mm/filemap: make buffered writes work with RWF_UNCACHED

mm/filemap: make buffered writes work with RWF_UNCACHED

If RWF_UNCACHED is set for a write, mark new folios being written with
uncached. This is done by passing in the fact that it's an uncached write
through the folio pointer. We can only get there when IOCB_UNCACHED was
allowed, which can only happen if the file system opts in. Opting in means
they need to check for the LSB in the folio pointer to know if it's an
uncached write or not. If it is, then FGP_UNCACHED should be used if
creating new folios is necessary.

Uncached writes will drop any folios they create upon writeback
completion, but leave folios that may exist in that range alone. Since
->write_begin() doesn't currently take any flags, and to avoid needing
to change the callback kernel wide, use the foliop being passed in to
->write_begin() to signal if this is an uncached write or not. File
systems can then use that to mark newly created folios as uncached.

This provides similar benefits to using RWF_UNCACHED with reads. Testing
buffered writes on 32 files:

writing bs 65536, uncached 0
  1s: 196035MB/sec
  2s: 132308MB/sec
  3s: 132438MB/sec
  4s: 116528MB/sec
  5s: 103898MB/sec
  6s: 108893MB/sec
  7s: 99678MB/sec
  8s: 106545MB/sec
  9s: 106826MB/sec
 10s: 101544MB/sec
 11s: 111044MB/sec
 12s: 124257MB/sec
 13s: 116031MB/sec
 14s: 114540MB/sec
 15s: 115011MB/sec
 16s: 115260MB/sec
 17s: 116068MB/sec
 18s: 116096MB/sec

where it's quite obvious where the page cache filled, and performance
dropped from to about half of where it started, settling in at around
115GB/sec. Meanwhile, 32 kswapds were running full steam trying to
reclaim pages.

Running the same test with uncached buffered writes:

writing bs 65536, uncached 1
  1s: 198974MB/sec
  2s: 189618MB/sec
  3s: 193601MB/sec
  4s: 188582MB/sec
  5s: 193487MB/sec
  6s: 188341MB/sec
  7s: 194325MB/sec
  8s: 188114MB/sec
  9s: 192740MB/sec
 10s: 189206MB/sec
 11s: 193442MB/sec
 12s: 189659MB/sec
 13s: 191732MB/sec
 14s: 190701MB/sec
 15s: 191789MB/sec
 16s: 191259MB/sec
 17s: 190613MB/sec
 18s: 191951MB/sec

and the behavior is fully predictable, performing the same throughout
even after the page cache would otherwise have fully filled with dirty
data. It's also about 65% faster, and using half the CPU of the system
compared to the normal buffered write.

Signed-off-by: Jens Axboe <axboe@kernel.dk>