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Date:   Thu, 30 Apr 2020 22:23:47 +0000
From:   Dennis Zhou <dennis@kernel.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     fdmanana@kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, dennis@kernel.org, tj@kernel.org,
        cl@linux.com, linux-btrfs@vger.kernel.org,
        Filipe Manana <fdmanana@suse.com>
Subject: Re: [PATCH] percpu: make pcpu_alloc() aware of current gfp context
Message-ID: <20200430222347.GA164259@google.com>
References: <20200430164356.15543-1-fdmanana@kernel.org>
 <20200430144018.c855f031b321d68e5c89b30c@linux-foundation.org>
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On Thu, Apr 30, 2020 at 02:40:18PM -0700, Andrew Morton wrote:
> On Thu, 30 Apr 2020 17:43:56 +0100 fdmanana@kernel.org wrote:
> 
> > From: Filipe Manana <fdmanana@suse.com>
> > 
> > Since 5.7-rc1, on btrfs we have a percpu counter initialization for which
> > we always pass a GFP_KERNEL gfp_t argument (this happens since commit
> > 2992df73268f78 ("btrfs: Implement DREW lock")).  That is safe in some
> > contextes but not on others where allowing fs reclaim could lead to a
> > deadlock because we are either holding some btrfs lock needed for a
> > transaction commit or holding a btrfs transaction handle open.  Because
> > of that we surround the call to the function that initializes the percpu
> > counter with a NOFS context using memalloc_nofs_save() (this is done at
> > btrfs_init_fs_root()).
> > 
> > However it turns out that this is not enough to prevent a possible
> > deadlock because percpu_alloc() determines if it is in an atomic context
> > by looking exclusively at the gfp flags passed to it (GFP_KERNEL in this
> > case) and it is not aware that a NOFS context is set.  Because it thinks
> > it is in a non atomic context it locks the pcpu_alloc_mutex, which can
> > result in a btrfs deadlock when pcpu_balance_workfn() is running, has
> > acquired that mutex and is waiting for reclaim, while the btrfs task that
> > called percpu_counter_init() (and therefore percpu_alloc()) is holding
> > either the btrfs commit_root semaphore or a transaction handle (done at
> > fs/btrfs/backref.c:iterate_extent_inodes()), which prevents reclaim from
> > finishing as an attempt to commit the current btrfs transaction will
> > deadlock.
> > 
> 
> Patch looks good and seems sensible, thanks.
> 

Acked-by: Dennis Zhou <dennis@kernel.org>

> But why did btrfs use memalloc_nofs_save()/restore() rather than
> s/GFP_KERNEL/GFP_NOFS/?

I would also like to know.

Thanks,
Dennis