On Wed, 2010-12-01 at 19:34 -0800, Hugh Dickins wrote:
> On Wed, 1 Dec 2010, Trond Myklebust wrote:
> >
> > --- a/mm/truncate.c
> > +++ b/mm/truncate.c
> > @@ -108,6 +108,10 @@ truncate_complete_page(struct address_space *mapping, struct page *page)
> > clear_page_mlock(page);
> > remove_from_page_cache(page);
> > ClearPageMappedToDisk(page);
> > +
> > + if (mapping->a_ops->freepage)
> > + mapping->a_ops->freepage(page);
> > +
> > page_cache_release(page); /* pagecache ref */
> > return 0;
> > }
>
> I think Linus recommended that one be done in remove_from_page_cache()
> to catch all instances: did that get overruled later for some reason?
I'm fine with doing that as long as everyone is happy that there is no
chance of races. I was a bit nervous given the discussion that ensued
from the vmscan case.
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -454,6 +454,7 @@ static int __remove_mapping(struct address_space *mapping, struct page *page)
> > BUG_ON(!PageLocked(page));
> > BUG_ON(mapping != page_mapping(page));
> >
> > + preempt_disable();
> > spin_lock_irq(&mapping->tree_lock);
> > /*
> > * The non racy check for a busy page.
> > @@ -492,10 +493,19 @@ static int __remove_mapping(struct address_space *mapping, struct page *page)
> > swp_entry_t swap = { .val = page_private(page) };
> > __delete_from_swap_cache(page);
> > spin_unlock_irq(&mapping->tree_lock);
> > + preempt_enable();
> > swapcache_free(swap, page);
> > } else {
> > + void (*freepage)(struct page *);
> > +
> > + freepage = mapping->a_ops->freepage;
> > +
> > __remove_from_page_cache(page);
> > spin_unlock_irq(&mapping->tree_lock);
> > + if (freepage != NULL)
> > + freepage(page);
> > + preempt_enable();
> > +
> > mem_cgroup_uncharge_cache_page(page);
> > }
> >
> > @@ -503,6 +513,7 @@ static int __remove_mapping(struct address_space *mapping, struct page *page)
> >
> > cannot_free:
> > spin_unlock_irq(&mapping->tree_lock);
> > + preempt_enable();
> > return 0;
> > }
>
> I took his "stop_machine()" explanation ("an idle period for everything.
> And just a preemption reschedule isn't enough for that") to imply that
> there's no need for your preempt_disable/preempt_enable there: they
> don't add anything to the module unload case, and they don't help the
> spin_unlock_irq issue (and you're already being rightly careful to note
> freepage in advance).
>
> But maybe I misunderstood.
Again, I was being cautious (I freely admit to not having studied
stop_machine()). If nobody disagrees with your interpretation, then I'm
very happy to drop the preempt disable/enable crud.
Cheers
Trond
--
Trond Myklebust
Linux NFS client maintainer
NetApp
Trond.Myklebust@xxxxxxxxxx
www.netapp.com
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