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Re: [PATCH 13/19] MM: set PF_FSTRANS while allocating per-cpu memory to

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 13/19] MM: set PF_FSTRANS while allocating per-cpu memory to avoid deadlock.
From: NeilBrown <neilb@xxxxxxx>
Date: Wed, 16 Apr 2014 16:22:01 +1000
Cc: linux-mm@xxxxxxxxx, linux-nfs@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140416054942.GD15995@dastard>
References: <20140416033623.10604.69237.stgit@xxxxxxxxxxxxxx> <20140416040336.10604.67456.stgit@xxxxxxxxxxxxxx> <20140416054942.GD15995@dastard>
On Wed, 16 Apr 2014 15:49:42 +1000 Dave Chinner <david@xxxxxxxxxxxxx> wrote:

> On Wed, Apr 16, 2014 at 02:03:36PM +1000, NeilBrown wrote:
> > lockdep reports a locking chain
> > 
> >   sk_lock-AF_INET --> rtnl_mutex --> pcpu_alloc_mutex
> > 
> > As sk_lock may be needed to reclaim memory, allowing that
> > reclaim while pcu_alloc_mutex is held can lead to deadlock.
> > So set PF_FSTRANS while it is help to avoid the FS reclaim.
> > 
> > pcpu_alloc_mutex can be taken when rtnl_mutex is held:
> > 
> >     [<ffffffff8117f979>] pcpu_alloc+0x49/0x960
> >     [<ffffffff8118029b>] __alloc_percpu+0xb/0x10
> >     [<ffffffff8193b9f7>] loopback_dev_init+0x17/0x60
> >     [<ffffffff81aaf30c>] register_netdevice+0xec/0x550
> >     [<ffffffff81aaf785>] register_netdev+0x15/0x30
> > 
> > Signed-off-by: NeilBrown <neilb@xxxxxxx>
> 
> This looks like a workaround to avoid passing a gfp mask around to
> describe the context in which the allocation is taking place.
> Whether or not that's the right solution, I can't say, but spreading
> this "we can turn off all reclaim of filesystem objects" mechanism
> all around the kernel doesn't sit well with me...

We are (effectively) passing a gfp mask around, except that it lives in
'current' rather than lots of other places.
I actually like the idea of discarding PF_MEMALLOC, PF_FSTRANS and
PF_MEMALLOC_NOIO, and just having current->gfp_allowed_mask (to match the
global variable of the same name).

> 
> And, again, PF_FSTRANS looks plainly wrong in this code - it sure
> isn't a fs transaction context we are worried about here...

So would PF_MEMALLOC_NOFS work for you?

NeilBrown

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