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Shutemov" To: Hugh Dickins Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 01/10] mm/thp: fix __split_huge_pmd_locked() on shmem migration entry (fwd) Message-ID: <20210609101759.fygmcxcj2jcruoiv@box.shutemov.name> References: <59d94b4-c0dd-310-894-be99416f3c92@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <59d94b4-c0dd-310-894-be99416f3c92@google.com> X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: EEC9E4202A1E Authentication-Results: imf26.hostedemail.com; dkim=pass header.d=shutemov-name.20150623.gappssmtp.com header.s=20150623 header.b=I85F3WMK; spf=none (imf26.hostedemail.com: domain of kirill@shutemov.name has no SPF policy when checking 209.85.167.53) smtp.mailfrom=kirill@shutemov.name; dmarc=none X-Stat-Signature: 1sbbhqwju6enf17gf3yyaq1tzxq1tu4r X-HE-Tag: 1623233863-285800 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 > To: Andrew Morton > Cc: Hugh Dickins , > Kirill A. Shutemov , > Yang Shi , Wang Yugui , > Matthew Wilcox , > Naoya Horiguchi , > Alistair Popple , Ralph Campbell , > Zi Yan , Miaohe Lin , > Minchan Kim , Jue Wang , > Peter Xu , Jan Kara , > Shakeel Butt , Oscar Salvador > 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 > Cc: Acked-by: Kirill A. Shutemov -- Kirill A. Shutemov