[dm-devel] [Regression/Behavior change]dm-flakey corrupt read bio, even the feature is drop_writes

[Lukas Herbolt]

Hi Qu,

Sorry for the confusion. Reading the email again and the code it seems
that the READS are really returned as -EIO if you set the drop_writes.
I just tested it and you are right.

If I was reading the fstest correctly the flakey is created as:
---
flakey: 0 409600 flakey 8:64 0 0 180 1 drop_writes
---

I believe the READs are dropped because it does not have any flags set.

---
        if (bio_data_dir(bio) == READ) {
            /* If flags were specified, only corrupt those that match. */
            if (fc->corrupt_bio_byte && (fc->corrupt_bio_rw == READ) &&
                all_corrupt_bio_flags_match(bio, fc))
                goto map_bio;
            else
                return -EIO;
        }
---

with conclusion of setting:
---
                /*
                 * Flag this bio as submitted while down.
                 */
                pb->bio_submitted = true;
---

I have quick test patch ready, but it probably broke more thing than
fixes so I will continue on it.
Just in case you want to test it. Diff is done again 4.8-rc1

--- a/drivers/md/dm-flakey.c
+++ b/drivers/md/dm-flakey.c
@@ -292,6 +292,11 @@ static int flakey_map(struct dm_target *ti,
struct bio *bio)
                 * Map reads as normal only if corrupt_bio_byte set.
                 */
                if (bio_data_dir(bio) == READ) {
+                        /* We should retunr all READS as ok in case
of DROP WRITES flag is set. */
+                       if (test_bit(DROP_WRITES, &fc->flags)) {
+                               pb->bio_submitted = false;
+                               goto map_bio;
+                       }
                        /* If flags were specified, only corrupt those
that match. */
                        if (fc->corrupt_bio_byte &&
(fc->corrupt_bio_rw == READ) &&
                            all_corrupt_bio_flags_match(bio, fc))



On Mon, Aug 22, 2016 at 10:05 AM, Lukas Herbolt <lherbolt at redhat.com> wrote:
> Hello,
>
> There is patch from Mike. It's part of current pull request to 4.8-rc1
> For more details check:
>  - https://www.redhat.com/archives/dm-devel/2016-July/msg00561.html
>  - https://www.redhat.com/archives/dm-devel/2016-August/msg00109.html
>
> Lukas
>
> On Mon, Aug 22, 2016 at 9:31 AM, Qu Wenruo <quwenruo at cn.fujitsu.com> wrote:
>> Hi, Mike and btrfs and dm guys
>>
>> When doing regression test on v4.8-rc1, we found that fstests/btrfs/056
>> always fails. With the following dmesg:
>> ---
>> Buffer I/O error on dev dm-0, logical block 1310704, async page read
>> Buffer I/O error on dev dm-0, logical block 16, async page read
>> Buffer I/O error on dev dm-0, logical block 16, async page read
>> ---
>>
>> And bisect leads to the following commits:
>> ---
>> commit 99f3c90d0d85708e7401a81ce3314e50bf7f2819
>> Author: Mike Snitzer <snitzer at redhat.com>
>> Date:   Fri Jul 29 13:19:55 2016 -0400
>>
>>     dm flakey: error READ bios during the down_interval
>> ---
>>
>> While according to the document of dm-flakey, it says that when using
>> drop_writes feature, read bios are not affected:
>> ---
>>   drop_writes:
>>         All write I/O is silently ignored.
>>         Read I/O is handled correctly.
>> ---
>>
>> If I understand the word "correctly" correctly, it should means READ I/0 is
>> handled without problem.
>>
>> However with this commit, it also corrupt the read bio, leading to the test
>> failure.
>>
>>
>> At least there are two fixes available here;
>> 1) Fix fstest scripts
>>    The related macro is "_flakey_drop_and_remount yes", which will
>>    check the fs during the "drop_writes" time.
>>
>>    Currently, only btrfs/056 calls "_flakey_drop_and_remount" with
>>    "yes". So other test cases are not affected.
>>
>>    However, even we move the fsck outside of the "drop_writes" range,
>>    although test case can pass without problem, but we will still
>>    get a dmesg error:
>>    "Buffer I/O error on dev dm-0, logical block 1310704, async page read"
>>
>> 2) Revert to flakey behavior to allow READ bio
>>    Then everything is back to the old good days.
>>
>> Not sure which one is correct for current use case, as I'm not familiar with
>> dm codes.
>>
>> Any idea to fix dm-flaky and keep the READ bio behavior?
>>
>> Thanks,
>> Qu
>>
>>
>>
>>
>>
>> --
>> dm-devel mailing list
>> dm-devel at redhat.com
>> https://www.redhat.com/mailman/listinfo/dm-devel
>
>
>
> --
> Lukas Herbolt
> RHCE, RH436, BSc, SSc
> Senior Technical Support Engineer
> Global Support Services (GSS)
> Email:    lherbolt at redhat.com



-- 
Lukas Herbolt
RHCE, RH436, BSc, SSc
Senior Technical Support Engineer
Global Support Services (GSS)
Email:    lherbolt at redhat.com