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Howlett" , Lokesh Gidra , akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, selinux@vger.kernel.org, surenb@google.com, 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-Queue-Id: 769A1100021 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: 6sq6dh3kpxt1hfsir56m5azf19fan5jp X-HE-Tag: 1707848092-673475 X-HE-Meta: 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 G9Xw4x52 X0VhHsEHHgH5aKKKOUOY/ZQsMyq6s6xbmeXvASdDBIg8F98hvOXmIspoV1uzCqLVnX9oOfhRNXLwLquAqFPf1+I/qfHNFX+xLD0JpANQKLQJJVKpSjceLV9HmwTOt6MJ6dxh8isQOrxk6edgT9gwc8BJAcFZ0xHJEVp0RfG4XBbKO2nwHVFtOqStXlsJjq+5MxUmCyDABGp1cD/bYseqI5Hbn0oGbnelx2O5v2JXNMS9dBVpKI/C4OGDekYfzSy3HyzAelm4S0Y8TsJLG8NdQ5RQi9Q== 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 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, opportunistically= use > > > > per-vma locks to lock vmas. On failure, attempt again inside mmap_l= ock > > > > critical section. > > > > > > > > Write-protect operation requires mmap_lock as it iterates over mult= iple > > > > 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 that you > needs to be accounted for in this change. > > If you hit a NULL VMA, you need to fall back to the mmap_lock() scenario > 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? > > This is a necessity to avoid a race of removal/replacement of a VMA in > the mmap(MAP_FIXED) case. In this case, we munmap() prior to mmap()'ing > an area - which means you could see a NULL when there never should have > been a null. > > Although this would be exceedingly rare, you need to handle this case. > > Sorry I missed this earlier, > Liam