The patch below does not apply to the 5.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From ec0abae6dcdf7ef88607c869bf35a4b63ce1b370 Mon Sep 17 00:00:00 2001

From: Ralph Campbell rcampbell@nvidia.com Date: Fri, 18 Sep 2020 21:20:24 -0700 Subject: [PATCH] mm/thp: fix __split_huge_pmd_locked() for migration PMD

A migrating transparent huge page has to already be unmapped. Otherwise, the page could be modified while it is being copied to a new page and data could be lost. The function __split_huge_pmd() checks for a PMD migration entry before calling __split_huge_pmd_locked() leading one to think that __split_huge_pmd_locked() can handle splitting a migrating PMD.

However, the code always increments the page->_mapcount and adjusts the memory control group accounting assuming the page is mapped.

Also, if the PMD entry is a migration PMD entry, the call to is_huge_zero_pmd(*pmd) is incorrect because it calls pmd_pfn(pmd) instead of migration_entry_to_pfn(pmd_to_swp_entry(pmd)). Fix these problems by checking for a PMD migration entry.

Fixes: 84c3fc4e9c56 ("mm: thp: check pmd migration entry in common path") Signed-off-by: Ralph Campbell rcampbell@nvidia.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Reviewed-by: Yang Shi shy828301@gmail.com Reviewed-by: Zi Yan ziy@nvidia.com Cc: Jerome Glisse jglisse@redhat.com Cc: John Hubbard jhubbard@nvidia.com Cc: Alistair Popple apopple@nvidia.com Cc: Christoph Hellwig hch@lst.de Cc: Jason Gunthorpe jgg@nvidia.com Cc: Bharata B Rao bharata@linux.ibm.com Cc: Ben Skeggs bskeggs@redhat.com Cc: Shuah Khan shuah@kernel.org Cc: stable@vger.kernel.org [4.14+] Link: https://lkml.kernel.org/r/20200903183140.19055-1-rcampbell@nvidia.com Signed-off-by: Linus Torvalds torvalds@linux-foundation.org

diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 7ff29cc3d55c..faadc449cca5 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2022,7 +2022,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, put_page(page); add_mm_counter(mm, mm_counter_file(page), -HPAGE_PMD_NR); return; - } else if (is_huge_zero_pmd(*pmd)) { + } else if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) { /* * FIXME: Do we want to invalidate secondary mmu by calling * mmu_notifier_invalidate_range() see comments below inside @@ -2116,30 +2116,34 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, pte = pte_offset_map(&_pmd, addr); BUG_ON(!pte_none(*pte)); set_pte_at(mm, addr, pte, entry); - atomic_inc(&page[i]._mapcount); - pte_unmap(pte); - } - - /* - * Set PG_double_map before dropping compound_mapcount to avoid - * false-negative page_mapped(). - */ - if (compound_mapcount(page) > 1 && !TestSetPageDoubleMap(page)) { - for (i = 0; i < HPAGE_PMD_NR; i++) + if (!pmd_migration) atomic_inc(&page[i]._mapcount); + pte_unmap(pte); }

- lock_page_memcg(page); - if (atomic_add_negative(-1, compound_mapcount_ptr(page))) { - /* Last compound_mapcount is gone. */ - __dec_lruvec_page_state(page, NR_ANON_THPS); - if (TestClearPageDoubleMap(page)) { - /* No need in mapcount reference anymore */ + if (!pmd_migration) { + /* + * Set PG_double_map before dropping compound_mapcount to avoid + * false-negative page_mapped(). + */ + if (compound_mapcount(page) > 1 && + !TestSetPageDoubleMap(page)) { for (i = 0; i < HPAGE_PMD_NR; i++) - atomic_dec(&page[i]._mapcount); + atomic_inc(&page[i]._mapcount); + } + + lock_page_memcg(page); + if (atomic_add_negative(-1, compound_mapcount_ptr(page))) { + /* Last compound_mapcount is gone. */ + __dec_lruvec_page_state(page, NR_ANON_THPS); + if (TestClearPageDoubleMap(page)) { + /* No need in mapcount reference anymore */ + for (i = 0; i < HPAGE_PMD_NR; i++) + atomic_dec(&page[i]._mapcount); + } } + unlock_page_memcg(page); } - unlock_page_memcg(page);

smp_wmb(); /* make pte visible before pmd */ pmd_populate(mm, pmd, pgtable);