linux-kernel - Re: [PATCH 1/6] mm: enable page walking API to lock vmas during the walk

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Message-ID: <CAJuCfpF6WcJBSix0PD0cOD_MaeLpfGz1ddS6Ug_M+g0QTfkdzw@mail.gmail.com>
Date:   Tue, 1 Aug 2023 13:28:56 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     akpm@...ux-foundation.org, jannh@...gle.com, willy@...radead.org,
        liam.howlett@...cle.com, david@...hat.com, peterx@...hat.com,
        ldufour@...ux.ibm.com, vbabka@...e.cz, michel@...pinasse.org,
        jglisse@...gle.com, mhocko@...e.com, hannes@...xchg.org,
        dave@...olabs.net, hughd@...gle.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/6] mm: enable page walking API to lock vmas during the walk

On Mon, Jul 31, 2023 at 1:24 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Mon, Jul 31, 2023 at 12:33 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > On Mon, 31 Jul 2023 at 12:31, Suren Baghdasaryan <surenb@...gle.com> wrote:
> > >
> > > I got the idea but a couple of modifications, if I may.
> >
> > Ack, sounds sane to me.
>
> Ok, I'll wait for more feedback today and will post an update tomorrow. Thanks!

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()

In all these walks we lock PTL when modifying the page table entries,
that's why I think we can skip locking the vma but maybe I'm missing
something? Could someone please check these 3 cases and confirm or
deny my claim?
Thanks,
Suren.

>
> >
> >              Linus