Re: [rfc][patch] fs: turn iprune_mutex into rwsem

[Andreas Dilger <adilger@sun.com>]

On Aug 15, 2009  15:57 -0400, Christoph Hellwig wrote:
> On Fri, Aug 14, 2009 at 05:25:05PM +0200, Nick Piggin wrote:
> > Now I think the main problem is having the filesystem block (and do IO
> > in inode reclaim. The problem is that this doesn't get accounted well
> > and penalizes a random allocator with a big latency spike caused by
> > work generated from elsewhere.
> > 
> > I think the best idea would be to avoid this. By design if possible,
> > or by deferring the hard work to an asynchronous context. If the latter,
> > then the fs would probably want to throttle creation of new work with
> > queue size of the deferred work, but let's not get into those details.
> 
> I don't really see a good way to avoid this.  For any filesystem that
> does some sort of preallocations we need to drop them in ->clear_inode.

One of the problems I've seen in the past is that filesystem memory reclaim
(in particular dentry/inode cleanup) cannot happen within filesystems due
to potential deadlocks.  This is particularly problematic when there is a
lot of memory pressure from within the kernel and very little from userspace
(e.g. updatedb or find).

However, many/most inodes/dentries in the filesystem could be discarded
quite easily and would not deadlock the system.  I wonder if it makes
sense to keep a mask in the inode that the filesystem could set that
determines whether it is safe to clean up the inode even though __GFP_FS
is not set?  That would potentially allow e.g. shrink_icache_memory()
to free a large number of "non-tricky" inodes if needed (e.g. ones without
locks/preallocation/expensive cleanup).

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.