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Thu, 10 Aug 2023 23:13:14 -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 23:13:00 -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: 57CEF8000E X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: 6ouxpbihue97borbuh6ez3hiuysytob5 X-HE-Tag: 1691734395-37910 X-HE-Meta: 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 5xlXos+r 2XBXgdFatIdtNfDcSkhGJ6MOpj6VC5rxE4CqCaTNM5zN8blp6vKl8wcocLm/RIR8+LOePaR2PlHVkOoIPTaRcDDAMMqWvnU4bL6GUhYa2/Wlqklf3tXHuhKbP1jH6KREPFA0vKtONr4VSFzV/CNJ9gZa5QDXHMXFL1/1aczv4YA7VI73ouAAp30PLh/iVgBEN5jF00YsIxzrtiOPwBchaceCv8FdnPtI+V4WTHNOIAhLsCN8DYrnBfIe/AdwTUz0qUIkIY2xZ/SdXC1WzSLETMmU1wCOUDc2i/Z7QsvdiDzBFsHDv0nt9iYbOGQ== 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 4:43=E2=80=AFPM Suren Baghdasaryan wrote: > > 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 sayin= g > > > > "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. So far it works without a glitch. Matthew, I think it's fine. If you post a fixup please add my Tested-by. Thanks, Suren. > > > > > > > > > > > > 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 = and > > > > that thread generates an assertion when it finds out the lock it ju= st > > > > 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?