linux-kernel - Re: [PATCH v17 03/14] shmem: Implement splice-read

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Date:   Tue, 14 Mar 2023 11:02:40 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     David Howells <dhowells@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Daniel Golle <daniel@...rotopia.org>,
        Guenter Roeck <groeck7@...il.com>,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v17 03/14] shmem: Implement splice-read

On Tue, Mar 14, 2023 at 9:43 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> The problem is that we might have swapped out the shmem folio.  So we
> don't want to clear the page, but ask swap to fill the page.

Doesn't shmem_swapin_folio() already basically do all that work?

The real oddity with shmem - compared to other filesystems - is that
the xarray has a value entry instead of being a real folio. And yes,
the current filemap code will then just ignore such entries as
"doesn't exist", and so the regular read iterators will all fail on
it.

But while filemap_get_read_batch() will stop at a value-folio, I feel
like filemap_create_folio() should be able to turn a value page into a
"real" page. Right now it already allocates said page, but then I
think filemap_add_folio() will return -EEXIST when said entry exists
as a value.

But *if* instead of -EEXIST we could just replace the value with the
(already locked) page, and have some sane way to pass that value
(which is the swap entry data) to readpage(), I think that should just
do it all.

Admittedly I really don't know this area very well, so I may be
*entirely* out to lunch.

But the whole "teach the filemap code to actually react to XA value
entries" would be how I'd solve the hole issue too. So I think there
are commonalities here.

             Linus
               Linus