Re: [f2fs-dev] [PATCH] f2fs: change maximum zstd compression buffer size

From: Chao Yu
Date: Mon May 04 2020 - 22:08:47 EST


On 2020-5-5 9:52, Chao Yu wrote:
On 2020-5-4 22:30, Jaegeuk Kim wrote:
From: Daeho Jeong <daehojeong@xxxxxxxxxx>

Current zstd compression buffer size is one page and header size less
than cluster size. By this, zstd compression always succeeds even if
the real compression data is failed to fit into the buffer size, and
eventually reading the cluster returns I/O error with the corrupted
compression data.

What's the root cause of this issue? I didn't get it.


Signed-off-by: Daeho Jeong <daehojeong@xxxxxxxxxx>
---
fs/f2fs/compress.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/compress.c b/fs/f2fs/compress.c
index 4c7eaeee52336..a9fa8049b295f 100644
--- a/fs/f2fs/compress.c
+++ b/fs/f2fs/compress.c
@@ -313,7 +313,7 @@ static int zstd_init_compress_ctx(struct compress_ctx *cc)
cc->private = workspace;
cc->private2 = stream;

- cc->clen = cc->rlen - PAGE_SIZE - COMPRESS_HEADER_SIZE;
+ cc->clen = ZSTD_compressBound(PAGE_SIZE << cc->log_cluster_size);

Max size of intermediate buffer has been set as below?

- zstd_init_compress_ctx
- params = ZSTD_getParams(F2FS_ZSTD_DEFAULT_CLEVEL, cc->rlen, 0)
- ZSTD_CStreamWorkspaceBound(params.cParams)
- outBuffSize = ZSTD_compressBound(blockSize) + 1;
- workspace_size = ... + outBuffSize + ...
- workspace = f2fs_kvmalloc(workspace_size)


In my machine, the value is 66572 which is much larger than size of dst buffer,
so, in where we can tell the real size of dst buffer to zstd compressor?
Otherwise, if compressed data size is larger than dst buffer size, when we flush
compressed data into dst buffer, we may suffer overflow.

return 0;
}

@@ -330,7 +330,7 @@ static int zstd_compress_pages(struct compress_ctx *cc)
ZSTD_inBuffer inbuf;
ZSTD_outBuffer outbuf;
int src_size = cc->rlen;
- int dst_size = src_size - PAGE_SIZE - COMPRESS_HEADER_SIZE;
+ int dst_size = cc->clen;
int ret;

inbuf.pos = 0;



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