Re: [PATCH 11/13] iomap: make buffered writes work with RWF_UNCACHED

[Matthew Wilcox <willy@infradead.org>]

On Fri, Nov 08, 2024 at 12:49:58PM -0700, Jens Axboe wrote:
> On 11/8/24 12:26 PM, Jens Axboe wrote:
> > On 11/8/24 11:46 AM, Matthew Wilcox wrote:
> >> On Fri, Nov 08, 2024 at 10:43:34AM -0700, Jens Axboe wrote:
> >>> +++ b/fs/iomap/buffered-io.c
> >>> @@ -959,6 +959,8 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
> >>>  		}
> >>>  		if (iter->iomap.flags & IOMAP_F_STALE)
> >>>  			break;
> >>> +		if (iter->flags & IOMAP_UNCACHED)
> >>> +			folio_set_uncached(folio);
> >>
> >> This seems like it'd convert an existing page cache folio into being
> >> uncached?  Is this just leftover from a previous version or is that a
> >> design decision you made?
> > 
> > I'll see if we can improve that. Currently both the read and write side
> > do drop whatever it touches. We could feasibly just have it drop
> > newly instantiated pages - iow, uncached just won't create new persistent
> > folios, but it'll happily use the ones that are there already.
> 
> Well that was nonsense on the read side, it deliberately only prunes
> entries that has uncached set. For the write side, this is a bit
> trickier. We'd essentially need to know if the folio populated by
> write_begin was found in the page cache, or create from new. Any way we
> can do that? One way is to change ->write_begin() so it takes a kiocb
> rather than a file, but that's an amount of churn I'd rather avoid!
> Maybe there's a way I'm just not seeing?

Umm.  We can solve it for iomap with a new FGP_UNCACHED flag and
checking IOMAP_UNCACHED in iomap_get_folio().  Not sure how we solve it
for other filesystems though.  Any filesystem which uses FGP_NOWAIT has
_a_ solution, but eg btrfs will need to plumb through a third boolean
flag (or, more efficiently, just start passing FGP flags to
prepare_one_folio()).