Re: [PATCH v2 01/10] mm/thp: fix __split_huge_pmd_locked() on shmem migration entry (fwd)

[Kirill A. Shutemov]

On Tue, Jun 08, 2021 at 09:05:21PM -0700, Hugh Dickins wrote:
> 
> 
> ---------- Forwarded message ----------
> Date: Tue, 8 Jun 2021 21:00:12 -0700 (PDT)
> From: Hugh Dickins <hughd@xxxxxxxxxx>
> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>,
>     Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>,
>     Yang Shi <shy828301@xxxxxxxxx>, Wang Yugui <wangyugui@xxxxxxxxxxxx>,
>     Matthew Wilcox <willy@xxxxxxxxxxxxx>,
>     Naoya Horiguchi <naoya.horiguchi@xxxxxxx>,
>     Alistair Popple <apopple@xxxxxxxxxx>, Ralph Campbell <rcampbell@xxxxxxxxxx>,
>     Zi Yan <ziy@xxxxxxxxxx>, Miaohe Lin <linmiaohe@xxxxxxxxxx>,
>     Minchan Kim <minchan@xxxxxxxxxx>, Jue Wang <juew@xxxxxxxxxx>,
>     Peter Xu <peterx@xxxxxxxxxx>, Jan Kara <jack@xxxxxxx>,
>     Shakeel Butt <shakeelb@xxxxxxxxxx>, Oscar Salvador <osalvador@xxxxxxx>
> Subject: [PATCH v2 01/10] mm/thp: fix __split_huge_pmd_locked() on shmem
>     migration entry
> 
> 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.
> 
> Fixes: e71769ae5260 ("mm: enable thp migration for shmem thp")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>


-- 
 Kirill A. Shutemov