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SubjectRe: [PATCH v2] lockdep: Fix fs_reclaim warning.
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On 8.02.2018 13:43, Tetsuo Handa wrote:
>>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
>

I think I've hit another incarnation of that one. The call stack is:
http://paste.opensuse.org/3f22d013

The cleaned up callstack of all the ? entries look like:

__lock_acquire+0x2d8a/0x4b70
lock_acquire+0x110/0x330
kmem_cache_alloc+0x29/0x2c0
__clear_extent_bit+0x488/0x800
try_release_extent_mapping+0x288/0x3c0
__btrfs_releasepage+0x6c/0x140
shrink_page_list+0x227e/0x3110
shrink_inactive_list+0x414/0xdb0
shrink_node_memcg+0x7c8/0x1250
shrink_node+0x2ae/0xb50
do_try_to_free_pages+0x2b1/0xe20
try_to_free_pages+0x205/0x570
__alloc_pages_nodemask+0xb91/0x2160
new_slab+0x27a/0x4e0
___slab_alloc+0x355/0x610
__slab_alloc+0x4c/0xa0
kmem_cache_alloc+0x22d/0x2c0
mempool_alloc+0xe1/0x280
bio_alloc_bioset+0x1d7/0x830
ext4_mpage_readpages+0x99f/0x1000 <-
__do_page_cache_readahead+0x4be/0x840
filemap_fault+0x8c8/0xfc0
ext4_filemap_fault+0x7d/0xb0
__do_fault+0x7a/0x150
__handle_mm_fault+0x1542/0x29d0
__do_page_fault+0x557/0xa30
async_page_fault+0x4c/0x60


There is no fs stacking going on here and that is 4.15-rc9.


> 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 */
>

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