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    Subject[PATCH 5.10 075/101] mm/thp: fix __split_huge_pmd_locked() on shmem migration entry
    Date
    From: Hugh Dickins <hughd@google.com>

    [ Upstream commit 99fa8a48203d62b3743d866fc48ef6abaee682be ]

    Patch series "mm/thp: fix THP splitting unmap BUGs and related", v10.

    Here is v2 batch of long-standing THP bug fixes that I had not got
    around to sending before, but prompted now by Wang Yugui's report
    https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@e16-tech.com/

    Wang Yugui has tested a rollup of these fixes applied to 5.10.39, and
    they have done no harm, but have *not* fixed that issue: something more
    is needed and I have no idea of what.

    This patch (of 7):

    Stressing huge tmpfs page migration racing hole punch often crashed on
    the VM_BUG_ON(!pmd_present) in pmdp_huge_clear_flush(), with DEBUG_VM=y
    kernel; or shortly afterwards, on a bad dereference in
    __split_huge_pmd_locked() when DEBUG_VM=n. They forgot to allow for pmd
    migration entries in the non-anonymous case.

    Full disclosure: those particular experiments were on a kernel with more
    relaxed mmap_lock and i_mmap_rwsem locking, and were not repeated on the
    vanilla kernel: it is conceivable that stricter locking happens to avoid
    those cases, or makes them less likely; but __split_huge_pmd_locked()
    already allowed for pmd migration entries when handling anonymous THPs,
    so this commit brings the shmem and file THP handling into line.

    And while there: use old_pmd rather than _pmd, as in the following
    blocks; and make it clearer to the eye that the !vma_is_anonymous()
    block is self-contained, making an early return after accounting for
    unmapping.

    Link: https://lkml.kernel.org/r/af88612-1473-2eaa-903-8d1a448b26@google.com
    Link: https://lkml.kernel.org/r/dd221a99-efb3-cd1d-6256-7e646af29314@google.com
    Fixes: e71769ae5260 ("mm: enable thp migration for shmem thp")
    Signed-off-by: Hugh Dickins <hughd@google.com>
    Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Cc: Yang Shi <shy828301@gmail.com>
    Cc: Wang Yugui <wangyugui@e16-tech.com>
    Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
    Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
    Cc: Alistair Popple <apopple@nvidia.com>
    Cc: Ralph Campbell <rcampbell@nvidia.com>
    Cc: Zi Yan <ziy@nvidia.com>
    Cc: Miaohe Lin <linmiaohe@huawei.com>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Jue Wang <juew@google.com>
    Cc: Peter Xu <peterx@redhat.com>
    Cc: Jan Kara <jack@suse.cz>
    Cc: Shakeel Butt <shakeelb@google.com>
    Cc: Oscar Salvador <osalvador@suse.de>
    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: this commit made intervening cleanups in
    pmdp_huge_clear_flush() redundant: here it's rediffed to skip them.

    Signed-off-by: Hugh Dickins <hughd@google.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    mm/huge_memory.c | 27 ++++++++++++++++++---------
    mm/pgtable-generic.c | 4 ++--
    2 files changed, 20 insertions(+), 11 deletions(-)

    diff --git a/mm/huge_memory.c b/mm/huge_memory.c
    index d9ade23ac2b2..be0ad6f7981e 100644
    --- a/mm/huge_memory.c
    +++ b/mm/huge_memory.c
    @@ -2031,7 +2031,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
    count_vm_event(THP_SPLIT_PMD);

    if (!vma_is_anonymous(vma)) {
    - _pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
    + old_pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
    /*
    * We are going to unmap this huge page. So
    * just go ahead and zap it
    @@ -2040,16 +2040,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
    zap_deposited_table(mm, pmd);
    if (vma_is_special_huge(vma))
    return;
    - page = pmd_page(_pmd);
    - if (!PageDirty(page) && pmd_dirty(_pmd))
    - set_page_dirty(page);
    - if (!PageReferenced(page) && pmd_young(_pmd))
    - SetPageReferenced(page);
    - page_remove_rmap(page, true);
    - put_page(page);
    + if (unlikely(is_pmd_migration_entry(old_pmd))) {
    + swp_entry_t entry;
    +
    + entry = pmd_to_swp_entry(old_pmd);
    + page = migration_entry_to_page(entry);
    + } else {
    + page = pmd_page(old_pmd);
    + if (!PageDirty(page) && pmd_dirty(old_pmd))
    + set_page_dirty(page);
    + if (!PageReferenced(page) && pmd_young(old_pmd))
    + SetPageReferenced(page);
    + page_remove_rmap(page, true);
    + put_page(page);
    + }
    add_mm_counter(mm, mm_counter_file(page), -HPAGE_PMD_NR);
    return;
    - } else if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
    + }
    +
    + 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
    diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
    index 9578db83e312..4e640baf9794 100644
    --- a/mm/pgtable-generic.c
    +++ b/mm/pgtable-generic.c
    @@ -135,8 +135,8 @@ pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma, unsigned long address,
    {
    pmd_t pmd;
    VM_BUG_ON(address & ~HPAGE_PMD_MASK);
    - VM_BUG_ON((pmd_present(*pmdp) && !pmd_trans_huge(*pmdp) &&
    - !pmd_devmap(*pmdp)) || !pmd_present(*pmdp));
    + VM_BUG_ON(pmd_present(*pmdp) && !pmd_trans_huge(*pmdp) &&
    + !pmd_devmap(*pmdp));
    pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);
    flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
    return pmd;
    --
    2.30.2
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