From: Lorenzo Stoakes <ljs@kernel•org>
To: Yang Shi <shy828301@gmail•com>
Cc: "David Hildenbrand (Arm)" <david@kernel•org>,
Suren Baghdasaryan <surenb@google•com>,
Barry Song <baohua@kernel•org>,
Matthew Wilcox <willy@infradead•org>,
akpm@linux-foundation•org, linux-mm@kvack•org,
liam@infradead•org, vbabka@kernel•org, rppt@kernel•org,
mhocko@suse•com, jack@suse•cz, pfalcato@suse•de,
wanglian@kylinos•cn, chentao@kylinos•cn, lianux.mm@gmail•com,
kunwu.chan@gmail•com, liyangouwen1@oppo•com, chrisl@kernel•org,
kasong@tencent•com, shikemeng@huaweicloud•com,
nphamcs@gmail•com, bhe@redhat•com, youngjun.park@lge•com,
linux-arm-kernel@lists•infradead.org,
linux-kernel@vger•kernel.org, loongarch@lists•linux.dev,
linuxppc-dev@lists•ozlabs.org, linux-riscv@lists•infradead.org,
linux-s390@vger•kernel.org, Nanzhe Zhao <nzzhao@126•com>
Subject: Re: [PATCH v2 0/5] mm: reduce mmap_lock contention and improve page fault performance
Date: Fri, 22 May 2026 16:37:54 +0100 [thread overview]
Message-ID: <ahB3DSwipVXw6tmK@lucifer> (raw)
In-Reply-To: <CAHbLzkq3S7NDYe4LXjurKNQU+40-wtqrD_PT18YcyHbAcNxiRQ@mail.gmail.com>
On Wed, May 20, 2026 at 02:39:49PM -0700, Yang Shi wrote:
> On Wed, May 20, 2026 at 3:34 AM David Hildenbrand (Arm)
> <david@kernel•org> wrote:
> >
> > On 5/19/26 14:53, Lorenzo Stoakes wrote:
> > > On Mon, May 18, 2026 at 12:56:59PM -0700, Suren Baghdasaryan wrote:
> > >
> > >>>
> > >>> I think we either need to fix `fork()`, or keep the current
> > >>> behavior of dropping the VMA lock before performing I/O.
> > >>
> > >> I see. So, this problem arises from the fact that we are changing the
> > >> pagefaults requiring I/O operation to hold VMA lock...
> > >> And you want to lock VMA on fork only if vma_is_anonymous(vma) ||
> > >> is_cow_mapping(vma->vm_flags). So, we will be blocking page faults for
> > >> anonymous and COW VMAs only while holding mmap_write_lock, preventing
> > >> any VMA modification. On the surface, that looks ok to me but I might
> > >> be missing some corner cases. If nobody sees any obvious issues, I
> > >> think it's worth a try.
> > >
> > > Not sure if you noticed but I did raise concerns ;)
> > >
> > > I wonder if you've confused the fault path and fork here, as I think Barry has
> > > been a little unclear on that.
> > >
> > > What's being suggested in this thread is to fundamentally change fork behaviour
> > > so it's different from the entire history of the kernel (or - presumably - at
> > > least recent history :)
> > I don't want fork() to become different in that regard.
> >
> > There is already a slight difference with vs. without per-VMA locks, because
> > there is a window in-between us taking the write mmap_lock and all the per-VMA
> > locks. I raised that previously [1] and assumed that it is probably fine.
> >
> > I also raised in the past why I think we must not allow concurrent page faults,
> > at least as soon as anonymous memory is involved [2].
>
> Thanks for sharing the context, it is quite helpful to understand the
> race conditions. Because Lorenzo also raised the concern about page
> fault race, I will reply to all the concerns regarding page fault race
> together in this thread.
>
> IIUC, there is already some sort of race with per vma lock. Before per
> vma lock, mmap_lock did lock everything. So page fault happened either
> before fork or after fork. But page fault can happen on other VMAs
> which have not been lock'ed yet during fork with per vma lock. For
> example, we have 3 VMAs, we lock the first VMA, but page fault still
> can happen on the other 2 VMAs during fork if they already have
> anon_vma. This is the status quo now, but it seems not harmful.
>
> The bad race shared by David is caused by racing with copy page. So it
> seems like it will be fine as long as we serialize copy page against
> page fault if I don't miss anything. Since we decide whether to copy
> page or not by checking vma->anon_vma, so it seems fine to not take
> vma lock if vma->anon_vma is NULL. This will not introduce more race
> either because setting up a new anon_vma in page fault or madvise
> requires taking mmap_lock according to the earlier discussions.
NAK. No.
We're not doing this, we're not changing how fork fundamentally behaves because
of concerns about the fault path.
I've delineated exactly why I think this is a problem and you're pressing ahead
without addressing those concerns.
So at this point I'm going to be a grumpy maintainer and just say no, stop
please :)
Let's fix this in the right place. You don't fix a leak in the roof by repairing
a shelf next door :)
Thanks, Lorenzo
>
> Thanks,
> Yang
>
> >
> > ... and I raised that this is pretty much slower by design right now: "Well, the
> > design decision that CONFIG_PER_VMA_LOCK made for now to make page faults fast
> > and to make blocking any page faults from happening to be slower ..." [3]
> >
> > [1] https://lore.kernel.org/all/970295ab-e85d-7af3-76e6-df53a5c52f8b@redhat.com/
> > [2] https://lore.kernel.org/all/7e3f35cc-59b9-bf12-b8b1-4ed78223844a@redhat.com/
> > [3] https://lore.kernel.org/all/2efa2c89-3765-721d-2c3c-00590054aa5b@redhat.com/
> >
> > --
> > Cheers,
> >
> > David
> >
next prev parent reply other threads:[~2026-05-22 15:38 UTC|newest]
Thread overview: 72+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-04-30 4:04 [PATCH v2 0/5] mm: reduce mmap_lock contention and improve page fault performance Barry Song (Xiaomi)
2026-04-30 4:04 ` [PATCH v2 1/5] mm/filemap: Retry fault by VMA lock if the lock was released for I/O Barry Song (Xiaomi)
2026-04-30 4:04 ` [PATCH v2 2/5] mm/swapin: Retry swapin " Barry Song (Xiaomi)
2026-04-30 4:04 ` [PATCH v2 3/5] mm: Move folio_lock_or_retry() and drop __folio_lock_or_retry() Barry Song (Xiaomi)
2026-04-30 4:04 ` [PATCH v2 4/5] mm: Don't retry page fault if folio is uptodate during swap-in Barry Song (Xiaomi)
2026-04-30 12:35 ` Matthew Wilcox
2026-05-01 16:11 ` Matthew Wilcox
2026-04-30 4:04 ` [PATCH v2 5/5] mm/filemap: Avoid retrying page faults on uptodate folios in filemap faults Barry Song (Xiaomi)
2026-04-30 12:37 ` [PATCH v2 0/5] mm: reduce mmap_lock contention and improve page fault performance Matthew Wilcox
2026-04-30 22:49 ` Barry Song
2026-05-01 14:56 ` Matthew Wilcox
2026-05-01 17:44 ` Barry Song
2026-05-01 17:57 ` Matthew Wilcox
2026-05-01 18:25 ` Barry Song
2026-05-01 19:39 ` Matthew Wilcox
2026-05-03 20:39 ` Barry Song
2026-05-03 13:13 ` Jan Kara
2026-05-03 19:55 ` Barry Song
2026-05-04 13:03 ` Jan Kara
2026-05-04 13:35 ` Barry Song
2026-05-04 14:15 ` Barry Song
2026-05-17 8:45 ` Barry Song
2026-05-18 9:46 ` Lorenzo Stoakes
2026-05-18 11:25 ` Barry Song
2026-05-18 16:17 ` Matthew Wilcox
2026-05-18 20:50 ` Barry Song
2026-05-18 19:56 ` Suren Baghdasaryan
2026-05-18 21:14 ` Barry Song
2026-05-19 12:45 ` Lorenzo Stoakes
2026-05-19 14:17 ` Liam R. Howlett
2026-05-19 22:01 ` Barry Song
2026-05-20 21:04 ` Matthew Wilcox
2026-05-20 21:14 ` Barry Song
2026-05-20 21:15 ` Matthew Wilcox
2026-05-20 21:35 ` David Hildenbrand (Arm)
2026-05-20 23:37 ` Barry Song
2026-05-22 15:53 ` Lorenzo Stoakes
2026-05-22 21:31 ` Barry Song
2026-05-22 2:33 ` Barry Song (Xiaomi)
2026-05-22 13:09 ` Matthew Wilcox
2026-05-22 13:36 ` Barry Song
2026-05-22 13:48 ` Barry Song
2026-05-22 15:42 ` Lorenzo Stoakes
2026-05-19 12:53 ` Lorenzo Stoakes
2026-05-19 21:18 ` Barry Song
2026-05-20 7:50 ` Lorenzo Stoakes
2026-05-20 9:07 ` Barry Song
2026-05-20 10:07 ` Lorenzo Stoakes
2026-05-20 16:20 ` Suren Baghdasaryan
2026-05-20 5:51 ` Suren Baghdasaryan
2026-05-22 15:39 ` Lorenzo Stoakes
2026-05-20 10:33 ` David Hildenbrand (Arm)
2026-05-20 12:55 ` Lorenzo Stoakes
2026-05-20 21:39 ` Yang Shi
2026-05-22 15:37 ` Lorenzo Stoakes [this message]
2026-05-19 12:43 ` Lorenzo Stoakes
2026-05-18 9:53 ` David Hildenbrand (Arm)
2026-05-19 13:42 ` Lorenzo Stoakes
2026-05-18 21:21 ` Yang Shi
2026-05-19 11:07 ` Barry Song
2026-05-19 13:34 ` Lorenzo Stoakes
2026-05-19 18:50 ` Yang Shi
2026-05-19 20:53 ` Yang Shi
2026-05-19 13:12 ` Lorenzo Stoakes
2026-05-19 13:39 ` Lorenzo Stoakes
2026-05-19 18:41 ` Yang Shi
2026-05-19 21:02 ` Yang Shi
2026-05-20 8:11 ` Lorenzo Stoakes
2026-05-01 15:52 ` Lorenzo Stoakes
2026-05-01 16:06 ` Matthew Wilcox
2026-05-01 17:09 ` Lorenzo Stoakes
2026-05-01 17:59 ` Barry Song
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