Re: [RFC 0/8] Cpuset aware writeback

[Andrew Morton <akpm@osdl.org>]

> On Wed, 17 Jan 2007 11:43:42 -0800 (PST) Christoph Lameter <clameter@sgi.com> wrote:
> On Tue, 16 Jan 2007, Andrew Morton wrote:
> 
> > Do what blockdevs do: limit the number of in-flight requests (Peter's
> > recent patch seems to be doing that for us) (perhaps only when PF_MEMALLOC
> > is in effect, to keep Trond happy) and implement a mempool for the NFS
> > request critical store.  Additionally:
> > 
> > - we might need to twiddle the NFS gfp_flags so it doesn't call the
> >   oom-killer on failure: just return NULL.
> > 
> > - consider going off-cpuset for critical allocations.  It's better than
> >   going oom.  A suitable implementation might be to ignore the caller's
> >   cpuset if PF_MEMALLOC.  Maybe put a WARN_ON_ONCE in there: we prefer that
> >   it not happen and we want to know when it does.
> 
> Given the intermediate  layers (network, additional gizmos (ip over xxx) 
> and the network cards) that will not be easy.

Paul has observed that it's already done.  But it seems to not be working.

> > btw, regarding the per-address_space node mask: I think we should free it
> > when the inode is clean (!mapping_tagged(PAGECACHE_TAG_DIRTY)).  Chances
> > are, the inode will be dirty for 30 seconds and in-core for hours.  We
> > might as well steal its nodemask storage and give it to the next file which
> > gets written to.  A suitable place to do all this is in
> > __mark_inode_dirty(I_DIRTY_PAGES), using inode_lock to protect
> > address_space.dirty_page_nodemask.
> 
> The inode lock is not taken when the page is dirtied.

The inode_lock is taken when the address_space's first page is dirtied.  It is
also taken when the address_space's last dirty page is cleaned.  So the place
where the inode is added to and removed from sb->s_dirty is, I think, exactly
the place where we want to attach and detach address_space.dirty_page_nodemask.

> The tree_lock
> is already taken when the mapping is dirtied and so I used that to
> avoid races adding and removing pointers to nodemasks from the address 
> space.
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