linux-kernel - Re: [PATCH v12 01/10] vfs, iomap: Fix generic_file_splice_read() to avoid reversion of ITER

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Date:   Wed, 8 Feb 2023 08:19:56 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...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,
        syzbot+a440341a59e3b7142895@...kaller.appspotmail.com,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v12 01/10] vfs, iomap: Fix generic_file_splice_read() to
 avoid reversion of ITER_PIPE

On Wed, Feb 08, 2023 at 04:09:51PM +0000, David Howells wrote:
> How about one of two different solutions?
> 
>  (1) Repurpose the function I proposed for generic_file_splice_read() but only
>      for splicing from O_DIRECT files; reading from non-O_DIRECT files would
>      use an ITER_PIPE as upstream.

Given the amounts of problems we had with O_DIRECT vs splice, and the
fact that even doing this is a bit pointless that seems sensible to me.

>      for splicing from O_DIRECT files, as (1), but also replace the splice
>      from a buffered file with something like the patch below.  This uses
>      filemap_get_pages() to do the reading and to get a bunch of folios from
>      the pagecache that we can then splice into the pipe directly.

I defintively like the idea of killing ITER_PIPE.  Isn't the 16
folios in a folio tree often much less than what we could fit into
a single pipe buf?  Unless you have a file system that can use
huge folios for buffered I/O and actually does this might significantly
limit performance.

With that in mind I'll try to find some time to review your actual
patch, but I'm a little busy at the moment.