linux-kernel - Re: [PATCH 1/3] mm: handle swap page faults under VMA lock if page is uncontended

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Date:   Tue, 2 May 2023 04:22:32 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, mhocko@...e.com,
        josef@...icpanda.com, jack@...e.cz, ldufour@...ux.ibm.com,
        laurent.dufour@...ibm.com, michel@...pinasse.org,
        liam.howlett@...cle.com, jglisse@...gle.com, vbabka@...e.cz,
        minchan@...gle.com, dave@...olabs.net, punit.agrawal@...edance.com,
        lstoakes@...il.com, hdanton@...a.com, apopple@...dia.com,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 1/3] mm: handle swap page faults under VMA lock if page
 is uncontended

On Mon, May 01, 2023 at 07:30:13PM -0700, Suren Baghdasaryan wrote:
> On Mon, May 1, 2023 at 7:02 PM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > On Mon, May 01, 2023 at 10:50:23AM -0700, Suren Baghdasaryan wrote:
> > > +++ b/mm/memory.c
> > > @@ -3711,11 +3711,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > >       if (!pte_unmap_same(vmf))
> > >               goto out;
> > >
> > > -     if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> > > -             ret = VM_FAULT_RETRY;
> > > -             goto out;
> > > -     }
> > > -
> > >       entry = pte_to_swp_entry(vmf->orig_pte);
> > >       if (unlikely(non_swap_entry(entry))) {
> > >               if (is_migration_entry(entry)) {
> >
> > You're missing the necessary fallback in the (!folio) case.
> > swap_readpage() is synchronous and will sleep.
> 
> True, but is it unsafe to do that under VMA lock and has to be done
> under mmap_lock?

... you were the one arguing that we didn't want to wait for I/O with
the VMA lock held?