[PATCH v3] xfs: avoid deadlock when trigger memory reclaim in ->writepages

Yafang Shao
2020-06-15T11:57:07Z

Recently there is a XFS deadlock on our server with an old kernel.
This deadlock is caused by allocating memory in xfs_map_blocks() while
doing writeback on behalf of memroy reclaim. Although this deadlock happens
on an old kernel, I think it could happen on the upstream as well. This
issue only happens once and can't be reproduced, so I haven't tried to
reproduce it on upsteam kernel.

Bellow is the call trace of this deadlock.
[480594.790087] INFO: task redis-server:16212 blocked for more than 120 seconds.
[480594.790087] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[480594.790088] redis-server D ffffffff8168bd60 0 16212 14347 0x00000004
[480594.790090] ffff880da128f070 0000000000000082 ffff880f94a2eeb0 ffff880da128ffd8
[480594.790092] ffff880da128ffd8 ffff880da128ffd8 ffff880f94a2eeb0 ffff88103f9d6c40
[480594.790094] 0000000000000000 7fffffffffffffff ffff88207ffc0ee8 ffffffff8168bd60
[480594.790096] Call Trace:
[480594.790101] [<ffffffff8168dce9>] schedule+0x29/0x70
[480594.790103] [<ffffffff8168b749>] schedule_timeout+0x239/0x2c0
[480594.790111] [<ffffffff8168d28e>] io_schedule_timeout+0xae/0x130
[480594.790114] [<ffffffff8168d328>] io_schedule+0x18/0x20
[480594.790116] [<ffffffff8168bd71>] bit_wait_io+0x11/0x50
[480594.790118] [<ffffffff8168b895>] __wait_on_bit+0x65/0x90
[480594.790121] [<ffffffff811814e1>] wait_on_page_bit+0x81/0xa0
[480594.790125] [<ffffffff81196ad2>] shrink_page_list+0x6d2/0xaf0
[480594.790130] [<ffffffff811975a3>] shrink_inactive_list+0x223/0x710
[480594.790135] [<ffffffff81198225>] shrink_lruvec+0x3b5/0x810
[480594.790139] [<ffffffff8119873a>] shrink_zone+0xba/0x1e0
[480594.790141] [<ffffffff81198c20>] do_try_to_free_pages+0x100/0x510
[480594.790143] [<ffffffff8119928d>] try_to_free_mem_cgroup_pages+0xdd/0x170
[480594.790145] [<ffffffff811f32de>] mem_cgroup_reclaim+0x4e/0x120
[480594.790147] [<ffffffff811f37cc>] __mem_cgroup_try_charge+0x41c/0x670
[480594.790153] [<ffffffff811f5cb6>] __memcg_kmem_newpage_charge+0xf6/0x180
[480594.790157] [<ffffffff8118c72d>] __alloc_pages_nodemask+0x22d/0x420
[480594.790162] [<ffffffff811d0c7a>] alloc_pages_current+0xaa/0x170
[480594.790165] [<ffffffff811db8fc>] new_slab+0x30c/0x320
[480594.790168] [<ffffffff811dd17c>] ___slab_alloc+0x3ac/0x4f0
[480594.790204] [<ffffffff81685656>] __slab_alloc+0x40/0x5c
[480594.790206] [<ffffffff811dfc43>] kmem_cache_alloc+0x193/0x1e0
[480594.790233] [<ffffffffa04fab67>] kmem_zone_alloc+0x97/0x130 [xfs]
[480594.790247] [<ffffffffa04f90ba>] _xfs_trans_alloc+0x3a/0xa0 [xfs]
[480594.790261] [<ffffffffa04f915c>] xfs_trans_alloc+0x3c/0x50 [xfs]
[480594.790276] [<ffffffffa04e958b>] xfs_iomap_write_allocate+0x1cb/0x390 [xfs]
[480594.790299] [<ffffffffa04d3616>] xfs_map_blocks+0x1a6/0x210 [xfs]
[480594.790312] [<ffffffffa04d416b>] xfs_do_writepage+0x17b/0x550 [xfs]
[480594.790314] [<ffffffff8118d881>] write_cache_pages+0x251/0x4d0 [xfs]
[480594.790338] [<ffffffffa04d3e05>] xfs_vm_writepages+0xc5/0xe0 [xfs]
[480594.790341] [<ffffffff8118ebfe>] do_writepages+0x1e/0x40
[480594.790343] [<ffffffff811837b5>] __filemap_fdatawrite_range+0x65/0x80
[480594.790346] [<ffffffff81183901>] filemap_write_and_wait_range+0x41/0x90
[480594.790360] [<ffffffffa04df2c6>] xfs_file_fsync+0x66/0x1e0 [xfs]
[480594.790363] [<ffffffff81231cf5>] do_fsync+0x65/0xa0
[480594.790365] [<ffffffff81231fe3>] SyS_fdatasync+0x13/0x20
[480594.790367] [<ffffffff81698d09>] system_call_fastpath+0x16/0x1b

Note that xfs_iomap_write_allocate() is replaced by xfs_convert_blocks() in
commit 4ad765edb02a ("xfs: move xfs_iomap_write_allocate to xfs_aops.c")
and write_cache_pages() is replaced by iomap_writepages() in
commit 598ecfbaa742 ("iomap: lift the xfs writeback code to iomap").
So for upsteam, the call trace should be,
xfs_vm_writepages
-> iomap_writepages
-> write_cache_pages
-> iomap_do_writepage
-> xfs_map_blocks
-> xfs_convert_blocks
-> xfs_bmapi_convert_delalloc
-> xfs_trans_alloc //It should alloc page with GFP_NOFS

I'm not sure whether it is proper to add the GFP_NOFS to all the
->writepags, so I only add it for XFS.

Stefan also reported that he saw this issue two times while under memory
pressure, So I add his reported-by.

Reported-by: Stefan Priebe - Profihost AG <s.priebe@profihost.ag>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>

---
v2 -> v3:
- retitle the subject from "iomap: avoid deadlock if memory reclaim is
triggered in writepage path"
- set GFP_NOFS only for XFS ->writepages

v1 -> v2:
- retitle the subject from "xfs: avoid deadlock when tigger memory reclam
in xfs_map_blocks()"
- set GFP_NOFS in iomap_do_writepage(), per Dave
---
fs/xfs/xfs_aops.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index b356118..1ccfbf2 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -573,9 +573,21 @@ static inline bool xfs_ioend_needs_workqueue(struct iomap_ioend *ioend)
struct writeback_control *wbc)
{
struct xfs_writepage_ctx wpc = { };
+ unsigned int nofs_flag;
+ int ret;

xfs_iflags_clear(XFS_I(mapping->host), XFS_ITRUNCATED);
- return iomap_writepages(mapping, wbc, &wpc.ctx, &xfs_writeback_ops);
+
+ /*
+ * We can allocate memory here while doing writeback on behalf of
+ * memory reclaim. To avoid memory allocation deadlocks set the
+ * task-wide nofs context for the following operations.
+ */
+ nofs_flag = memalloc_nofs_save();
+ ret = iomap_writepages(mapping, wbc, &wpc.ctx, &xfs_writeback_ops);
+ memalloc_nofs_restore(nofs_flag);
+
+ return ret;
}

STATIC int
--
1.8.3.1