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SubjectRe: [PATCH 1/6] cpuset write dirty map
On Tue, 18 Sep 2007, Ethan Solomita wrote:

> > Does it have to be atomic? atomic is weak and can fail.
> >
> > If some callers can do GFP_KERNEL and some can only do GFP_ATOMIC then we
> > should at least pass the gfp_t into this function so it can do the stronger
> > allocation when possible.
> I was going to say that sanity would be improved by just allocing the
> nodemask at inode alloc time. A failure here could be a problem because
> below cpuset_intersects_dirty_nodes() assumes that a NULL nodemask
> pointer means that there are no dirty nodes, thus preventing dirty pages
> from getting written to disk. i.e. This must never fail.

Hmmm. It should assume that there is no tracking thus any node can be
dirty? Match by default?

> Given that we allocate it always at the beginning, I'm leaning towards
> just allocating it within mapping no matter its size. It will make the
> code much much simpler, and save me writing all the comments we've been
> discussing. 8-)
> How disastrous would this be? Is the need to support a 1024 node system
> with 1,000,000 open mostly-read-only files thus needing to spend 120MB
> of extra memory on my nodemasks a real scenario and a showstopper?

Consider that a 1024 node system has more than 4TB of memory. If that
system is running as a fileserver then you get into some issues. But then
120MB are not that big of a deal. Its more the cache footprint issue I
would think. Having a NULL there avoids touching a 128 byte nodemask. I
think your approach should be fine.

> >> +void cpuset_clear_dirty_nodes(struct address_space *mapping)
> >> +{
> >> + nodemask_t *nodes = mapping->dirty_nodes;
> >> +
> >> + if (nodes) {
> >> + mapping->dirty_nodes = NULL;
> >> + kfree(nodes);
> >> + }
> >> +}
> >
> > Can this race with cpuset_update_dirty_nodes()? And with itself? If not,
> > a comment which describes the locking requirements would be good.
> I'll add a comment. Such a race should not be possible. It is called
> only from clear_inode() which is used when the inode is being freed
> "with extreme prejudice" (from its comments). I can add a check that
> i_state I_FREEING is set. Would that do?

There is already a comment saying that it cannot happen.
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