linux-kernel - Re: [PATCH v2 01/10] mm/thp: fix __split_huge_pmd

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Date:   Wed, 9 Jun 2021 13:17:59 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/10] mm/thp: fix __split_huge_pmd_locked() on shmem
 migration entry (fwd)

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@...gle.com>
> To: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Hugh Dickins <hughd@...gle.com>,
>     Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>,
>     Yang Shi <shy828301@...il.com>, Wang Yugui <wangyugui@...-tech.com>,
>     Matthew Wilcox <willy@...radead.org>,
>     Naoya Horiguchi <naoya.horiguchi@....com>,
>     Alistair Popple <apopple@...dia.com>, Ralph Campbell <rcampbell@...dia.com>,
>     Zi Yan <ziy@...dia.com>, Miaohe Lin <linmiaohe@...wei.com>,
>     Minchan Kim <minchan@...nel.org>, Jue Wang <juew@...gle.com>,
>     Peter Xu <peterx@...hat.com>, Jan Kara <jack@...e.cz>,
>     Shakeel Butt <shakeelb@...gle.com>, Oscar Salvador <osalvador@...e.de>
> 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@...gle.com>
> Cc: <stable@...r.kernel.org>

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>


-- 
 Kirill A. Shutemov