[PATCH] lockdep: Fix fs_reclaim warning.

[Tetsuo Handa]

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

Since no fs locks are held, doing GFP_KERNEL allocation should be safe
as long as there is PF_MEMALLOC safeguard (

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

) which prevents infinite recursion.

This warning seems to be caused by commit d92a8cfcb37ecd13
("locking/lockdep: Rework FS_RECLAIM annotation") which moved the
location of

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

check added by commit cf40bd16fdad42c0 ("lockdep: annotate reclaim context
(__GFP_NOFS)"). Since __kmalloc_reserve() from __alloc_skb() adds
__GFP_NOMEMALLOC | __GFP_NOWARN to gfp_mask, __need_fs_reclaim() is
failing to return false despite PF_MEMALLOC context (and resulted in
lockdep warning).

Since there was no PF_MEMALLOC safeguard as of cf40bd16fdad42c0, checking
__GFP_NOMEMALLOC might make sense. But since this safeguard was added by
commit 341ce06f69abfafa ("page allocator: calculate the alloc_flags for
allocation only once"), checking __GFP_NOMEMALLOC no longer makes sense.
Thus, let's remove __GFP_NOMEMALLOC check and allow __need_fs_reclaim() to
return false.

Reported-by: Dave Jones <davej@xxxxxxxxxxxxxxxxx>
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxxxx>
---
 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 76c9688..7804b0e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3583,7 +3583,7 @@ 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