[PATCH v3 21/24] erofs: introduce LZ4 decompression inplace

Gao Xiang
2019-07-22T02:51:59Z

compressed data will be usually loaded into last pages of
the extent (the last page for 4k) for in-place decompression
(more specifically, in-place IO), as ilustration below,

start of compressed logical extent
| end of this logical extent
| |
______v___________________________v________
... | page 6 | page 7 | page 8 | page 9 | ...
|__________|__________|__________|__________|
. ^ . ^
. |compressed|
. | data |
. . .
|< dstsize >|<margin>|
oend iend
op ip

Therefore, it's possible to do decompression inplace (thus no
memcpy at all) if the margin is sufficient and safe enough [1],
and it can be implemented only for fixed-size output compression
compared with fixed-size input compression.

No memcpy for most of in-place IO (about 99% of enwik9) after
decompression inplace is implemented and sequential read will
be improved of course (see the following patches for test results).

[1] https://github.com/lz4/lz4/commit/b17f578a919b7e6b078cede2d52be29dd48c8e8c
https://github.com/lz4/lz4/commit/5997e139f53169fa3a1c1b4418d2452a90b01602

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
---
fs/erofs/decompressor.c | 36 ++++++++++++++++++++++++++++++++----
fs/erofs/erofs_fs.h | 2 +-
2 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/fs/erofs/decompressor.c b/fs/erofs/decompressor.c
index 2ee10bb7e440..18fcbd75423e 100644
--- a/fs/erofs/decompressor.c
+++ b/fs/erofs/decompressor.c
@@ -14,6 +14,9 @@
#endif

#define LZ4_MAX_DISTANCE_PAGES (DIV_ROUND_UP(LZ4_DISTANCE_MAX, PAGE_SIZE) + 1)
+#ifndef LZ4_DECOMPRESS_INPLACE_MARGIN
+#define LZ4_DECOMPRESS_INPLACE_MARGIN(srcsize) (((srcsize) >> 8) + 32)
+#endif

struct z_erofs_decompressor {
/*
@@ -112,7 +115,7 @@ static int lz4_decompress(struct z_erofs_decompress_req *rq, u8 *out)
{
unsigned int inputmargin, inlen;
u8 *src;
- bool copied;
+ bool copied, support_0padding;
int ret;

if (rq->inputsize > PAGE_SIZE)
@@ -120,13 +123,38 @@ static int lz4_decompress(struct z_erofs_decompress_req *rq, u8 *out)

src = kmap_atomic(*rq->in);
inputmargin = 0;
+ support_0padding = false;
+
+ /* decompression inplace is only safe when 0padding is enabled */
+ if (EROFS_SB(rq->sb)->requirements & EROFS_REQUIREMENT_LZ4_0PADDING) {
+ support_0padding = true;
+
+ while (!src[inputmargin & ~PAGE_MASK])
+ if (!(++inputmargin & ~PAGE_MASK))
+ break;
+
+ if (inputmargin >= rq->inputsize) {
+ kunmap_atomic(src);
+ return -EIO;
+ }
+ }

copied = false;
inlen = rq->inputsize - inputmargin;
if (rq->inplace_io) {
- src = generic_copy_inplace_data(rq, src, inputmargin);
- inputmargin = 0;
- copied = true;
+ const uint oend = (rq->pageofs_out +
+ rq->outputsize) & ~PAGE_MASK;
+ const uint nr = PAGE_ALIGN(rq->pageofs_out +
+ rq->outputsize) >> PAGE_SHIFT;
+
+ if (rq->partial_decoding || !support_0padding ||
+ rq->out[nr - 1] != rq->in[0] ||
+ rq->inputsize - oend <
+ LZ4_DECOMPRESS_INPLACE_MARGIN(inlen)) {
+ src = generic_copy_inplace_data(rq, src, inputmargin);
+ inputmargin = 0;
+ copied = true;
+ }
}

ret = LZ4_decompress_safe_partial(src + inputmargin, out,
diff --git a/fs/erofs/erofs_fs.h b/fs/erofs/erofs_fs.h
index e418725abfd6..8c64e78ceaca 100644
--- a/fs/erofs/erofs_fs.h
+++ b/fs/erofs/erofs_fs.h
@@ -17,7 +17,7 @@
* incompatible with this kernel version.
*/
#define EROFS_REQUIREMENT_LZ4_0PADDING 0x00000001
-#define EROFS_ALL_REQUIREMENTS 0
+#define EROFS_ALL_REQUIREMENTS EROFS_REQUIREMENT_LZ4_0PADDING

struct erofs_super_block {
/* 0 */__le32 magic; /* in the little endian */
--
2.17.1