Messages in this thread Patch in this message | | | From | Sergey Senozhatsky <> | Subject | [RFC][PATCH] zram: try harder to store user data on compression error | Date | Thu, 18 May 2017 11:17:14 +0900 |
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__zram_bvec_write() overwrites the existing compressed object only when it successfully compresses a new one. When zram_compress() fails, we propagate the error, but never touch the old object, so we keep the old data for that particular page and all reads that could hit that index later will read stale data. This can cause problems. By example,
- Overwriting a file
over-write block 1 aaa->xxx OK over-write block 2 bbb->yyy OK over-write block 3 ccc->zzz error << the block still has 'ccc' data.
- Now, reading the file
read block 1 xxx OK read block 2 yyy OK read block 3 ccc OK << we should not return OK here. Because "xxxyyyccc" is not correct file.
As a solution, when we fail to compress a new page (S/W or H/W compressor error, depending on particular setup) try to store the page uncompressed (PAGE_SIZE-ed zs_pool object). This, at least, lets us to return valid and correct data: read block 1 xxx - OK, compressed read block 2 yyy - OK, compressed read block 3 zzz - OK, uncompressed (because compressing back-end returned an error)
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> --- drivers/block/zram/zram_drv.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c index 372602c7da49..ba3e36bbb619 100644 --- a/drivers/block/zram/zram_drv.c +++ b/drivers/block/zram/zram_drv.c @@ -723,10 +723,12 @@ static int zram_compress(struct zram *zram, struct zcomp_strm **zstrm, kunmap_atomic(src); if (unlikely(ret)) { + /* + * We failed to compress the page, try to store it + * uncompressed (if there is enough memory). + */ pr_err("Compression failed! err=%d\n", ret); - if (entry) - zram_entry_free(zram, entry); - return ret; + comp_len = PAGE_SIZE; } if (unlikely(comp_len > max_zpage_size)) -- 2.13.0
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