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

Christoph Hellwig
2023-02-08T16:20:17Z

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.