The patch titled
Subject: zram: try harder to store user data on compression error
has been added to the -mm tree. Its filename is
zram-try-harder-to-store-user-data-on-compression-error.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/zram-try-harder-to-store-user-data-on-compression-error.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/zram-try-harder-to-store-user-data-on-compression-error.patch
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From: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
Subject: zram: try harder to store user data on compression error
__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. For 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, allows 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)
Link: http://lkml.kernel.org/r/20170518021714.3229-1-sergey.senozhatsky@xxxxxxxxx
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
drivers/block/zram/zram_drv.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
diff -puN drivers/block/zram/zram_drv.c~zram-try-harder-to-store-user-data-on-compression-error drivers/block/zram/zram_drv.c
--- a/drivers/block/zram/zram_drv.c~zram-try-harder-to-store-user-data-on-compression-error
+++ a/drivers/block/zram/zram_drv.c
@@ -732,10 +732,12 @@ compress_again:
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))
_
Patches currently in -mm which might be from sergey.senozhatsky.work@xxxxxxxxx are
zram-try-harder-to-store-user-data-on-compression-error.patch
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