btrfs: zstd: avoid a copy in zstd_decompress_bio()

btrfs: zstd: avoid a copy in zstd_decompress_bio()

zstd_decompress_bio() gives zstd a sectorsize-sized scratch buffer, and
btrfs_decompress_buf2page() then copies the part overlapping the read bio
into the destination folios. Every delivered byte is written twice.

Instead, choose the output buffer per streaming call. zstd_map_dest()
kmaps the current page-bounded segment of the read bio, so zstd writes
into the page cache directly. The scratch buffer is kept only for output
with no destination: the prefix before a read starting inside a
compressed extent, which zstd cannot skip, and gaps left by folios
already in the page cache.

Varying the output buffer across calls is safe: btrfs uses the default
ZSTD_bm_buffered mode, where the sliding window lives in the dstream's
internal buffer and the caller's dst is a pure sink. The read bio's
iterator must still advance by exactly the bytes delivered, since
btrfs_decompress_bio() zero-fills from it; that used to happen inside
btrfs_decompress_buf2page() and is now an explicit bio_advance(), made
only for output that reached a folio.

bio_iter_iovec() exposes at most one base page, so direct output is
page-bounded. Compared to the old sectorsize-sized chunks, this can
increase stream calls when sectorsize exceeds PAGE_SIZE, but eliminates
the extra btrfs copy for output delivered to the read bio; the 64 KiB
sectorsize row below shows the copy still wins there.

Benchmarked the change in 2-vCPU x86-64 KVM guests (4 KiB pages, RAM
disk) using a 64 MiB zstd-compressed file. Results are medians of seven
cold-cache reads in each of six interleaved A/B boot pairs; mincore
confirmed zero resident pages before every run.

Normal sequential reads with readahead produced:

  sectorsize       base       patched    reduction
  4 KiB          8.678 ms     8.004 ms       7.80%
  16 KiB         8.216 ms     7.934 ms       3.64%
  64 KiB         7.875 ms     7.344 ms       6.88%

Random 4 KiB preads at 4 KiB sectorsize, means of six interleaved A/B
boot pairs, patched better in all six:

  base           patched        gain
  264.33 MB/s    272.67 MB/s     3.2%

Signed-off-by: Usama Arif <usama.arif@linux.dev>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>