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

[Suren Baghdasaryan <surenb@google.com>]

On Thu, May 4, 2023 at 10:03 PM Huang, Ying <ying.huang@intel.com> wrote:
>
> Yosry Ahmed <yosryahmed@google.com> writes:
>
> > On Wed, May 3, 2023 at 12:57 PM Suren Baghdasaryan <surenb@google.com> wrote:
> >>
> >> On Wed, May 3, 2023 at 1:34 AM Yosry Ahmed <yosryahmed@google.com> wrote:
> >> >
> >> > On Tue, May 2, 2023 at 4:05 PM Suren Baghdasaryan <surenb@google.com> wrote:
> >> > >
> >> > > On Tue, May 2, 2023 at 3:31 PM Matthew Wilcox <willy@infradead.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@infradead.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@infradead.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@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?
> >> > > > > > >
> >> > > > > > > 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.
> >>
> >> Uh, you are correct. If this branch is common, we could use the same
> >> "drop the lock and retry" pattern inside read_swap_cache_async(). That
> >> would be quite easy to implement.
> >> Thanks for checking on it!
> >
> >
> > I am honestly not sure how common this is.
> >
> > +Ying who might have a better idea.
>
> Checked the code and related history.  It seems that we can just pass
> "synchronous = false" to swap_readpage() in read_swap_cache_async().
> "synchronous = true" was introduced in commit 23955622ff8d ("swap: add
> block io poll in swapin path") to reduce swap read latency for block
> devices that can be polled.  But in commit 9650b453a3d4 ("block: ignore
> RWF_HIPRI hint for sync dio"), the polling is deleted.  So, we don't
> need to pass "synchronous = true" to swap_readpage() during
> swapin_readahead(), because we will wait the IO to complete in
> folio_lock_or_retry().

Thanks for investigating, Ying! It sounds like we can make some
simplifications here. I'll double-check and if I don't find anything
else, will change to "synchronous = false" in the next version of the
patchset.

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