linux-kernel - Re: kmemleak report after 9082e87bfbf8 ("block: remove struct bio

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Date:	Mon, 6 Jun 2016 17:12:46 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Christoph Hellwig <hch@....de>
Cc:	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jens Axboe <axboe@...com>, Larry.Finger@...inger.net,
	bart.vanassche@...disk.com, drysdale@...gle.com
Subject: Re: kmemleak report after 9082e87bfbf8 ("block: remove struct
 bio_batch")

On Mon, Jun 06, 2016 at 04:13:34PM +0200, Christoph Hellwig wrote:
> I've got a few reports of this over the weekend, but it still doesn't
> make much sense to me.
> 
> Could it be that kmemleak can't deal with the bio_batch logic?  I've
> tried to look at the various bio and biovec number entries in
> /proc/slabinfo, and while they keep changing a bit during the
> system runtime there doesn't seem to be a persistent increase
> even after lots of mkfs calls.

I think the reported leaks settle after about 10-20min (2-3 kmemleak
periodic scans), so checking /proc/slabinfo may not be sufficient if
the leak is not growing. The leaks also do not seem to disappear,
otherwise kmemleak would no longer report them (e.g. after kfree, even
if they had been previously reported).

What kmemleak reports is objects for which it cannot find a pointer (to
anywhere inside that object; e.g. list_heads are handled). False
positives are indeed present sometimes but for cases where pointers are
stored in non-tracked objects like alloc_pages().

It seems that this leak reports always come in pairs. The first one:

unreferenced object 0xffff880262cbe900 (size 256):
  comm "NetworkManager", pid 516, jiffies 4294895670 (age 2479.340s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 c0 f3 ab 8a 00 88 ff ff  ................
    02 20 00 20 00 00 00 00 11 00 00 00 00 00 00 00  . . ............
  backtrace:
    [<ffffffff811d244e>] kmem_cache_alloc+0xfe/0x250
    [<ffffffff81175b42>] mempool_alloc+0x72/0x190
    [<ffffffff812dc9f6>] bio_alloc_bioset+0xb6/0x240
    [<ffffffff812eca7f>] next_bio+0x1f/0x50
    [<ffffffff812ecf1a>] blkdev_issue_zeroout+0xea/0x1d0
    [<ffffffffc025b610>] ext4_issue_zeroout+0x40/0x50 [ext4]
    [<ffffffffc028c58d>] ext4_ext_map_blocks+0x144d/0x1bb0 [ext4]
    [<ffffffff811839f4>] release_pages+0x254/0x310
    [<ffffffff8118437a>] __pagevec_release+0x2a/0x40
    [<ffffffffc025b297>] mpage_prepare_extent_to_map+0x227/0x2c0 [ext4]
    [<ffffffffc025b793>] ext4_map_blocks+0x173/0x5d0 [ext4]
    [<ffffffffc025f1d0>] ext4_writepages+0x700/0xd40 [ext4]
    [<ffffffff8121312e>] legitimize_mnt+0xe/0x50
    [<ffffffff811d244e>] kmem_cache_alloc+0xfe/0x250
    [<ffffffff81173fd5>] __filemap_fdatawrite_range+0xc5/0x100
    [<ffffffff81174113>] filemap_write_and_wait_range+0x33/0x70

is the first mempool_alloc() in bio_alloc_bioset() for struct bio and
front_pad.

The second report:

unreferenced object 0xffff880036488600 (size 256):
  comm "NetworkManager", pid 516, jiffies 4294895670 (age 2479.348s)
  hex dump (first 32 bytes):
    80 39 08 00 00 ea ff ff 00 10 00 00 00 00 00 00  .9..............
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff811d244e>] kmem_cache_alloc+0xfe/0x250
    [<ffffffff812dc8b7>] bvec_alloc+0x57/0xe0
    [<ffffffff812dcaaf>] bio_alloc_bioset+0x16f/0x240
    [<ffffffff812eca7f>] next_bio+0x1f/0x50
    [<ffffffff812ecf1a>] blkdev_issue_zeroout+0xea/0x1d0
    [<ffffffffc025b610>] ext4_issue_zeroout+0x40/0x50 [ext4]
    [<ffffffffc028c58d>] ext4_ext_map_blocks+0x144d/0x1bb0 [ext4]
    [<ffffffff811839f4>] release_pages+0x254/0x310
    [<ffffffff8118437a>] __pagevec_release+0x2a/0x40
    [<ffffffffc025b297>] mpage_prepare_extent_to_map+0x227/0x2c0 [ext4]
    [<ffffffffc025b793>] ext4_map_blocks+0x173/0x5d0 [ext4]
    [<ffffffffc025f1d0>] ext4_writepages+0x700/0xd40 [ext4]
    [<ffffffff8121312e>] legitimize_mnt+0xe/0x50
    [<ffffffff811d244e>] kmem_cache_alloc+0xfe/0x250
    [<ffffffff81173fd5>] __filemap_fdatawrite_range+0xc5/0x100
    [<ffffffff81174113>] filemap_write_and_wait_range+0x33/0x70

is for the struct bio_vec allocation in bvec_alloc() (the one going via
kmem_cache_alloc).

IIUC, the bio object above allocated via next_bio() ->
bio_alloc_bioset() is returned to __blkdev_issue_zeroout() which
eventually submits them either directly for the last one or via
next_bio().

Regarding bio chaining, I can't figure out what the first bio allocated
in __blkdev_issue_zeroout() is chained to since bio == NULL initially.
Subsequent next_bio() allocations are linked to the previous ones via
bio_chain() but somehow kmemleak loses track of the first one, hence the
subsequent bios are reported as leaks. That's unless the chaining should
be the other way around:

diff --git a/block/blk-lib.c b/block/blk-lib.c
index 23d7f301a196..3bf78b7b74cc 100644
--- a/block/blk-lib.c
+++ b/block/blk-lib.c
@@ -15,7 +15,7 @@ static struct bio *next_bio(struct bio *bio, int rw, unsigned int nr_pages,
 	struct bio *new = bio_alloc(gfp, nr_pages);
 
 	if (bio) {
-		bio_chain(bio, new);
+		bio_chain(new, bio);
 		submit_bio(rw, bio);
 	}
 

Also confusing is that chaining is done via bio->bi_private, however
this is overridden in other places like submit_bio_wait().

However, since I don't fully understand this code, this chaining may not
even be essential to struct bio freeing (and I'm investigating the wrong
path).

-- 
Catalin