RE: [PATCH 1/14] mm: invalidate_mapping_pages flush cleancache

[Chris Mason]

Excerpts from Hugh Dickins's message of 2011-05-31 13:05:27 -0400:
> On Tue, 31 May 2011, Dan Magenheimer wrote:
> > > 
> > > truncate_inode_pages_range() and invalidate_inode_pages2_range()
> > > call cleancache_flush_inode(mapping) before and after: shouldn't
> > > invalidate_mapping_pages() be doing the same?
> > 
> > I don't claim to be an expert on VFS, and so I have cc'ed
> > Chris Mason who originally placed the cleancache hooks
> > in VFS, but I think this patch is unnecessary.  Instead
> > of flushing ALL of the cleancache pages belonging to
> > the inode with cleancache_flush_inode, the existing code
> > eventually calls __delete_from_page_cache on EACH page
> > that is being invalidated.
> 
> On each one that's in pagecache (and satisfies the other "can we
> do it easily?" conditions peculiar to invalidate_mapping_pages()).
> But there may be other slots in the range that don't reach
> __delete_from_page_cache() e.g. because not currently in pagecache,
> but whose cleancache ought to be flushed.  I think that's what a
> caller of invalidate_mapping_pages(), e.g. drop caches, expects.

We call invalidate_mapping_pages from prune_icache, so if we drop the
cleancache there we lose the cache entries any time the inode is dropped
from ram.

Is there a specific case you're thinking of where we want to drop the
cleancache but don't have the pages?

O_DIRECT perhaps?

-chris
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