From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754925AbdERCRT (ORCPT ); Wed, 17 May 2017 22:17:19 -0400 Received: from mail-pf0-f193.google.com ([209.85.192.193]:35573 "EHLO mail-pf0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753397AbdERCRS (ORCPT ); Wed, 17 May 2017 22:17:18 -0400 From: Sergey Senozhatsky X-Google-Original-From: Sergey Senozhatsky To: Minchan Kim , Andrew Morton Cc: linux-kernel@vger.kernel.org, Sergey Senozhatsky , Sergey Senozhatsky Subject: [RFC][PATCH] zram: try harder to store user data on compression error Date: Thu, 18 May 2017 11:17:14 +0900 Message-Id: <20170518021714.3229-1-sergey.senozhatsky@gmail.com> X-Mailer: git-send-email 2.13.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org __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 --- 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