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Subject[PATCH v2] lockdep: Fix fs_reclaim warning.
From
Date
>From 361d37a7d36978020dfb4c11ec1f4800937ccb68 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Thu, 8 Feb 2018 10:35:35 +0900
Subject: [PATCH v2] lockdep: Fix fs_reclaim warning.

Dave Jones reported fs_reclaim lockdep warnings.

============================================
WARNING: possible recursive locking detected
4.15.0-rc9-backup-debug+ #1 Not tainted
--------------------------------------------
sshd/24800 is trying to acquire lock:
(fs_reclaim){+.+.}, at: [<0000000084f438c2>] fs_reclaim_acquire.part.102+0x5/0x30

but task is already holding lock:
(fs_reclaim){+.+.}, at: [<0000000084f438c2>] fs_reclaim_acquire.part.102+0x5/0x30

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(fs_reclaim);
lock(fs_reclaim);

*** DEADLOCK ***

May be due to missing lock nesting notation

2 locks held by sshd/24800:
#0: (sk_lock-AF_INET6){+.+.}, at: [<000000001a069652>] tcp_sendmsg+0x19/0x40
#1: (fs_reclaim){+.+.}, at: [<0000000084f438c2>] fs_reclaim_acquire.part.102+0x5/0x30

stack backtrace:
CPU: 3 PID: 24800 Comm: sshd Not tainted 4.15.0-rc9-backup-debug+ #1
Call Trace:
dump_stack+0xbc/0x13f
__lock_acquire+0xa09/0x2040
lock_acquire+0x12e/0x350
fs_reclaim_acquire.part.102+0x29/0x30
kmem_cache_alloc+0x3d/0x2c0
alloc_extent_state+0xa7/0x410
__clear_extent_bit+0x3ea/0x570
try_release_extent_mapping+0x21a/0x260
__btrfs_releasepage+0xb0/0x1c0
btrfs_releasepage+0x161/0x170
try_to_release_page+0x162/0x1c0
shrink_page_list+0x1d5a/0x2fb0
shrink_inactive_list+0x451/0x940
shrink_node_memcg.constprop.88+0x4c9/0x5e0
shrink_node+0x12d/0x260
try_to_free_pages+0x418/0xaf0
__alloc_pages_slowpath+0x976/0x1790
__alloc_pages_nodemask+0x52c/0x5c0
new_slab+0x374/0x3f0
___slab_alloc.constprop.81+0x47e/0x5a0
__slab_alloc.constprop.80+0x32/0x60
__kmalloc_track_caller+0x267/0x310
__kmalloc_reserve.isra.40+0x29/0x80
__alloc_skb+0xee/0x390
sk_stream_alloc_skb+0xb8/0x340
tcp_sendmsg_locked+0x8e6/0x1d30
tcp_sendmsg+0x27/0x40
inet_sendmsg+0xd0/0x310
sock_write_iter+0x17a/0x240
__vfs_write+0x2ab/0x380
vfs_write+0xfb/0x260
SyS_write+0xb6/0x140
do_syscall_64+0x1e5/0xc05
entry_SYSCALL64_slow_path+0x25/0x25

This warning is caused by commit d92a8cfcb37ecd13 ("locking/lockdep: Rework
FS_RECLAIM annotation") which replaced lockdep_set_current_reclaim_state()/
lockdep_clear_current_reclaim_state() in __perform_reclaim() and
lockdep_trace_alloc() in slab_pre_alloc_hook() with fs_reclaim_acquire()/
fs_reclaim_release(). Since __kmalloc_reserve() from __alloc_skb() adds
__GFP_NOMEMALLOC | __GFP_NOWARN to gfp_mask, and all reclaim path simply
propagates __GFP_NOMEMALLOC, fs_reclaim_acquire() in slab_pre_alloc_hook()
is trying to grab the 'fake' lock again when __perform_reclaim() already
grabbed the 'fake' lock.

The

/* this guy won't enter reclaim */
if ((current->flags & PF_MEMALLOC) && !(gfp_mask & __GFP_NOMEMALLOC))
return false;

test which causes slab_pre_alloc_hook() to try to grab the 'fake' lock
was added by commit cf40bd16fdad42c0 ("lockdep: annotate reclaim context
(__GFP_NOFS)"). But that test is outdated because PF_MEMALLOC thread won't
enter reclaim regardless of __GFP_NOMEMALLOC after commit 341ce06f69abfafa
("page allocator: calculate the alloc_flags for allocation only once")
added the PF_MEMALLOC safeguard (

/* Avoid recursion of direct reclaim */
if (p->flags & PF_MEMALLOC)
goto nopage;

in __alloc_pages_slowpath()).

Thus, let's fix outdated test by removing __GFP_NOMEMALLOC test and allow
__need_fs_reclaim() to return false.

Reported-and-tested-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Nick Piggin <npiggin@gmail.com>
---
mm/page_alloc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 81e18ce..19fb76b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3590,7 +3590,7 @@ static bool __need_fs_reclaim(gfp_t gfp_mask)
return false;

/* this guy won't enter reclaim */
- if ((current->flags & PF_MEMALLOC) && !(gfp_mask & __GFP_NOMEMALLOC))
+ if (current->flags & PF_MEMALLOC)
return false;

/* We're only interested __GFP_FS allocations for now */
--
1.8.3.1
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