Re: [patch 1/7] cpusets: add dirty map to struct address_space

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 4 Nov 2008 15:20:56 -0600 (CST)
Christoph Lameter <cl@linux-foundation.org> wrote:

> On Tue, 4 Nov 2008, Andrew Morton wrote:
> 
> >> +#ifdef CONFIG_CPUSETS
> >> +#if MAX_NUMNODES <= BITS_PER_LONG
> >> +	nodemask_t		dirty_nodes;	/* nodes with dirty pages */
> >> +#else
> >> +	nodemask_t		*dirty_nodes;	/* pointer to mask, if dirty */
> >> +#endif
> >> +#endif
> >>  } __attribute__((aligned(sizeof(long))));
> >
> > eek.  Increasing the size of the address_space (and hence of the inode)
> > is a moderately big deal - there can be millions of these in memory.
> 
> Well this is adding only a single word to the inode structure.

multiplied by millions.

> >> @@ -72,6 +72,8 @@ struct writeback_control {
> >>  	 * so we use a single control to update them
> >>  	 */
> >>  	unsigned no_nrwrite_index_update:1;
> >> +
> >> +	nodemask_t *nodes;		/* Nodemask to writeback */
> >
> > This one doesn't get ifdefs?
> 
> The structure is typically allocated temporarily on the stack.

An ifdef also helps catch the presence of unnecesary code.

> >> +	nodemask_t *nodes = mapping->dirty_nodes;
> >> +	int node = page_to_nid(page);
> >> +
> >> +	if (!nodes) {
> >> +		nodes = kmalloc(sizeof(nodemask_t), GFP_ATOMIC);
> >
> > erk, OK, called from __set_page_dirty, needs to be atomic.
> >
> > What are the consequences when this allocation fails?
> 
> Dirty tracking will not occur. All nodes are assumed to be dirty.

It won't oops?

> We discussed this earlier 
> http://www.ussg.iu.edu/hypermail/linux/kernel/0709.1/2291.html

Well, if it isn't in the changelog and isn't in code comments, we get
to discuss it again.

A great amount of mailing list discussion is a Huge Honking Big Fat
Hint that the original code was insufficently understandable.