Re: [PATCH -mm] memcg: do not trigger OOM fromadd_to_page_cache_locked

[Johannes Weiner]

On Tue, Nov 27, 2012 at 09:05:30AM +0900, Kamezawa Hiroyuki wrote:
> (2012/11/26 22:18), Michal Hocko wrote:
> >[CCing also Johannes - the thread started here:
> >https://lkml.org/lkml/2012/11/21/497]
> >
> >On Mon 26-11-12 01:38:55, azurIt wrote:
> >>>This is hackish but it should help you in this case. Kamezawa, what do
> >>>you think about that? Should we generalize this and prepare something
> >>>like mem_cgroup_cache_charge_locked which would add __GFP_NORETRY
> >>>automatically and use the function whenever we are in a locked context?
> >>>To be honest I do not like this very much but nothing more sensible
> >>>(without touching non-memcg paths) comes to my mind.
> >>
> >>
> >>I installed kernel with this patch, will report back if problem occurs
> >>again OR in few weeks if everything will be ok. Thank you!
> >
> >Now that I am looking at the patch closer it will not work because it
> >depends on other patch which is not merged yet and even that one would
> >help on its own because __GFP_NORETRY doesn't break the charge loop.
> >Sorry I have missed that...
> >
> >The patch bellow should help though. (it is based on top of the current
> >-mm tree but I will send a backport to 3.2 in the reply as well)
> >---
> > From 7796f942d62081ad45726efd90b5292b80e7c690 Mon Sep 17 00:00:00 2001
> >From: Michal Hocko <mhocko@xxxxxxx>
> >Date: Mon, 26 Nov 2012 11:47:57 +0100
> >Subject: [PATCH] memcg: do not trigger OOM from add_to_page_cache_locked
> >
> >memcg oom killer might deadlock if the process which falls down to
> >mem_cgroup_handle_oom holds a lock which prevents other task to
> >terminate because it is blocked on the very same lock.
> >This can happen when a write system call needs to allocate a page but
> >the allocation hits the memcg hard limit and there is nothing to reclaim
> >(e.g. there is no swap or swap limit is hit as well and all cache pages
> >have been reclaimed already) and the process selected by memcg OOM
> >killer is blocked on i_mutex on the same inode (e.g. truncate it).
> >
> >Process A
> >[<ffffffff811109b8>] do_truncate+0x58/0xa0		# takes i_mutex
> >[<ffffffff81121c90>] do_last+0x250/0xa30
> >[<ffffffff81122547>] path_openat+0xd7/0x440
> >[<ffffffff811229c9>] do_filp_open+0x49/0xa0
> >[<ffffffff8110f7d6>] do_sys_open+0x106/0x240
> >[<ffffffff8110f950>] sys_open+0x20/0x30
> >[<ffffffff815b5926>] system_call_fastpath+0x18/0x1d
> >[<ffffffffffffffff>] 0xffffffffffffffff
> >
> >Process B
> >[<ffffffff8110a9c1>] mem_cgroup_handle_oom+0x241/0x3b0
> >[<ffffffff8110b5ab>] T.1146+0x5ab/0x5c0
> >[<ffffffff8110c22e>] mem_cgroup_cache_charge+0xbe/0xe0
> >[<ffffffff810ca28c>] add_to_page_cache_locked+0x4c/0x140
> >[<ffffffff810ca3a2>] add_to_page_cache_lru+0x22/0x50
> >[<ffffffff810ca45b>] grab_cache_page_write_begin+0x8b/0xe0
> >[<ffffffff81193a18>] ext3_write_begin+0x88/0x270
> >[<ffffffff810c8fc6>] generic_file_buffered_write+0x116/0x290
> >[<ffffffff810cb3cc>] __generic_file_aio_write+0x27c/0x480
> >[<ffffffff810cb646>] generic_file_aio_write+0x76/0xf0           # takes ->i_mutex
> >[<ffffffff8111156a>] do_sync_write+0xea/0x130
> >[<ffffffff81112183>] vfs_write+0xf3/0x1f0
> >[<ffffffff81112381>] sys_write+0x51/0x90
> >[<ffffffff815b5926>] system_call_fastpath+0x18/0x1d
> >[<ffffffffffffffff>] 0xffffffffffffffff
> >
> >This is not a hard deadlock though because administrator can still
> >intervene and increase the limit on the group which helps the writer to
> >finish the allocation and release the lock.
> >
> >This patch heals the problem by forbidding OOM from page cache charges
> >(namely add_ro_page_cache_locked). mem_cgroup_cache_charge_no_oom helper
> >function is defined which adds GFP_MEMCG_NO_OOM to the gfp mask which
> >then tells mem_cgroup_charge_common that OOM is not allowed for the
> >charge. No OOM from this path, except for fixing the bug, also make some
> >sense as we really do not want to cause an OOM because of a page cache
> >usage.
> >As a possibly visible result add_to_page_cache_lru might fail more often
> >with ENOMEM but this is to be expected if the limit is set and it is
> >preferable than OOM killer IMO.
> >
> >__GFP_NORETRY is abused for this memcg specific flag because it has been
> >used to prevent from OOM already (since not-merged-yet "memcg: reclaim
> >when more than one page needed"). The only difference is that the flag
> >doesn't prevent from reclaim anymore which kind of makes sense because
> >the global memory allocator triggers reclaim as well. The retry without
> >any reclaim on __GFP_NORETRY doesn't make much sense anyway because this
> >is effectively a busy loop with allowed OOM in this path.
> >
> >Reported-by: azurIt <azurit@xxxxxxxx>
> >Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> 
> As a short term fix, I think this patch will work enough and seems simple enough.
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

Yes, let's do this for now.

> >diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> >index 10e667f..aac9b21 100644
> >--- a/include/linux/gfp.h
> >+++ b/include/linux/gfp.h
> >@@ -152,6 +152,9 @@ struct vm_area_struct;
> >  /* 4GB DMA on some platforms */
> >  #define GFP_DMA32	__GFP_DMA32
> >
> >+/* memcg oom killer is not allowed */
> >+#define GFP_MEMCG_NO_OOM	__GFP_NORETRY

Could we leave this within memcg, please?  An extra flag to
mem_cgroup_cache_charge() or the like.  GFP flags are about
controlling the page allocator, this seems abusive.  We have an oom
flag down in try_charge, maybe just propagate this up the stack?

> >diff --git a/mm/filemap.c b/mm/filemap.c
> >index 83efee7..ef14351 100644
> >--- a/mm/filemap.c
> >+++ b/mm/filemap.c
> >@@ -447,7 +447,13 @@ int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
> >  	VM_BUG_ON(!PageLocked(page));
> >  	VM_BUG_ON(PageSwapBacked(page));
> >
> >-	error = mem_cgroup_cache_charge(page, current->mm,
> >+	/*
> >+	 * Cannot trigger OOM even if gfp_mask would allow that normally
> >+	 * because we might be called from a locked context and that
> >+	 * could lead to deadlocks if the killed process is waiting for
> >+	 * the same lock.
> >+	 */
> >+	error = mem_cgroup_cache_charge_no_oom(page, current->mm,
> >  					gfp_mask & GFP_RECLAIM_MASK);
> >  	if (error)
> >  		goto out;

Shmem does not use this function but also charges under the i_mutex in
the write path and fallocate at least.
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