[PATCH] memcg: calling reclaim_high(GFP_KERNEL) in GFP_NOFS context deadlocks

Dave Chinner
2022-09-29T21:54:54Z

From: Dave Chinner <dchinner@redhat.com>

This should be more obvious, but gfpflags_allow_blocking() is not
the same thing as a GFP_KERNEL reclaim contexts. The former checks
GFP_DIRECT_RECLAIM which tells us if direct reclaim is allowed. The
latter (GFP_KERNEL) allows blocking on anything, including
filesystem and IO structures during reclaim.

However, we do lots of memory allocation from various filesystems we
are under GFP_NOFS contexts, including page cache folios. Hence if
direct reclaim in GFP_NOFS context waits on filesystem progress
(e.g. waits on folio writeback) then memory reclaim can deadlock.

e.g. page cache allocation (which is GFP_NOFS context) gets stuck
waiting on page writeback like so:

[ 75.943494] task:test_write state:D stack:12560 pid: 3728 ppid: 3613 flags:0x00004002
[ 75.944788] Call Trace:
[ 75.945183] <TASK>
[ 75.945543] __schedule+0x2f9/0xa30
[ 75.946118] ? __mod_memcg_lruvec_state+0x41/0x90
[ 75.946895] schedule+0x5a/0xc0
[ 75.947397] io_schedule+0x42/0x70
[ 75.947992] folio_wait_bit_common+0x159/0x3d0
[ 75.948732] ? dio_warn_stale_pagecache.part.0+0x50/0x50
[ 75.949505] folio_wait_writeback+0x28/0x80
[ 75.950163] shrink_page_list+0x96e/0xc30
[ 75.950843] shrink_lruvec+0x558/0xb80
[ 75.951440] shrink_node+0x2c6/0x700
[ 75.952059] do_try_to_free_pages+0xd5/0x570
[ 75.952771] try_to_free_mem_cgroup_pages+0x105/0x220
[ 75.953548] reclaim_high.constprop.0+0xa3/0xf0
[ 75.954209] mem_cgroup_handle_over_high+0x8f/0x280
[ 75.955025] ? kmem_cache_alloc_lru+0x1c6/0x3f0
[ 75.955781] try_charge_memcg+0x6c3/0x820
[ 75.956436] ? __mem_cgroup_threshold+0x16/0x150
[ 75.957204] charge_memcg+0x76/0xf0
[ 75.957810] __mem_cgroup_charge+0x29/0x80
[ 75.958464] __filemap_add_folio+0x225/0x590
[ 75.959112] ? scan_shadow_nodes+0x30/0x30
[ 75.959794] filemap_add_folio+0x37/0xa0
[ 75.960432] __filemap_get_folio+0x1fd/0x340
[ 75.961141] ? xas_load+0x5/0xa0
[ 75.961712] iomap_write_begin+0x103/0x6a0
[ 75.962390] ? filemap_dirty_folio+0x5c/0x80
[ 75.963106] ? iomap_write_end+0xa2/0x2b0
[ 75.963744] iomap_file_buffered_write+0x17c/0x380
[ 75.964546] xfs_file_buffered_write+0xb1/0x2e0
[ 75.965286] ? xfs_file_buffered_write+0x2b2/0x2e0
[ 75.966097] vfs_write+0x2ca/0x3d0
[ 75.966702] __x64_sys_pwrite64+0x8c/0xc0
[ 75.967349] do_syscall_64+0x35/0x80

At this point, the system has 58 pending XFS IO completions that are
stuck waiting for workqueue progress:

[ 1664.460579] workqueue xfs-conv/dm-0: flags=0x4c
[ 1664.461332] pwq 48: cpus=24 node=3 flags=0x0 nice=0 active=58/256 refcnt=59
[ 1664.461335] pending: xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io, xfs_end_io

and nothing is making progress. The reason progress is not being
made is not clear from what I can gather from the steaming corpse,
but it is clear that the memcg reclaim code should not be blocking
on filesystem related objects in GFP_NOFS allocation contexts.

We have the reclaim context parameters right there when we call
mem_cgroup_handle_over_high(), so pass them down the stack so memcg
reclaim doesn't cause deadlocks. This makes the reclaim deadlocks in
the test I've been running go away.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
include/linux/memcontrol.h | 4 ++--
include/linux/resume_user_mode.h | 2 +-
mm/memcontrol.c | 6 +++---
3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 6257867fbf95..575bb8cfc810 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -919,7 +919,7 @@ unsigned long mem_cgroup_get_zone_lru_size(struct lruvec *lruvec,
return READ_ONCE(mz->lru_zone_size[zone_idx][lru]);
}

-void mem_cgroup_handle_over_high(void);
+void mem_cgroup_handle_over_high(gfp_t gfp_mask);

unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg);

@@ -1433,7 +1433,7 @@ static inline void folio_memcg_unlock(struct folio *folio)
{
}

-static inline void mem_cgroup_handle_over_high(void)
+static inline void mem_cgroup_handle_over_high(gfp_t gfp_mask)
{
}

diff --git a/include/linux/resume_user_mode.h b/include/linux/resume_user_mode.h
index 285189454449..f8f3e958e9cf 100644
--- a/include/linux/resume_user_mode.h
+++ b/include/linux/resume_user_mode.h
@@ -55,7 +55,7 @@ static inline void resume_user_mode_work(struct pt_regs *regs)
}
#endif

- mem_cgroup_handle_over_high();
+ mem_cgroup_handle_over_high(GFP_KERNEL);
blkcg_maybe_throttle_current();

rseq_handle_notify_resume(NULL, regs);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b69979c9ced5..09fbebff9796 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2491,7 +2491,7 @@ static unsigned long calculate_high_delay(struct mem_cgroup *memcg,
* Scheduled by try_charge() to be executed from the userland return path
* and reclaims memory over the high limit.
*/
-void mem_cgroup_handle_over_high(void)
+void mem_cgroup_handle_over_high(gfp_t gfp_mask)
{
unsigned long penalty_jiffies;
unsigned long pflags;
@@ -2519,7 +2519,7 @@ void mem_cgroup_handle_over_high(void)
*/
nr_reclaimed = reclaim_high(memcg,
in_retry ? SWAP_CLUSTER_MAX : nr_pages,
- GFP_KERNEL);
+ gfp_mask);

/*
* memory.high is breached and reclaim is unable to keep up. Throttle
@@ -2755,7 +2755,7 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
if (current->memcg_nr_pages_over_high > MEMCG_CHARGE_BATCH &&
!(current->flags & PF_MEMALLOC) &&
gfpflags_allow_blocking(gfp_mask)) {
- mem_cgroup_handle_over_high();
+ mem_cgroup_handle_over_high(gfp_mask);
}
return 0;
}
--
2.37.2