Re: [PATCH] bio: decrease bi_iter.bi_size by len in the fail path

[Ming Lei]

On Thu, May 29, 2014 at 1:21 AM, Maurizio Lombardi <mlombard@xxxxxxxxxx> wrote:
> Hi Ming,
>
> On Thu, May 29, 2014 at 12:59:19AM +0800, Ming Lei wrote:
>>
>> Actually, the correct thing may be like what did in the
>> attached patch, as Maurizio discussed with me[1].
>>
>> Very interestingly, I have reproduced the problem one time
>> with ext4/271 ext4/301 ext4/305, but won't with the attached
>> patch after running it for 3 rounds.
>>
>> [tom@localhost xfstests]$ sudo ./check ext4/271 ext4/301 ext4/305
>> FSTYP         -- ext4
>> PLATFORM      -- Linux/x86_64 localhost 3.15.0-rc7-next-20140527+
>> MKFS_OPTIONS  -- /dev/vdc
>> MOUNT_OPTIONS -- -o acl,user_xattr /dev/vdc /mnt/scratch
>>
>> ext4/271 1s ... 1s
>> ext4/301 31s ... 32s
>> ext4/305 181s ... 180s
>> Ran: ext4/271 ext4/301 ext4/305
>> Passed all 3 tests
>>
>> Jet, could you test the attached patch?
>>
>> [1], https://lkml.org/lkml/2014/5/27/327
>
> There is a little mistake in your patch, you removed bio->bi_iter.bi_size += len;
> after the "done" label,
> but be careful that at line 747 there is a "goto done"... bi_size should be incremented
> before jumping there.

Good catch, thanks Maurizio.

Jet, please test the attached patch in this mail and ignore previous
one.

The story behind the patch should be like below:

- one page is added in __bio_add_page() 'successfully',
and bio->bi_phys_segments is equal to queue_max_segments(q),
but it should have been rejected since the last vector isn't covered

- next time, __bio_add_page() is called to add one page, but this
time blk_recount_segments() can figure out the actual physical
segments and find it is more than max segments, so failure is
triggered, but the bio->bi_phys_segments is updated with
max segments plus one

- the oops is triggered and reported by Jet, :-)


Thanks,
-- 
Ming Lei
diff --git a/block/bio.c b/block/bio.c
index 0443694..f9bae56 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -744,6 +744,7 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
 				}
 			}
 
+			bio->bi_iter.bi_size += len;
 			goto done;
 		}
 	}
@@ -761,6 +762,7 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
 	bvec->bv_offset = offset;
 	bio->bi_vcnt++;
 	bio->bi_phys_segments++;
+	bio->bi_iter.bi_size += len;
 
 	/*
 	 * Perform a recount if the number of segments is greater
@@ -802,7 +804,6 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
 		bio->bi_flags &= ~(1 << BIO_SEG_VALID);
 
  done:
-	bio->bi_iter.bi_size += len;
 	return len;
 
  failed:
@@ -810,6 +811,7 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
 	bvec->bv_len = 0;
 	bvec->bv_offset = 0;
 	bio->bi_vcnt--;
+	bio->bi_iter.bi_size -= len;
 	blk_recount_segments(q, bio);
 	return 0;
 }