linux-kernel - Re: [patch 5/9] mm + fs: prepare for non-page entries in page cache radix trees

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Message-ID: <20140122174744.GC4407@cmpxchg.org>
Date:	Wed, 22 Jan 2014 12:47:44 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Bob Liu <bob.liu@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Luigi Semenzato <semenzato@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Metin Doslu <metin@...usdata.com>,
	Michel Lespinasse <walken@...gle.com>,
	Ozgun Erdogan <ozgun@...usdata.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Roman Gushchin <klamm@...dex-team.ru>,
	Ryan Mallon <rmallon@...il.com>, Tejun Heo <tj@...nel.org>,
	Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 5/9] mm + fs: prepare for non-page entries in page cache
 radix trees

On Mon, Jan 13, 2014 at 11:01:32AM +0900, Minchan Kim wrote:
> On Fri, Jan 10, 2014 at 01:10:39PM -0500, Johannes Weiner wrote:
> > shmem mappings already contain exceptional entries where swap slot
> > information is remembered.
> > 
> > To be able to store eviction information for regular page cache,
> > prepare every site dealing with the radix trees directly to handle
> > entries other than pages.
> > 
> > The common lookup functions will filter out non-page entries and
> > return NULL for page cache holes, just as before.  But provide a raw
> > version of the API which returns non-page entries as well, and switch
> > shmem over to use it.
> > 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Reviewed-by: Minchan Kim <minchan@...nel.org>

Thanks, Minchan!

> > @@ -890,6 +973,73 @@ repeat:
> >  EXPORT_SYMBOL(find_or_create_page);
> >  
> >  /**
> > + * __find_get_pages - gang pagecache lookup
> > + * @mapping:	The address_space to search
> > + * @start:	The starting page index
> > + * @nr_pages:	The maximum number of pages
> > + * @pages:	Where the resulting pages are placed
> 
> where is @indices?

Fixed :)

> > @@ -894,6 +894,53 @@ EXPORT_SYMBOL(__pagevec_lru_add);
> >  
> >  /**
> >   * pagevec_lookup - gang pagecache lookup
> 
>       __pagevec_lookup?
> 
> > + * @pvec:	Where the resulting entries are placed
> > + * @mapping:	The address_space to search
> > + * @start:	The starting entry index
> > + * @nr_pages:	The maximum number of entries
> 
>       missing @indices?
> 
> > + *
> > + * pagevec_lookup() will search for and return a group of up to
> > + * @nr_pages pages and shadow entries in the mapping.  All entries are
> > + * placed in @pvec.  pagevec_lookup() takes a reference against actual
> > + * pages in @pvec.
> > + *
> > + * The search returns a group of mapping-contiguous entries with
> > + * ascending indexes.  There may be holes in the indices due to
> > + * not-present entries.
> > + *
> > + * pagevec_lookup() returns the number of entries which were found.
> 
>       __pagevec_lookup

Yikes, all three fixed.

> > @@ -22,6 +22,22 @@
> >  #include <linux/cleancache.h>
> >  #include "internal.h"
> >  
> > +static void clear_exceptional_entry(struct address_space *mapping,
> > +				    pgoff_t index, void *entry)
> > +{
> > +	/* Handled by shmem itself */
> > +	if (shmem_mapping(mapping))
> > +		return;
> > +
> > +	spin_lock_irq(&mapping->tree_lock);
> > +	/*
> > +	 * Regular page slots are stabilized by the page lock even
> > +	 * without the tree itself locked.  These unlocked entries
> > +	 * need verification under the tree lock.
> > +	 */
> 
> Could you explain why repeated spin_lock with irq disabled isn't problem
> in truncation path?

To modify the cache tree, we have to take the IRQ-safe tree_lock, this
is no different than removing a page (see truncate_complete_page).
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