAGE(!PageHead(page), page); diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c index 11df03e71288..08e283ad4660 100644 --- a/mm/page_vma_mapped.c +++ b/mm/page_vma_mapped.c @@ -208,6 +208,17 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) pvmw->ptl = NULL; } } else if (!pmd_present(pmde)) { + /* + * If PVMW_SYNC, take and drop THP pmd lock so that we + * cannot return prematurely, while zap_huge_pmd() has + * cleared *pmd but not decremented compound_mapcount(). + */ + if ((pvmw->flags & PVMW_SYNC) && + PageTransCompound(pvmw->page)) { + spinlock_t *ptl = pmd_lock(mm, pvmw->pmd); + + spin_unlock(ptl); + } return false; } if (!map_pte(pvmw)) diff --git a/mm/rmap.c b/mm/rmap.c index 70872d5b203c..5df055654e63 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1348,6 +1348,15 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, unsigned long start = address, end; enum ttu_flags flags = (enum ttu_flags)arg;
+ /* + * When racing against e.g. zap_pte_range() on another cpu, + * in between its ptep_get_and_clear_full() and page_remove_rmap(), + * try_to_unmap() may return false when it is about to become true, + * if page table locking is skipped: use TTU_SYNC to wait for that. + */ + if (flags & TTU_SYNC) + pvmw.flags = PVMW_SYNC; + /* munlock has nothing to gain from examining un-locked vmas */ if ((flags & TTU_MUNLOCK) && !(vma->vm_flags & VM_LOCKED)) return true; @@ -1723,7 +1732,13 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags) else rmap_walk(page, &rwc);
- return !page_mapcount(page) ? true : false; + /* + * When racing against e.g. zap_pte_range() on another cpu, + * in between its ptep_get_and_clear_full() and page_remove_rmap(), + * try_to_unmap() may return false when it is about to become true, + * if page table locking is skipped: use TTU_SYNC to wait for that. + */ + return !page_mapcount(page); }
/**