Re: [PATCH 005 of 13] md: Allow stripes to be expanded in preparation for expanding an array.

[Neil Brown <neilb@suse.de>]

On Thursday March 16, akpm@osdl.org wrote:
> NeilBrown <neilb@suse.de> wrote:
> >
> > +	/* Got them all.
> >  +	 * Return the new ones and free the old ones.
> >  +	 * At this point, we are holding all the stripes so the array
> >  +	 * is completely stalled, so now is a good time to resize
> >  +	 * conf->disks.
> >  +	 */
> >  +	ndisks = kzalloc(newsize * sizeof(struct disk_info), GFP_NOIO);
> >  +	if (ndisks) {
> >  +		for (i=0; i<conf->raid_disks; i++)
> >  +			ndisks[i] = conf->disks[i];
> >  +		kfree(conf->disks);
> >  +		conf->disks = ndisks;
> >  +	} else
> >  +		err = -ENOMEM;
> >  +	while(!list_empty(&newstripes)) {
> >  +		nsh = list_entry(newstripes.next, struct stripe_head, lru);
> >  +		list_del_init(&nsh->lru);
> >  +		for (i=conf->raid_disks; i < newsize; i++)
> >  +			if (nsh->dev[i].page == NULL) {
> >  +				struct page *p = alloc_page(GFP_NOIO);
> >  +				nsh->dev[i].page = p;
> >  +				if (!p)
> >  +					err = -ENOMEM;
> >  +			}
> >  +		release_stripe(nsh);
> >  +	}
> >  +	while(!list_empty(&oldstripes)) {
> >  +		osh = list_entry(oldstripes.next, struct stripe_head, lru);
> >  +		list_del(&osh->lru);
> >  +		kmem_cache_free(conf->slab_cache, osh);
> >  +	}
> >  +	kmem_cache_destroy(conf->slab_cache);
> >  +	conf->slab_cache = sc;
> >  +	conf->active_name = 1-conf->active_name;
> >  +	conf->pool_size = newsize;
> >  +	return err;
> >  +}
> 
> Are you sure the -ENOMEM handling here is solid?  It
> looks.... strange.

The philosophy of the -ENOMEM handling is (awkwardly?) embodied in the
comment
	 * Finally we add new pages.  This could fail, but we leave
	 * the stripe cache at it's new size, just with some pages empty.

at the top of the function.  The core function here is making some
data structures bigger.  In each case, having a bigger data structure
than required is no big deal.  So we try to increase the size of each
of them (the stripe_head cache, the 'disks' array, and the pages
allocated to each stripe.
If any of there fail we return -ENOMEM, but may allow others to
succeed.

Does that help?

NeilBrown
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