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From: Chris Li <chrisl@kernel.org>
Date: Fri, 26 Apr 2024 16:16:01 -0700
Message-ID: <CANeU7Q=YYFWPBMHPPeOQDxO9=yAiQP8w90e2mO0U+hBuzCV1RQ@mail.gmail.com>
Subject: Re: [PATCH 0/8] mm/swap: optimize swap cache search space
To: "Huang, Ying" <ying.huang@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>, Kairui Song <ryncsn@gmail.com>, linux-mm@kvack.org, 
	Kairui Song <kasong@tencent.com>, Andrew Morton <akpm@linux-foundation.org>, 
	Barry Song <v-songbaohua@oppo.com>, Ryan Roberts <ryan.roberts@arm.com>, Neil Brown <neilb@suse.de>, 
	Minchan Kim <minchan@kernel.org>, Hugh Dickins <hughd@google.com>, 
	David Hildenbrand <david@redhat.com>, Yosry Ahmed <yosryahmed@google.com>, linux-fsdevel@vger.kernel.org, 
	linux-kernel@vger.kernel.org
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Hi Ying,

On Tue, Apr 23, 2024 at 7:26=E2=80=AFPM Huang, Ying <ying.huang@intel.com> =
wrote:
>
> Hi, Matthew,
>
> Matthew Wilcox <willy@infradead.org> writes:
>
> > On Mon, Apr 22, 2024 at 03:54:58PM +0800, Huang, Ying wrote:
> >> Is it possible to add "start_offset" support in xarray, so "index"
> >> will subtract "start_offset" before looking up / inserting?
> >
> > We kind of have that with XA_FLAGS_ZERO_BUSY which is used for
> > XA_FLAGS_ALLOC1.  But that's just one bit for the entry at 0.  We could
> > generalise it, but then we'd have to store that somewhere and there's
> > no obvious good place to store it that wouldn't enlarge struct xarray,
> > which I'd be reluctant to do.
> >
> >> Is it possible to use multiple range locks to protect one xarray to
> >> improve the lock scalability?  This is why we have multiple "struct
> >> address_space" for one swap device.  And, we may have same lock
> >> contention issue for large files too.
> >
> > It's something I've considered.  The issue is search marks.  If we dele=
te
> > an entry, we may have to walk all the way up the xarray clearing bits a=
s
> > we go and I'd rather not grab a lock at each level.  There's a convenie=
nt
> > 4 byte hole between nr_values and parent where we could put it.
> >
> > Oh, another issue is that we use i_pages.xa_lock to synchronise
> > address_space.nrpages, so I'm not sure that a per-node lock will help.
>
> Thanks for looking at this.
>
> > But I'm conscious that there are workloads which show contention on
> > xa_lock as their limiting factor, so I'm open to ideas to improve all
> > these things.
>
> I have no idea so far because my very limited knowledge about xarray.

For the swap file usage, I have been considering an idea to remove the
index part of the xarray from swap cache. Swap cache is different from
file cache in a few aspects.
For one if we want to have a folio equivalent of "large swap entry".
Then the natural alignment of those swap offset on does not make
sense. Ideally we should be able to write the folio to un-aligned swap
file locations.

The other aspect for swap files is that, we already have different
data structures organized around swap offset, swap_map and
swap_cgroup. If we group the swap related data structure together. We
can add a pointer to a union of folio or a shadow swap entry. We can
use atomic updates on the swap struct member or breakdown the access
lock by ranges just like swap cluster does.

I want to discuss those ideas in the upcoming LSF/MM meet up as well.

Chris