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

[Dennis Zhou]

On Thu, Apr 30, 2020 at 02:40:18PM -0700, Andrew Morton wrote:
> On Thu, 30 Apr 2020 17:43:56 +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.
> > 
> 
> Patch looks good and seems sensible, thanks.
> 

Acked-by: Dennis Zhou <dennis@xxxxxxxxxx>

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

I would also like to know.

Thanks,
Dennis