linux-kernel - Re: [PATCH v3 26/35] mm: fall back to mmap_lock if vma->anon

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAB=+i9ToNQ-se1XWGOGbiM_0QRBYjuhCF5A8Cdvius89t8r7Ag@mail.gmail.com>
Date:   Fri, 17 Feb 2023 19:21:21 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org,
        michel@...pinasse.org, jglisse@...gle.com, mhocko@...e.com,
        vbabka@...e.cz, hannes@...xchg.org, mgorman@...hsingularity.net,
        dave@...olabs.net, liam.howlett@...cle.com, peterz@...radead.org,
        ldufour@...ux.ibm.com, paulmck@...nel.org, mingo@...hat.com,
        will@...nel.org, luto@...nel.org, songliubraving@...com,
        peterx@...hat.com, david@...hat.com, dhowells@...hat.com,
        hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev,
        punit.agrawal@...edance.com, lstoakes@...il.com,
        peterjung1337@...il.com, rientjes@...gle.com, chriscli@...gle.com,
        axelrasmussen@...gle.com, joelaf@...gle.com, minchan@...gle.com,
        rppt@...nel.org, jannh@...gle.com, shakeelb@...gle.com,
        tatashin@...gle.com, edumazet@...gle.com, gthelen@...gle.com,
        gurua@...gle.com, arjunroy@...gle.com, soheil@...gle.com,
        leewalsh@...gle.com, posk@...gle.com,
        michalechner92@...glemail.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v3 26/35] mm: fall back to mmap_lock if vma->anon_vma is
 not yet set

On Fri, Feb 17, 2023 at 11:15 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Thu, Feb 16, 2023 at 11:43 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
> >
> > On Thu, Feb 16, 2023 at 7:44 AM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Wed, Feb 15, 2023 at 09:17:41PM -0800, Suren Baghdasaryan wrote:
> > > > When vma->anon_vma is not set, page fault handler will set it by either
> > > > reusing anon_vma of an adjacent VMA if VMAs are compatible or by
> > > > allocating a new one. find_mergeable_anon_vma() walks VMA tree to find
> > > > a compatible adjacent VMA and that requires not only the faulting VMA
> > > > to be stable but also the tree structure and other VMAs inside that tree.
> > > > Therefore locking just the faulting VMA is not enough for this search.
> > > > Fall back to taking mmap_lock when vma->anon_vma is not set. This
> > > > situation happens only on the first page fault and should not affect
> > > > overall performance.
> > >
> > > I think I asked this before, but don't remember getting an aswer.
> > > Why do we defer setting anon_vma to the first fault?  Why don't we
> > > set it up at mmap time?
> >
> > Yeah, I remember that conversation Matthew and I could not find the
> > definitive answer at the time. I'll look into that again or maybe
> > someone can answer it here.
>
> After looking into it again I'm still under the impression that
> vma->anon_vma is populated lazily (during the first page fault rather
> than at mmap time) to avoid doing extra work for areas which are never
> faulted. Though I might be missing some important detail here.

I think this is because the kernel cannot merge VMAs that have
different anon_vmas?

Enabling lazy population of anon_vma could potentially increase the
chances of merging VMAs.

> > In the end rather than changing that logic I decided to skip
> > vma->anon_vma==NULL cases because I measured them being less than
> > 0.01% of all page faults, so ROI from changing that would be quite
> > low. But I agree that the logic is weird and maybe we can improve
> > that. I will have to review that again when I'm working on eliminating
> > all these special cases we skip, like swap/userfaults/etc.