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

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Date:   Wed, 3 May 2023 01:34:13 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Matthew Wilcox <willy@...radead.org>, 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 Tue, May 2, 2023 at 4:05 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Tue, May 2, 2023 at 3:31 PM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > On Tue, May 02, 2023 at 09:36:03AM -0700, Suren Baghdasaryan wrote:
> > > On Tue, May 2, 2023 at 8:03 AM Matthew Wilcox <willy@...radead.org> wrote:
> > > >
> > > > On Mon, May 01, 2023 at 10:04:56PM -0700, Suren Baghdasaryan wrote:
> > > > > On Mon, May 1, 2023 at 8:22 PM Matthew Wilcox <willy@...radead.org> wrote:
> > > > > >
> > > > > > 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?
> > > > >
> > > > > Well, that discussion was about waiting in folio_lock_or_retry() with
> > > > > the lock being held. I argued against it because currently we drop
> > > > > mmap_lock lock before waiting, so if we don't drop VMA lock we would
> > > > > be changing the current behavior which might introduce new
> > > > > regressions. In the case of swap_readpage and swapin_readahead we
> > > > > already wait with mmap_lock held, so waiting with VMA lock held does
> > > > > not introduce new problems (unless there is a need to hold mmap_lock).
> > > > >
> > > > > That said, you are absolutely correct that this situation can be
> > > > > improved by dropping the lock in these cases too. I just didn't want
> > > > > to attack everything at once. I believe after we agree on the approach
> > > > > implemented in https://lore.kernel.org/all/20230501175025.36233-3-surenb@google.com
> > > > > for dropping the VMA lock before waiting, these cases can be added
> > > > > easier. Does that make sense?
> > > >
> > > > OK, I looked at this path some more, and I think we're fine.  This
> > > > patch is only called for SWP_SYNCHRONOUS_IO which is only set for
> > > > QUEUE_FLAG_SYNCHRONOUS devices, which are brd, zram and nvdimms
> > > > (both btt and pmem).  So the answer is that we don't sleep in this
> > > > path, and there's no need to drop the lock.
> > >
> > > Yes but swapin_readahead does sleep, so I'll have to handle that case
> > > too after this.
> >
> > Sleeping is OK, we do that in pXd_alloc()!  Do we block on I/O anywhere
> > in swapin_readahead()?  It all looks like async I/O to me.
>
> Hmm. I thought that we have synchronous I/O in the following paths:
>     swapin_readahead()->swap_cluster_readahead()->swap_readpage()
>     swapin_readahead()->swap_vma_readahead()->swap_readpage()
> but just noticed that in both cases swap_readpage() is called with the
> synchronous parameter being false. So you are probably right here...

In both swap_cluster_readahead() and swap_vma_readahead() it looks
like if the readahead window is 1 (aka we are not reading ahead), then
we jump to directly calling read_swap_cache_async() passing do_poll =
true, which means we may end up calling swap_readpage() passing
synchronous = true.

I am not familiar with readahead heuristics, so I am not sure how
common this is, but it's something to think about.

> Does that mean swapin_readahead() might return a page which does not
> have its content swapped-in yet?
>