linux-kernel - RE: [PATCH 1/14] mm: invalidate_mapping

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Date:	Tue, 31 May 2011 10:05:27 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	chris.mason@...cle.com
Subject: RE: [PATCH 1/14] mm: invalidate_mapping_pages flush cleancache

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.

> And since __delete_from_page_cache
> calls cleancache_flush_page, only that subset of pages
> in the mapping that invalidate_mapping_pages() would
> invalidate (which, from the comment above the routine
> indicates, is only *unlocked* pages) is removed from
> cleancache.

It's nice to target the particular range asked for, rather than
throwing away all the cleancache for the whole mapping, I can
see that (though that's a defect in the cleancache_flush_inode()
interface).  But then why do truncate_inode_pages_range() and
invalidate_inode_pages2_range() throw it all away, despite
going down to __delete_from_page_cache on individual pages found?

Maybe the right patch is to remove cleancache_flush_inode() from
the two instead of adding it to the one?  But I think not.

Hugh
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