Re: [PATCH for 3.2.34] memcg: do not trigger OOM fromadd_to_page_cache_locked

[Michal Hocko]

On Wed 06-02-13 02:17:21, azurIt wrote:
> >5-memcg-fix-1.patch is not complete. It doesn't contain the folloup I
> >mentioned in a follow up email. Here is the full patch:
> 
> 
> Here is the log where OOM, again, killed MySQL server [search for "(mysqld)"]:
> http://www.watchdog.sk/lkml/oom_mysqld6

[...]
WARNING: at mm/memcontrol.c:2409 T.1149+0x2d9/0x610()
Hardware name: S5000VSA
gfp_mask:4304 nr_pages:1 oom:0 ret:2
Pid: 3545, comm: apache2 Tainted: G        W    3.2.37-grsec #1
Call Trace:
 [<ffffffff8105502a>] warn_slowpath_common+0x7a/0xb0
 [<ffffffff81055116>] warn_slowpath_fmt+0x46/0x50
 [<ffffffff81108163>] ? mem_cgroup_margin+0x73/0xa0
 [<ffffffff8110b6f9>] T.1149+0x2d9/0x610
 [<ffffffff812af298>] ? blk_finish_plug+0x18/0x50
 [<ffffffff8110c6b4>] mem_cgroup_cache_charge+0xc4/0xf0
 [<ffffffff810ca6bf>] add_to_page_cache_locked+0x4f/0x140
 [<ffffffff810ca7d2>] add_to_page_cache_lru+0x22/0x50
 [<ffffffff810cad32>] filemap_fault+0x252/0x4f0
 [<ffffffff810eab18>] __do_fault+0x78/0x5a0
 [<ffffffff810edcb4>] handle_pte_fault+0x84/0x940
 [<ffffffff810e2460>] ? vma_prio_tree_insert+0x30/0x50
 [<ffffffff810f2508>] ? vma_link+0x88/0xe0
 [<ffffffff810ee6a8>] handle_mm_fault+0x138/0x260
 [<ffffffff8102709d>] do_page_fault+0x13d/0x460
 [<ffffffff810f46fc>] ? do_mmap_pgoff+0x3dc/0x430
 [<ffffffff815b61ff>] page_fault+0x1f/0x30
---[ end trace 8817670349022007 ]---
apache2 invoked oom-killer: gfp_mask=0x0, order=0, oom_adj=0, oom_score_adj=0
apache2 cpuset=uid mems_allowed=0
Pid: 3545, comm: apache2 Tainted: G        W    3.2.37-grsec #1
Call Trace:
 [<ffffffff810ccd2e>] dump_header+0x7e/0x1e0
 [<ffffffff810ccc2f>] ? find_lock_task_mm+0x2f/0x70
 [<ffffffff810cd1f5>] oom_kill_process+0x85/0x2a0
 [<ffffffff810cd8a5>] out_of_memory+0xe5/0x200
 [<ffffffff810cda7d>] pagefault_out_of_memory+0xbd/0x110
 [<ffffffff81026e76>] mm_fault_error+0xb6/0x1a0
 [<ffffffff8102734e>] do_page_fault+0x3ee/0x460
 [<ffffffff810f46fc>] ? do_mmap_pgoff+0x3dc/0x430
 [<ffffffff815b61ff>] page_fault+0x1f/0x30

The first trace comes from the debugging WARN and it clearly points to
a file fault path. __do_fault pre-charges a page in case we need to
do CoW (copy-on-write) for the returned page. This one falls back to
memcg OOM and never returns ENOMEM as I have mentioned earlier. 
However, the fs fault handler (filemap_fault here) can fallback to
page_cache_read if the readahead (do_sync_mmap_readahead) fails
to get page to the page cache. And we can see this happening in
the first trace. page_cache_read then calls add_to_page_cache_lru
and eventually gets to add_to_page_cache_locked which calls
mem_cgroup_cache_charge_no_oom so we will get ENOMEM if oom should
happen. This ENOMEM gets to the fault handler and kaboom.

So the fix is really much more complex than I thought. Although
add_to_page_cache_locked sounded like a good place it turned out to be
not in fact.

We need something more clever appaerently. One way would be not misusing
__GFP_NORETRY for GFP_MEMCG_NO_OOM and give it a real flag. We have 32
bits for those flags in gfp_t so there should be some room there. 
Or we could do this per task flag, same we do for NO_IO in the current
-mm tree.
The later one seems easier wrt. gfp_mask passing horror - e.g.
__generic_file_aio_write doesn't pass flags and it can be called from
unlocked contexts as well.

I have to think about it some more.
-- 
Michal Hocko
SUSE Labs
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