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Tue, 13 Feb 2024 10:57:11 -0800 (PST) MIME-Version: 1.0 References: <20240213001920.3551772-1-lokeshgidra@google.com> <20240213001920.3551772-4-lokeshgidra@google.com> <20240213033307.zbhrpjigco7vl56z@revolver> <20240213170609.s3queephdyxzrz7j@revolver> <20240213184905.tp4i2ifbglfzlwi6@revolver> In-Reply-To: <20240213184905.tp4i2ifbglfzlwi6@revolver> From: Suren Baghdasaryan Date: Tue, 13 Feb 2024 10:56:58 -0800 Message-ID: Subject: Re: [PATCH v5 3/3] userfaultfd: use per-vma locks in userfaultfd operations To: "Liam R. Howlett" , Suren Baghdasaryan , Lokesh Gidra , akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, selinux@vger.kernel.org, kernel-team@android.com, aarcange@redhat.com, peterx@redhat.com, david@redhat.com, axelrasmussen@google.com, bgeffon@google.com, willy@infradead.org, jannh@google.com, kaleshsingh@google.com, ngeoffray@google.com, timmurray@google.com, rppt@kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 8B6DA20020 X-Stat-Signature: sk78kmdk8ue4if9wxci7yss7eg347iu8 X-Rspam-User: X-HE-Tag: 1707850633-731807 X-HE-Meta: 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 MR8f122h XEuYYwZPgeHeSUPmQUdDoYkEzq9OQJ9R33fm7ORtiQBZODg32CK+2UFn3xjht4ZYnYmlvf00I3mE6Q7hbr93U6B4NZHoVryeQtKVpdTBbQE4ladEI9kr24FoINACnLThLxpYf6G3GSCLtWTPK5QNHaz9eHj9f80pc1O2tqBpmCANK4XE0tSMzPXmE9NXxknm9gnQQsA+kvUnENPUl3DlKrtuOgZPvfg3NtcDsCkjf3GoKif4SRvch36MOIWBZRGr+X3cn1QLwhDTUNxUlvZjNcSSuRGJdkDj3lronLHQrvJl4qSorkYs6pjXKhVjZoi5zjKin 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: List-Subscribe: List-Unsubscribe: On Tue, Feb 13, 2024 at 10:49=E2=80=AFAM Liam R. Howlett wrote: > > * Suren Baghdasaryan [240213 13:25]: > > On Tue, Feb 13, 2024 at 10:14=E2=80=AFAM Lokesh Gidra wrote: > > > > > > On Tue, Feb 13, 2024 at 9:06=E2=80=AFAM Liam R. Howlett wrote: > > > > > > > > * Lokesh Gidra [240213 06:25]: > > > > > On Mon, Feb 12, 2024 at 7:33=E2=80=AFPM Liam R. Howlett wrote: > > > > > > > > > > > > * Lokesh Gidra [240212 19:19]: > > > > > > > All userfaultfd operations, except write-protect, opportunist= ically use > > > > > > > per-vma locks to lock vmas. On failure, attempt again inside = mmap_lock > > > > > > > critical section. > > > > > > > > > > > > > > Write-protect operation requires mmap_lock as it iterates ove= r multiple > > > > > > > vmas. > > > > > > > > > > > > > > Signed-off-by: Lokesh Gidra > > > > > > > --- > > > > > > > fs/userfaultfd.c | 13 +- > > > > > > > include/linux/userfaultfd_k.h | 5 +- > > > > > > > mm/userfaultfd.c | 392 ++++++++++++++++++++++++= ++-------- > > > > > > > 3 files changed, 312 insertions(+), 98 deletions(-) > > > > > > > > > > > > > ... > > > > > > > > I just remembered an issue with the mmap tree that exists today tha= t you > > > > needs to be accounted for in this change. > > > > > > > > If you hit a NULL VMA, you need to fall back to the mmap_lock() sce= nario > > > > today. > > > > > > Unless I'm missing something, isn't that already handled in the patch= ? > > > We get the VMA outside mmap_lock critical section only via > > > lock_vma_under_rcu() (in lock_vma() and find_and_lock_vmas()) and in > > > both cases if we get NULL in return, we retry in mmap_lock critical > > > section with vma_lookup(). Wouldn't that suffice? > > > > I think that case is handled correctly by lock_vma(). > > Yeah, it looks good. I had a bit of a panic as I forgot to check that > and I was thinking of a previous version. I rechecked and v5 looks > good. > > > > > Sorry for coming back a bit late. The overall patch looks quite good > > but the all these #ifdef CONFIG_PER_VMA_LOCK seem unnecessary to me. > > Why find_and_lock_vmas() and lock_mm_and_find_vmas() be called the > > same name (find_and_lock_vmas()) and in one case it would lock only > > the VMA and in the other case it takes mmap_lock? Similarly > > unlock_vma() would in one case unlock the VMA and in the other drop > > the mmap_lock? That would remove all these #ifdefs from the code. > > Maybe this was already discussed? > > Yes, I don't think we should be locking the mm in lock_vma(), as it > makes things hard to follow. > > We could use something like uffd_prepare(), uffd_complete() but I > thought of those names rather late in the cycle, but I've already caused > many iterations of this patch set and that clean up didn't seem as vital > as simplicity and clarity of the locking code. Maybe lock_vma_for_uffd()/unlock_vma_for_uffd()? Whatever name is better I'm fine with it but all these #ifdef's sprinkled around don't contribute to the readability. Anyway, I don't see this as a blocker, just nice to have. > > Thanks, > Liam > > -- > To unsubscribe from this group and stop receiving emails from it, send an= email to kernel-team+unsubscribe@android.com. >