Recent 3.x kernels: Memory leak causing OOMs

Tue Apr 1 02:19:59 PDT 2014

Right, so the machine has died again this morning.

Excuse me if I ignore this merge window, I seem to have some debugging
to do on my own because no one else is interested in my reports of this,
and I need to get this fixed.  Modern Linux is totally unusable for me
at the moment.

On Mon, Mar 17, 2014 at 07:33:16PM +0000, Russell King - ARM Linux wrote:
> On Mon, Mar 17, 2014 at 06:18:13PM +0000, Catalin Marinas wrote:
> > On Mon, Mar 17, 2014 at 06:07:48PM +1100, NeilBrown wrote:
> > > On Sat, 15 Mar 2014 10:19:52 +0000 Russell King - ARM Linux
> > > <linux at arm.linux.org.uk> wrote:
> > > > unreferenced object 0xc3c3f880 (size 256):
> > > >   comm "md2_resync", pid 4680, jiffies 638245 (age 8615.570s)
> > > >   hex dump (first 32 bytes):
> > > >     00 00 00 00 00 00 00 00 00 00 00 00 01 00 00 f0  ................
> > > >     00 00 00 00 10 00 00 00 00 00 00 00 00 00 00 00  ................
> > > >   backtrace:
> > > >     [<c008d4f0>] __save_stack_trace+0x34/0x40
> > > >     [<c008d5f0>] create_object+0xf4/0x214
> > > >     [<c02da114>] kmemleak_alloc+0x3c/0x6c
> > > >     [<c008c0d4>] __kmalloc+0xd0/0x124
> > > >     [<c00bb124>] bio_alloc_bioset+0x4c/0x1a4
> > > >     [<c021206c>] r1buf_pool_alloc+0x40/0x148
> > > >     [<c0061160>] mempool_alloc+0x54/0xfc
> > > >     [<c0211938>] sync_request+0x168/0x85c
> > > >     [<c021addc>] md_do_sync+0x75c/0xbc0
> > > >     [<c021b594>] md_thread+0x138/0x154
> > > >     [<c0037b48>] kthread+0xb0/0xbc
> > > >     [<c0013190>] ret_from_fork+0x14/0x24
> > > >     [<ffffffff>] 0xffffffff
> > > > 
> > > > with 3077 of these in the debug file.  3075 are for "md2_resync" and
> > > > two are for "md4_resync".
> > > > 
> > > > /proc/slabinfo shows for this bucket:
> > > > kmalloc-256         3237   3450    256   15    1 : tunables  120   60    0 : slabdata    230    230      0
> > > > 
> > > > but this would only account for about 800kB of memory usage, which itself
> > > > is insignificant - so this is not the whole story.
> > > > 
> > > > It seems that this is the culpret for the allocations:
> > > >         for (j = pi->raid_disks ; j-- ; ) {
> > > >                 bio = bio_kmalloc(gfp_flags, RESYNC_PAGES);
> > > > 
> > > > Since RESYNC_PAGES will be 64K/4K=16, each struct bio_vec is 12 bytes
> > > > (12 * 16 = 192) plus the size of struct bio, which would fall into this
> > > > bucket.
> > > > 
> > > > I don't see anything obvious - it looks like it isn't every raid check
> > > > which loses bios.  Not quite sure what to make of this right now.
> > > 
> > > I can't see anything obvious either.
> > > 
> > > The bios allocated there are stored in a r1_bio and those pointers are never
> > > changed.
> > > If the r1_bio wasn't freed then when the data-check finished, mempool_destroy
> > > would complain that the pool wasn't completely freed.
> > > And when the r1_bio is freed, all the bios are put as well.
> > 
> > It could be a false positive, there are areas that kmemleak doesn't scan
> > like page allocations and the pointer reference graph it tries to build
> > would fail.
> > 
> > What's interesting to see is the first few leaks reported as they are
> > always reported in the order of allocation. In this case, the
> > bio_kmalloc() returned pointer is stored in r1_bio. Is the r1_bio
> > reported as a leak as well?
> 
> I'd assume that something else would likely have a different size.
> All leaks are of 256 bytes.  Also...
> 
> $ grep kmemleak_alloc kmemleak-20140315 -A2 |sort | uniq -c |less
>    3081 --
>    3082     [<c008c0d4>] __kmalloc+0xd0/0x124
>    3082     [<c00bb124>] bio_alloc_bioset+0x4c/0x1a4
>    3082     [<c02da114>] kmemleak_alloc+0x3c/0x6c
> 
> seems pretty conclusive that it's just one spot.
> 
> > The sync_request() function eventually gets rid of the r1_bio as it is a
> > variable on the stack. But it is stored in a bio->bi_private variable
> > and that's where I lost track of where pointers are referenced from.
> > 
> > A simple way to check whether it's a false positive is to do a:
> > 
> > echo dump=<unref obj addr> > /sys/kernel/debug/kmemleak
> > 
> > If an object was reported as a leak but later on kmemleak doesn't know
> > about it, it means that it was freed and hence a false positive (maybe I
> > should add this as a warning in kmemleak if certain amount of leaked
> > objects freeing is detected).
> 
> So doing that with the above leaked bio produces:
> 
> kmemleak: Object 0xc3c3f880 (size 256):
> kmemleak:   comm "md2_resync", pid 4680, jiffies 638245
> kmemleak:   min_count = 1
> kmemleak:   count = 0
> kmemleak:   flags = 0x3
> kmemleak:   checksum = 1042746691
> kmemleak:   backtrace:
>      [<c008d4f0>] __save_stack_trace+0x34/0x40
>      [<c008d5f0>] create_object+0xf4/0x214
>      [<c02da114>] kmemleak_alloc+0x3c/0x6c
>      [<c008c0d4>] __kmalloc+0xd0/0x124
>      [<c00bb124>] bio_alloc_bioset+0x4c/0x1a4
>      [<c021206c>] r1buf_pool_alloc+0x40/0x148
>      [<c0061160>] mempool_alloc+0x54/0xfc
>      [<c0211938>] sync_request+0x168/0x85c
>      [<c021addc>] md_do_sync+0x75c/0xbc0
>      [<c021b594>] md_thread+0x138/0x154
>      [<c0037b48>] kthread+0xb0/0xbc
>      [<c0013190>] ret_from_fork+0x14/0x24
>      [<ffffffff>] 0xffffffff
> 
> -- 
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> improving, and getting towards what was expected from it.
> 
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-- 
FTTC broadband for 0.8mile line: now at 9.7Mbps down 460kbps up... slowly
improving, and getting towards what was expected from it.