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Thu, 29 Jun 2023 09:39:53 -0700 (PDT) MIME-Version: 1.0 References: <20230628172529.744839-1-surenb@google.com> <20230628172529.744839-7-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Thu, 29 Jun 2023 09:39:41 -0700 Message-ID: Subject: Re: [PATCH v5 6/6] mm: handle userfaults under VMA lock To: Peter Xu Cc: akpm@linux-foundation.org, willy@infradead.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, ying.huang@intel.com, david@redhat.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: 94AB640015 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: zb8kzdobosg18m55gf6e3d78qp3rxkxi X-HE-Tag: 1688056794-74683 X-HE-Meta: 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 d20no8tO qFv/yV780mSCfmxdUNGKu/bnTRbb8xqThu0axs8OUMfbj5a12qTuRBtFvOy6yxP08K33wtIN8NOjY9mOIBpR/IrHZSxW3yns1h6Kf4hOLGEdtapk55Pl3XDKUFBViIQxV2Rafhy6O0rv8cgfYpW69mXAsyHM08i53aT5/X2J+BRuapMkk2OAhXj4Md4GoHUMmcGncknmJaEEklSz7P/wi10a8Sax8XVef24kVNFZbbWGWH1/O1Q64BYsUaMQrNfpscBPDIxf1+eC3no7+JFjPrqaCIg== 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, Jun 29, 2023 at 9:33=E2=80=AFAM Peter Xu wrote: > > On Wed, Jun 28, 2023 at 05:19:31PM -0700, Suren Baghdasaryan wrote: > > On Wed, Jun 28, 2023 at 10:32=E2=80=AFAM Peter Xu w= rote: > > > > > > On Wed, Jun 28, 2023 at 10:25:29AM -0700, Suren Baghdasaryan wrote: > > > > Enable handle_userfault to operate under VMA lock by releasing VMA = lock > > > > instead of mmap_lock and retrying. Note that FAULT_FLAG_RETRY_NOWAI= T > > > > should never be used when handling faults under per-VMA lock protec= tion > > > > because that would break the assumption that lock is dropped on ret= ry. > > > > > > > > Signed-off-by: Suren Baghdasaryan > > > > > > Maybe the sanitize_fault_flags() changes suite more in patch 3, but n= ot a > > > big deal I guess. > > > > IIUC FAULT_FLAG_RETRY_NOWAIT comes into play in this patchset only in > > the context of uffds, therefore that check seems to be needed when we > > enable per-VMA lock uffd support, which is this patch. Does that make > > sense? > > I don't see why uffd is special in this regard, as e.g. swap also checks > NOWAIT when folio_lock_or_retry() so I assume it's also used there. > > IMHO the "NOWAIT should never apply with VMA_LOCK so far" assumption star= ts > from patch 3 where it conditionally releases the vma lock when > !(RETRY|COMPLETE); that is the real place where it can start to go wrong = if > anyone breaks the assumption. Um, yes, you are right as usual. It was clear to me from the code that NOWAIT is not used with swap under VMA_LOCK, that's why I didn't consider this check earlier. Yeah, patch 3 seems like a more appropriate place for it. I'll move it and post a new patchset later today or tomorrow morning with your Acks. Thanks, Suren. > > Thanks, > > -- > Peter Xu >