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

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Date:   Tue, 7 Feb 2023 21:26:12 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        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

Subject nitpick:  this does not ouch iomap at all.

> Fix this by replacing the use of an ITER_PIPE iterator with an ITER_BVEC
> iterator for which reversion won't free the buffers.  Bulk allocate all the
> buffers we think we're going to use in advance, do the read synchronously
> and only then trim the buffer down.  The pages we did use get pushed into
> the pipe.
> 
> This is more efficient by virtue of doing a bulk page allocation, but
> slightly less efficient by ignoring any partial page in the pipe.

For the usual case of a buffered read into the iter, this completely
changes the semantics:

 - before the pagecache pages just grew a new reference and were
   added to the pipe buffer, and I/O was done without an extra
   copy
 - with this patch you always allocate an extra set of pages for
   the pipe and copy to it

So I very much doubt this makes anything more efficient, and I don't
think we can just do it.

We'll have to fix reverting of pipe buffers, just as I already pointed
out in your cifs series that tries to play the same game.