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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 1 Aug 2023 15:13:56 -0700
Message-ID: <CAJuCfpGN+YE+w08cmMw9py_QrPBWNMVG=JGt=3sAWPZKhmnDTg@mail.gmail.com>
Subject: Re: [PATCH 1/6] mm: enable page walking API to lock vmas during the walk
To:     Peter Xu <peterx@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        akpm@linux-foundation.org, jannh@google.com, willy@infradead.org,
        liam.howlett@oracle.com, david@redhat.com, ldufour@linux.ibm.com,
        vbabka@suse.cz, michel@lespinasse.org, jglisse@google.com,
        mhocko@suse.com, hannes@cmpxchg.org, dave@stgolabs.net,
        hughd@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        stable@vger.kernel.org
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On Tue, Aug 1, 2023 at 2:46=E2=80=AFPM Suren Baghdasaryan <surenb@google.co=
m> wrote:
>
> On Tue, Aug 1, 2023 at 2:34=E2=80=AFPM Peter Xu <peterx@redhat.com> wrote=
:
> >
> > On Tue, Aug 01, 2023 at 01:28:56PM -0700, Suren Baghdasaryan wrote:
> > > I have the new patchset ready but I see 3 places where we walk the
> > > pages after mmap_write_lock() while *I think* we can tolerate
> > > concurrent page faults (don't need to lock the vmas):
> > >
> > > s390_enable_sie()
> > > break_ksm()
> > > clear_refs_write()
> >
> > This one doesn't look right to be listed - tlb flushing is postponed af=
ter
> > pgtable lock released, so I assume the same issue can happen like fork(=
):
> > where we can have race coditions to corrupt data if, e.g., thread A
> > writting with a writable (unflushed) tlb, alongside with thread B CoWin=
g.
>
> Ah, good point.
>
> >
> > It'll indeed be nice to know whether break_ksm() can avoid that lock_vm=
a
> > parameter across quite a few function jumps. I don't yet see an immedia=
te
> > issue with this one..  No idea on s390_enable_sie(), but to make it sim=
ple
> > and safe I'd simply leave it with the write vma lock to match the mmap
> > write lock.
>
> Thanks for taking a look, Peter!
>
> Ok, let me keep all three of them with vma locking in place to be safe
> and will post v2 for further discussion.

v2 posted at https://lore.kernel.org/all/20230801220733.1987762-1-surenb@go=
ogle.com/

> Thanks,
> Suren.
>
> >
> > Thanks,
> >
> > --
> > Peter Xu
> >