iput() in reclaim context

[Hugh Dickins]

On Tue, 20 May 2008, Andrew Morton wrote:
> On Wed, 21 May 2008 10:15:32 +0400 Evgeniy Polyakov <johnpol@xxxxxxxxxxx> wrote:
> > On Tue, May 20, 2008 at 04:28:16PM -0700, Andrew Morton (akpm@xxxxxxxxxxxxxxxxxxxx) wrote:
> > > It's more than efficiency.  There are lots and lots of things we cannot
> > > do in direct-reclaim context.
> > > 
> > > ...
> > > 
> > > c) Cannot run iput().  Or at least, we couldn't five or six years
> > >    ago.  afaik nobody has investigated whether the situation is now
> > >    better or worse.

I happened to notice your remark in the buffer heads defrag thread.
Do you remember what that limitation was about?

Because just a few months ago I discovered a shmem race which I fixed
by doing igrab+iput in shmem_writepage, in the reclaim context.  Feeling
guilty now: I'd better investigate, but would welcome a starting pointer.

(If I'm lucky, it'll be that the generic code in vmscan.c cannot
use iput, but particular filesystems might themselves be safe to.)

Thanks,
Hugh
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