Re: [patch 6/9] mm + fs: store shadow entries in page cache

Johannes Weiner
2013-11-27T17:09:14Z

On Tue, Nov 26, 2013 at 10:17:16AM +1100, Dave Chinner wrote:
> On Sun, Nov 24, 2013 at 06:38:25PM -0500, Johannes Weiner wrote:
> > Reclaim will be leaving shadow entries in the page cache radix tree
> > upon evicting the real page. As those pages are found from the LRU,
> > an iput() can lead to the inode being freed concurrently. At this
> > point, reclaim must no longer install shadow pages because the inode
> > freeing code needs to ensure the page tree is really empty.
> >
> > Add an address_space flag, AS_EXITING, that the inode freeing code
> > sets under the tree lock before doing the final truncate. Reclaim
> > will check for this flag before installing shadow pages.
> >
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ....
> > @@ -545,10 +546,25 @@ static void evict(struct inode *inode)
> > */
> > inode_wait_for_writeback(inode);
> >
> > + /*
> > + * Page reclaim can not do iput() and thus can race with the
> > + * inode teardown. Tell it when the address space is exiting,
> > + * so that it does not install eviction information after the
> > + * final truncate has begun.
> > + *
> > + * As truncation uses a lockless tree lookup, acquire the
> > + * spinlock to make sure any ongoing tree modification that
> > + * does not see AS_EXITING is completed before starting the
> > + * final truncate.
> > + */
> > + spin_lock_irq(&inode->i_data.tree_lock);
> > + mapping_set_exiting(&inode->i_data);
> > + spin_unlock_irq(&inode->i_data.tree_lock);
> > +
> > if (op->evict_inode) {
> > op->evict_inode(inode);
> > } else {
> > - if (inode->i_data.nrpages)
> > + if (inode->i_data.nrpages || inode->i_data.nrshadows)
> > truncate_inode_pages(&inode->i_data, 0);
> > clear_inode(inode);
> > }
>
> Ok, so what I see here is that we need a wrapper function that
> handles setting the AS_EXITING flag and doing the "final"
> truncate_inode_pages() call, and the locking for the AS_EXITING flag
> moved into mapping_set_exiting()
>
> That is, because this AS_EXITING flag and it's locking constraints
> are directly related to the upcoming truncate_inode_pages() call,
> I'd prefer to see a helper that captures that relationship used
> in all the filesystem code. e.g:
>
> void truncate_inode_pages_final(struct address_space *mapping)
> {
> spin_lock_irq(&mapping->tree_lock);
> mapping_set_exiting(mapping);
> spin_unlock_irq(&mapping->tree_lock);
> if (inode->i_data.nrpages || inode->i_data.nrshadows)
> truncate_inode_pages_range(mapping, 0, (loff_t)-1);
> }
>
> And document it in Documentation/filesystems/porting as a mandatory
> function to be called from ->evict_inode() implementations before
> calling clear_inode(). You can then replace all the direct calls to
> truncate_inode_pages() in the evict_inode() path with a call to
> truncate_inode_pages_final().

Ok, fair enough. I'll add a BUG_ON(!mapping_exiting(&inode->i_data))
to the inode sanity checks on final teardown to make sure filesystems
don't miss the change to truncate_inode_pages_final().

> As it is, I'd really like to see that unconditional irq disable go
> away from this code - disabling and enabling interrupts for every
> single inode we reclaim is going to add significant overhead to this
> hot code path. And given that:
>
> > +static inline void mapping_set_exiting(struct address_space *mapping)
> > +{
> > + set_bit(AS_EXITING, &mapping->flags);
> > +}
> > +
> > +static inline int mapping_exiting(struct address_space *mapping)
> > +{
> > + return test_bit(AS_EXITING, &mapping->flags);
> > +}
>
> these atomic bit ops, why do we need to take the tree_lock and
> disable irqs in evict() to set this bit if there's nothing to
> truncate on the inode? i.e. something like this:
>
> void truncate_inode_pages_final(struct address_space *mapping)
> {
> mapping_set_exiting(mapping);
> if (inode->i_data.nrpages || inode->i_data.nrshadows) {
> /*
> * spinlock barrier to ensure all modifications are
> * complete before we do the final truncate
> */
> spin_lock_irq(&mapping->tree_lock);
> spin_unlock_irq(&mapping->tree_lock);
> truncate_inode_pages_range(mapping, 0, (loff_t)-1);
> }

That would almost work, but we need to enforce ordering of the counter
reads and updates or truncation might read 0 on both while racing with
reclaim.

Reclaim would have to do:

spin_lock_irq(&mapping->tree_lock)
if !mapping_exiting():
swap shadow entry
mapping->nrshadows++
smp_wmb()
mapping->nrpages--
spin_unlock_irq(&mapping->tree_lock)

and the final truncate side would have to do

mapping_set_exiting()
nrpages = mapping->nrpages
smp_rmb()
nrshadows = mapping->nrshadows
if (nrpages || nrshadows)
spin_lock_irq(&mapping->tree_lock)
spin_unlock_irq(&mapping->tree_lock)
truncate