Re: [PATCHSET v2 0/15] Uncached buffered IO

[Matthew Wilcox]

On Sun, Nov 10, 2024 at 08:27:52AM -0700, Jens Axboe wrote:
> 5 years ago I posted patches adding support for RWF_UNCACHED, as a way
> to do buffered IO that isn't page cache persistent. The approach back
> then was to have private pages for IO, and then get rid of them once IO
> was done. But that then runs into all the issues that O_DIRECT has, in
> terms of synchronizing with the page cache.

Today's a holiday, and I suspect you're going to do a v3 before I have
a chance to do a proper review of this version of the series.

I think "uncached" isn't quite the right word.  Perhaps 'RWF_STREAMING'
so that userspace is indicating that this is a streaming I/O and the
kernel gets to choose what to do with that information.

Also, do we want to fail I/Os to filesystems which don't support
it?  I suppose really sophisticated userspace might fall back to
madvise(DONTNEED), but isn't most userspace going to just clear the flag
and retry the I/O?

Um.  Now I've looked, we also have posix_fadvise(POSIX_FADV_NOREUSE),
which is currently a noop.  But would we be better off honouring
POSIX_FADV_NOREUSE than introducing RWF_UNCACHED?  I'll think about this
some more while I'm offline.