[PATCH] btrfs: zstd: avoid a copy in zstd_decompress_bio()

[Usama Arif <usama.arif@linux.dev>]

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>
---
 fs/btrfs/zstd.c | 69 +++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 56 insertions(+), 13 deletions(-)

diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c
index 86919293fd546..c5aece6be6093 100644
--- a/fs/btrfs/zstd.c
+++ b/fs/btrfs/zstd.c
@@ -580,10 +580,48 @@ int zstd_compress_bio(struct list_head *ws, struct compressed_bio *cb)
 	return ret;
 }
 
+/*
+ * Map the destination for the next chunk of output.
+ *
+ * @decompressed is the offset of the next output byte inside the fully
+ * decompressed extent.  If that offset has reached the current destination
+ * segment, its page-bounded bio_vec is kmapped so that zstd can write into the
+ * page cache directly, and the number of bytes writable there is returned.
+ * Otherwise @kaddr_ret is set to NULL and the number of bytes to skip before
+ * that segment is returned.  This covers both the initial prefix and gaps in
+ * the destination bio.
+ */
+static u32 zstd_map_dest(struct compressed_bio *cb, u32 decompressed,
+			 void **kaddr_ret)
+{
+	struct bio *orig_bio = &cb->orig_bbio->bio;
+	struct bio_vec bvec;
+	u32 bvec_offset;
+	u32 off;
+
+	bvec = bio_iter_iovec(orig_bio, orig_bio->bi_iter);
+	/*
+	 * cb->start may underflow, but subtracting that value can still give us
+	 * the correct offset inside the full decompressed extent.
+	 */
+	bvec_offset = page_offset(bvec.bv_page) + bvec.bv_offset - cb->start;
+
+	if (decompressed < bvec_offset) {
+		*kaddr_ret = NULL;
+		return bvec_offset - decompressed;
+	}
+
+	off = decompressed - bvec_offset;
+	ASSERT(off < bvec.bv_len);
+	*kaddr_ret = bvec_kmap_local(&bvec) + off;
+	return bvec.bv_len - off;
+}
+
 int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 {
 	struct btrfs_fs_info *fs_info = cb_to_fs_info(cb);
 	struct workspace *workspace = list_entry(ws, struct workspace, list);
+	struct bio *orig_bio = &cb->orig_bbio->bio;
 	struct folio_iter fi;
 	size_t srclen = bio_get_size(&cb->bbio.bio);
 	zstd_dstream *stream;
@@ -591,7 +629,6 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 	const unsigned int min_folio_size = btrfs_min_folio_size(fs_info);
 	unsigned long folio_in_index = 0;
 	unsigned long total_folios_in = DIV_ROUND_UP(srclen, min_folio_size);
-	unsigned long buf_start;
 	unsigned long total_out = 0;
 
 	bio_first_folio(&fi, &cb->bbio.bio, 0);
@@ -615,15 +652,26 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 	workspace->in_buf.pos = 0;
 	workspace->in_buf.size = min_t(size_t, srclen, min_folio_size);
 
-	workspace->out_buf.dst = workspace->buf;
-	workspace->out_buf.pos = 0;
-	workspace->out_buf.size = fs_info->sectorsize;
-
-	while (1) {
+	while (orig_bio->bi_iter.bi_size) {
 		size_t ret2;
+		void *kaddr;
+		u32 dstlen;
+
+		dstlen = zstd_map_dest(cb, total_out, &kaddr);
+		if (kaddr) {
+			workspace->out_buf.dst = kaddr;
+			workspace->out_buf.size = dstlen;
+		} else {
+			workspace->out_buf.dst = workspace->buf;
+			workspace->out_buf.size = min_t(u32, dstlen,
+							fs_info->sectorsize);
+		}
+		workspace->out_buf.pos = 0;
 
 		ret2 = zstd_decompress_stream(stream, &workspace->out_buf,
 				&workspace->in_buf);
+		if (kaddr)
+			kunmap_local(kaddr);
 		if (unlikely(zstd_is_error(ret2))) {
 			struct btrfs_inode *inode = cb->bbio.inode;
 
@@ -634,14 +682,9 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 			ret = -EIO;
 			goto done;
 		}
-		buf_start = total_out;
 		total_out += workspace->out_buf.pos;
-		workspace->out_buf.pos = 0;
-
-		ret = btrfs_decompress_buf2page(workspace->out_buf.dst,
-				total_out - buf_start, cb, buf_start);
-		if (ret == 0)
-			break;
+		if (kaddr)
+			bio_advance(orig_bio, workspace->out_buf.pos);
 
 		if (workspace->in_buf.pos >= srclen)
 			break;

base-commit: 421066905cbceca1f78cba5f7d92b4980317ab2b
-- 
2.53.0-Meta