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

From: Matthew Wilcox <willy@infradead.org>
To: Suren Baghdasaryan <surenb@google.com>
Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@suse.com,
	josef@toxicpanda.com, jack@suse.cz, ldufour@linux.ibm.com,
	laurent.dufour@fr.ibm.com, michel@lespinasse.org,
	liam.howlett@oracle.com, jglisse@google.com, vbabka@suse.cz,
	minchan@google.com, dave@stgolabs.net,
	punit.agrawal@bytedance.com, lstoakes@gmail.com,
	hdanton@sina.com, apopple@nvidia.com, linux-mm@kvack.org,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	kernel-team@android.com
Subject: Re: [PATCH 1/3] mm: handle swap page faults under VMA lock if page is uncontended
Date: Tue, 2 May 2023 04:22:32 +0100	[thread overview]
Message-ID: <ZFCB+G9KSNE+J9cZ@casper.infradead.org> (raw)
In-Reply-To: <CAJuCfpHfAFx9rjv0gHK77LbP-8gd-kFnWw=aqfQTP6pH=zvMNg@mail.gmail.com>

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@infradead.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?