Re: [PATCH v2] lockdep: Fix fs_reclaim warning.

[Tetsuo Handa]

Peter, are you OK with this patch?

Tetsuo Handa wrote:
> From 361d37a7d36978020dfb4c11ec1f4800937ccb68 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> 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@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 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
>