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Message-ID: <CAJuCfpFpOT3KOBCjcnTo9D3AWq_GT9bKNyQ4xcwLJh1UPF6FFA@mail.gmail.com>
Date: Tue, 6 Sep 2022 17:58:18 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Peter Xu <peterx@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Michel Lespinasse <michel@...pinasse.org>,
Jerome Glisse <jglisse@...gle.com>,
Michal Hocko <mhocko@...e.com>,
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Matthew Wilcox <willy@...radead.org>,
"Liam R. Howlett" <liam.howlett@...cle.com>,
Peter Zijlstra <peterz@...radead.org>,
Laurent Dufour <ldufour@...ux.ibm.com>,
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LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH RESEND 19/28] mm: disallow do_swap_page to handle page
faults under VMA lock
On Tue, Sep 6, 2022 at 1:22 PM Peter Xu <peterx@...hat.com> wrote:
>
> On Tue, Sep 06, 2022 at 01:08:10PM -0700, Suren Baghdasaryan wrote:
> > On Tue, Sep 6, 2022 at 12:39 PM Peter Xu <peterx@...hat.com> wrote:
> > >
> > > On Thu, Sep 01, 2022 at 10:35:07AM -0700, Suren Baghdasaryan wrote:
> > > > Due to the possibility of do_swap_page dropping mmap_lock, abort fault
> > > > handling under VMA lock and retry holding mmap_lock. This can be handled
> > > > more gracefully in the future.
> > > >
> > > > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > > > ---
> > > > mm/memory.c | 5 +++++
> > > > 1 file changed, 5 insertions(+)
> > > >
> > > > diff --git a/mm/memory.c b/mm/memory.c
> > > > index 9ac9944e8c62..29d2f49f922a 100644
> > > > --- a/mm/memory.c
> > > > +++ b/mm/memory.c
> > > > @@ -3738,6 +3738,11 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > > > vm_fault_t ret = 0;
> > > > void *shadow = NULL;
> > > >
> > > > + if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> > > > + ret = VM_FAULT_RETRY;
> > > > + goto out;
> > > > + }
> > > > +
> > >
> > > May want to fail early similarly for handle_userfault() too for similar
> > > reason. Thanks,
> >
> > I wasn't aware of a similar issue there. Will have a closer look. Thanks!
>
> Sure.
>
> Just in case this would be anything helpful - handle_userfault() will both
> assert at the entry (mmap_assert_locked) and will in most cases release
> read lock along the way when waiting for page fault resolutions.
>
> And userfaultfd should work on anonymous memory for either missing mode or
> write protect mode.
Got it. Thanks for the explanation. It definitely helps!
>
> Thanks,
>
> --
> Peter Xu
>
> --
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