Re: [RFC 0/8] Cpuset aware writeback

[Christoph Lameter]

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.

> 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 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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