In zlib_compress_pages(), if the last looped call to zlib_deflate ends
between 1 and 5 bytes short of a page boundary, the final call with
Z_FINISH is unable to write the final 6 bytes of the zlib stream and bails
out. This causes compress_file_range() in inode.c to distrust the
compressor and flag ths inode nocompress, with the end result that the
rest of a potentially highly-compressible file is stored in expanded form.
You can demonstrate this with the following script, setting MOUNTPOINT to
an otherwise-unused btrfs mount with compress=zlib:
MOUNTPOINT=/mnt
sync; df $MOUNTPOINT
{ head -c 6630 /usr/src/linux/Documentation/BUG-HUNTING;
cat /dev/zero; } | head -c 1310720 > $MOUNTPOINT/test
sync; df $MOUNTPOINT
rm -f $MOUNTPOINT/test
The selection of 6630 bytes from BUG-HUNTING plus nulls provides for a
first 128KiB chunk that compresses to just over 4 KiB, and the other nine
chunks compress easily within a single page each, so this should compress
to 11 pages or 44 KiB; instead it uses significantly more, depending on
how many threads were able to compress chunks before nocompress was set.
The following patch fixes this by moving the Z_FINISH calls to inside the
page read/write loop, allowing it to call as many times as needed.
Signed-off-by: Danielle Church <dchurch@xxxxxxxxxxxxxxx>
---
fs/btrfs/zlib.c | 26 +++++++++++++++++---------
1 file changed, 17 insertions(+), 9 deletions(-)
diff --git a/fs/btrfs/zlib.c b/fs/btrfs/zlib.c
index fb22fd8..1dc0455 100644
--- a/fs/btrfs/zlib.c
+++ b/fs/btrfs/zlib.c
@@ -89,6 +89,7 @@ static int zlib_compress_pages(struct list_head *ws,
struct page *in_page = NULL;
struct page *out_page = NULL;
unsigned long bytes_left;
+ int deflate_flush = Z_SYNC_FLUSH;
*out_pages = 0;
*total_out = 0;
@@ -120,8 +121,12 @@ static int zlib_compress_pages(struct list_head *ws,
workspace->strm.avail_out = PAGE_CACHE_SIZE;
workspace->strm.avail_in = min(len, PAGE_CACHE_SIZE);
- while (workspace->strm.total_in < len) {
- ret = zlib_deflate(&workspace->strm, Z_SYNC_FLUSH);
+ while (deflate_flush != Z_FINISH || ret != Z_STREAM_END) {
+ ret = zlib_deflate(&workspace->strm, deflate_flush);
+ /* we're all done, including the stream end */
+ if (ret == Z_STREAM_END && deflate_flush == Z_FINISH)
+ break;
+
if (ret != Z_OK) {
printk(KERN_DEBUG "BTRFS: deflate in loop returned %d\n",
ret);
@@ -159,9 +164,12 @@ static int zlib_compress_pages(struct list_head *ws,
workspace->strm.avail_out = PAGE_CACHE_SIZE;
workspace->strm.next_out = cpage_out;
}
- /* we're all done */
- if (workspace->strm.total_in >= len)
- break;
+ /* we're all done with input data; keep looping until stream end is written */
+ if (workspace->strm.total_in >= len) {
+ deflate_flush = Z_FINISH;
+ workspace->strm.avail_in = 0;
+ continue;
+ }
/* we've read in a full page, get a new one */
if (workspace->strm.avail_in == 0) {
@@ -181,11 +189,11 @@ static int zlib_compress_pages(struct list_head *ws,
workspace->strm.next_in = data_in;
}
}
- workspace->strm.avail_in = 0;
- ret = zlib_deflate(&workspace->strm, Z_FINISH);
- zlib_deflateEnd(&workspace->strm);
+ ret = zlib_deflateEnd(&workspace->strm);
- if (ret != Z_STREAM_END) {
+ if (ret != Z_OK) {
+ printk(KERN_DEBUG "BTRFS: deflateEnd returned %d\n",
+ ret);
ret = -EIO;
goto out;
}
--
2.2.1
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