From: Hugh Dickins <hughd@xxxxxxxxxx> Subject: mm/thp: try_to_unmap() use TTU_SYNC for safe splitting 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@xxxxxxxxxx Fixes: fec89c109f3a ("thp: rewrite freeze_page()/unfreeze_page() with generic rmap walkers") Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Alistair Popple <apopple@xxxxxxxxxx> Cc: Jan Kara <jack@xxxxxxx> Cc: Jue Wang <juew@xxxxxxxxxx> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx> Cc: Minchan Kim <minchan@xxxxxxxxxx> Cc: Naoya Horiguchi <naoya.horiguchi@xxxxxxx> Cc: Oscar Salvador <osalvador@xxxxxxx> Cc: Peter Xu <peterx@xxxxxxxxxx> Cc: Ralph Campbell <rcampbell@xxxxxxxxxx> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> Cc: Wang Yugui <wangyugui@xxxxxxxxxxxx> Cc: Yang Shi <shy828301@xxxxxxxxx> Cc: Zi Yan <ziy@xxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/rmap.h | 1 + mm/huge_memory.c | 2 +- mm/page_vma_mapped.c | 11 +++++++++++ mm/rmap.c | 17 ++++++++++++++++- 4 files changed, 29 insertions(+), 2 deletions(-) --- a/include/linux/rmap.h~mm-thp-try_to_unmap-use-ttu_sync-for-safe-splitting +++ a/include/linux/rmap.h @@ -91,6 +91,7 @@ enum ttu_flags { TTU_SPLIT_HUGE_PMD = 0x4, /* split huge PMD if any */ TTU_IGNORE_MLOCK = 0x8, /* ignore mlock */ + TTU_SYNC = 0x10, /* avoid racy checks with PVMW_SYNC */ TTU_IGNORE_HWPOISON = 0x20, /* corrupted page is recoverable */ TTU_BATCH_FLUSH = 0x40, /* Batch TLB flushes where possible * and caller guarantees they will --- a/mm/huge_memory.c~mm-thp-try_to_unmap-use-ttu_sync-for-safe-splitting +++ a/mm/huge_memory.c @@ -2350,7 +2350,7 @@ void vma_adjust_trans_huge(struct vm_are static void unmap_page(struct page *page) { - enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | + enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_SYNC | TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD; bool unmap_success; --- a/mm/page_vma_mapped.c~mm-thp-try_to_unmap-use-ttu_sync-for-safe-splitting +++ a/mm/page_vma_mapped.c @@ -212,6 +212,17 @@ restart: 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)) --- a/mm/rmap.c~mm-thp-try_to_unmap-use-ttu_sync-for-safe-splitting +++ a/mm/rmap.c @@ -1405,6 +1405,15 @@ static bool try_to_unmap_one(struct page struct mmu_notifier_range range; enum ttu_flags flags = (enum ttu_flags)(long)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; @@ -1777,7 +1786,13 @@ bool try_to_unmap(struct page *page, enu 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); } /** _
