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Thu, 10 Aug 2023 16:43:45 -0700 (PDT) MIME-Version: 1.0 References: <20230630211957.1341547-1-surenb@google.com> <0ab6524a-6917-efe2-de69-f07fb5cdd9d2@redhat.com> In-Reply-To: From: Suren Baghdasaryan Date: Thu, 10 Aug 2023 16:43:34 -0700 Message-ID: Subject: Re: [PATCH v7 0/6] Per-VMA lock support for swap and userfaults To: Matthew Wilcox Cc: David Hildenbrand , akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@suse.com, josef@toxicpanda.com, jack@suse.cz, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, michel@lespinasse.org, liam.howlett@oracle.com, jglisse@google.com, vbabka@suse.cz, minchan@google.com, dave@stgolabs.net, punit.agrawal@bytedance.com, lstoakes@gmail.com, hdanton@sina.com, apopple@nvidia.com, peterx@redhat.com, ying.huang@intel.com, yuzhao@google.com, dhowells@redhat.com, hughd@google.com, viro@zeniv.linux.org.uk, brauner@kernel.org, pasha.tatashin@soleen.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 228D9120012 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: joyoiecz9ndn3budxhnrtz7z6rh94m9r X-HE-Tag: 1691711026-466037 X-HE-Meta: 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 VASj9Wwg WNDmp+qU8DwPdKTwgC5x8T1WDqlVy+bs627r6GS+gDT986yTr4BNxwYC/DlTzv2pcUWE0Hu5zZ+LYZ+2vAiDZ9je6+Fuib/4R/zCV+TfbqP+Pd1xdoV3XR7yIaisqq5WQ3dNoV61hX1DeW3a3tcfPZoZsltIUb+SXuHcJfc1zj4bRGB9fwKGjlIJccdXNCeqQzlGA7Us8s3yHoZpe/kvyULq5gQ== 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 Thu, Aug 10, 2023 at 4:29=E2=80=AFPM Suren Baghdasaryan wrote: > > On Thu, Aug 10, 2023 at 3:16=E2=80=AFPM Matthew Wilcox wrote: > > > > On Thu, Aug 10, 2023 at 06:24:15AM +0000, Suren Baghdasaryan wrote: > > > Ok, I think I found the issue. wp_page_shared() -> > > > fault_dirty_shared_page() can drop mmap_lock (see the comment saying > > > "Drop the mmap_lock before waiting on IO, if we can...", therefore we > > > have to ensure we are not doing this under per-VMA lock. > > > > ... or we could change maybe_unlock_mmap_for_io() the same way > > that we changed folio_lock_or_retry(): > > > > +++ b/mm/internal.h > > @@ -706,7 +706,7 @@ static inline struct file *maybe_unlock_mmap_for_io= (struct vm_fault *vmf, > > if (fault_flag_allow_retry_first(flags) && > > !(flags & FAULT_FLAG_RETRY_NOWAIT)) { > > fpin =3D get_file(vmf->vma->vm_file); > > - mmap_read_unlock(vmf->vma->vm_mm); > > + release_fault_lock(vmf); > > } > > return fpin; > > } > > > > What do you think? > > This is very tempting... Let me try that and see if anything explodes, > but yes, this would be ideal. Ok, so far looks good, the problem is not reproducible. I'll run some more exhaustive testing today. > > > > > > > I think what happens is that this path is racing with another page > > > fault which took mmap_lock for read. fault_dirty_shared_page() > > > releases this lock which was taken by another page faulting thread an= d > > > that thread generates an assertion when it finds out the lock it just > > > took got released from under it. > > > > I'm confused that our debugging didn't catch this earlier. lockdep > > should always catch this. > > Maybe this condition is rare enough?