Re: [PATCH v3] fs/buffer.c: Revoke LRU when trying to drop buffers

[Matthew Wilcox <willy@infradead.org>]

On Mon, Mar 15, 2021 at 04:41:38PM -0700, Andrew Morton wrote:
> > When a buffer is added to the LRU list, a reference is taken which is
> > not dropped until the buffer is evicted from the LRU list. This is the
> > correct behavior, however this LRU reference will prevent the buffer
> > from being dropped. This means that the buffer can't actually be dropped
> > until it is selected for eviction. There's no bound on the time spent
> > on the LRU list, which means that the buffer may be undroppable for
> > very long periods of time. Given that migration involves dropping
> > buffers, the associated page is now unmigratible for long periods of
> > time as well. CMA relies on being able to migrate a specific range
> > of pages, so these types of failures make CMA significantly
> > less reliable, especially under high filesystem usage.
>
> It looks like patch this turns drop_buffers() into a very expensive
> operation.  And that expensive operation occurs under the
> address_space-wide private_lock, which is more ouch.

This patch set is obsoleted by Minchan Kim's more recent patch-set.