[PATCH] memcg: do not trigger OOM from add_to_page_cache_locked

[Michal Hocko]

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 grows oom
argument which is pushed down the call chain.

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.

Changes since v1
- do not abuse gfp_flags and rather use oom parameter directly as per
  Johannes
- handle also shmem write fauls resp. fallocate properly as per Johannes

Reported-by: azurIt <azurit@xxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
 include/linux/memcontrol.h |    5 +++--
 mm/filemap.c               |    9 +++++++--
 mm/memcontrol.c            |    9 ++++-----
 mm/shmem.c                 |   15 ++++++++++++---
 4 files changed, 26 insertions(+), 12 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 095d2b4..8f48d5e 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -63,7 +63,7 @@ extern void mem_cgroup_commit_charge_swapin(struct page *page,
 extern void mem_cgroup_cancel_charge_swapin(struct mem_cgroup *memcg);
 
 extern int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
-					gfp_t gfp_mask);
+					gfp_t gfp_mask, bool oom);
 
 struct lruvec *mem_cgroup_zone_lruvec(struct zone *, struct mem_cgroup *);
 struct lruvec *mem_cgroup_page_lruvec(struct page *, struct zone *);
@@ -210,7 +210,8 @@ static inline int mem_cgroup_newpage_charge(struct page *page,
 }
 
 static inline int mem_cgroup_cache_charge(struct page *page,
-					struct mm_struct *mm, gfp_t gfp_mask)
+					struct mm_struct *mm, gfp_t gfp_mask,
+					bool oom)
 {
 	return 0;
 }
diff --git a/mm/filemap.c b/mm/filemap.c
index 83efee7..ef8fbd5 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -447,8 +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,
-					gfp_mask & GFP_RECLAIM_MASK);
+	/*
+	 * 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(page, current->mm, gfp_mask, false);
 	if (error)
 		goto out;
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 02ee2f7..26690d6 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3709,11 +3709,10 @@ out:
  * < 0 if the cgroup is over its limit
  */
 static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
-				gfp_t gfp_mask, enum charge_type ctype)
+				gfp_t gfp_mask, enum charge_type ctype, bool oom)
 {
 	struct mem_cgroup *memcg = NULL;
 	unsigned int nr_pages = 1;
-	bool oom = true;
 	int ret;
 
 	if (PageTransHuge(page)) {
@@ -3742,7 +3741,7 @@ int mem_cgroup_newpage_charge(struct page *page,
 	VM_BUG_ON(page->mapping && !PageAnon(page));
 	VM_BUG_ON(!mm);
 	return mem_cgroup_charge_common(page, mm, gfp_mask,
-					MEM_CGROUP_CHARGE_TYPE_ANON);
+					MEM_CGROUP_CHARGE_TYPE_ANON, true);
 }
 
 /*
@@ -3851,7 +3850,7 @@ void mem_cgroup_commit_charge_swapin(struct page *page,
 }
 
 int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
-				gfp_t gfp_mask)
+				gfp_t gfp_mask, bool oom)
 {
 	struct mem_cgroup *memcg = NULL;
 	enum charge_type type = MEM_CGROUP_CHARGE_TYPE_CACHE;
@@ -3863,7 +3862,7 @@ int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
 		return 0;
 
 	if (!PageSwapCache(page))
-		ret = mem_cgroup_charge_common(page, mm, gfp_mask, type);
+		ret = mem_cgroup_charge_common(page, mm, gfp_mask, type, oom);
 	else { /* page is swapcache/shmem */
 		ret = __mem_cgroup_try_charge_swapin(mm, page,
 						     gfp_mask, &memcg);
diff --git a/mm/shmem.c b/mm/shmem.c
index 55054a7..ba59cfa 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -760,7 +760,7 @@ int shmem_unuse(swp_entry_t swap, struct page *page)
 	 * the shmem_swaplist_mutex which might hold up shmem_writepage().
 	 * Charged back to the user (not to caller) when swap account is used.
 	 */
-	error = mem_cgroup_cache_charge(page, current->mm, GFP_KERNEL);
+	error = mem_cgroup_cache_charge(page, current->mm, GFP_KERNEL, true);
 	if (error)
 		goto out;
 	/* No radix_tree_preload: swap entry keeps a place for page in tree */
@@ -1152,8 +1152,16 @@ repeat:
 				goto failed;
 		}
 
+		 /*
+                  * Cannot trigger OOM even if gfp_mask would allow that
+                  * normally because we might be called from a locked
+                  * context (i_mutex held) if this is a write lock or
+                  * fallocate and that could lead to deadlocks if the
+                  * killed process is waiting for the same lock.
+		  */
 		error = mem_cgroup_cache_charge(page, current->mm,
-						gfp & GFP_RECLAIM_MASK);
+						gfp & GFP_RECLAIM_MASK,
+						sgp < SGP_WRITE);
 		if (!error) {
 			error = shmem_add_to_page_cache(page, mapping, index,
 						gfp, swp_to_radix_entry(swap));
@@ -1209,7 +1217,8 @@ repeat:
 		SetPageSwapBacked(page);
 		__set_page_locked(page);
 		error = mem_cgroup_cache_charge(page, current->mm,
-						gfp & GFP_RECLAIM_MASK);
+						gfp & GFP_RECLAIM_MASK,
+						sgp < SGP_WRITE);
 		if (error)
 			goto decused;
 		error = radix_tree_preload(gfp & GFP_RECLAIM_MASK);
-- 
1.7.10.4

-- 
Michal Hocko
SUSE Labs
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/