Re: [PATCH] percpu: make pcpu_alloc() aware of current gfp context

[Tejun Heo]

On Thu, Apr 30, 2020 at 05:43:56PM +0100, fdmanana@xxxxxxxxxx wrote:
> From: Filipe Manana <fdmanana@xxxxxxxx>
> 
> 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.
...
> This could be fixed by making btrfs pass GFP_NOFS instead of GFP_KERNEL to
> percpu_counter_init() in contextes where it is not reclaim safe, however
> that type of approach is discouraged since memalloc_[nofs|noio]_save()
> were introduced.  Therefore this change makes pcpu_alloc() look up into
> an existing nofs/noio context before deciding whether it is in an atomic
> context or not.
> 
> Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks.

-- 
tejun