[PATCH] slab: cleanup kmem_getpages

[PATCH] slab: cleanup kmem_getpages

[Christoph Hellwig]

The last ifdef addition hit the ugliness treshold on this functions, so:

 - rename the varibale i to nr_pages so it's somewhat descriptive
 - remove the addr variable and do the page_address call at the very end
 - instead of ifdef'ing the whole alloc_pages_node call just make the
   __GFP_COMP addition to flags conditional
 - rewrite the __GFP_COMP comment to make sense


Signed-off-by: Christoph Hellwig <hch@lst.de>

Index: linux-2.6/mm/slab.c
===================================================================
--- linux-2.6.orig/mm/slab.c	2006-04-13 16:22:12.000000000 +0200
+++ linux-2.6/mm/slab.c	2006-04-13 16:53:15.000000000 +0200
@@ -1452,31 +1452,30 @@
 static void *kmem_getpages(struct kmem_cache *cachep, gfp_t flags, int nodeid)
 {
 	struct page *page;
-	void *addr;
-	int i;
+	int nr_pages;
 
-	flags |= cachep->gfpflags;
 #ifndef CONFIG_MMU
-	/* nommu uses slab's for process anonymous memory allocations, so
-	 * requires __GFP_COMP to properly refcount higher order allocations"
+	/*
+	 * Nommu uses slab's for process anonymous memory allocations, and thus
+	 * requires __GFP_COMP to properly refcount higher order allocations
 	 */
-	page = alloc_pages_node(nodeid, (flags | __GFP_COMP), cachep->gfporder);
-#else
-	page = alloc_pages_node(nodeid, flags, cachep->gfporder);
+	flags |= __GFP_COMP;
 #endif
+	flags |= cachep->gfpflags;
+
+	page = alloc_pages_node(nodeid, flags, cachep->gfporder);
 	if (!page)
 		return NULL;
-	addr = page_address(page);
 
-	i = (1 << cachep->gfporder);
+	nr_pages = (1 << cachep->gfporder);
 	if (cachep->flags & SLAB_RECLAIM_ACCOUNT)
-		atomic_add(i, &slab_reclaim_pages);
-	add_page_state(nr_slab, i);
-	while (i--) {
+		atomic_add(nr_pages, &slab_reclaim_pages);
+	add_page_state(nr_slab, nr_pages);
+	while (nr_pages--) {
 		__SetPageSlab(page);
 		page++;
 	}
-	return addr;
+	return page_address(page);
 }
 
 /*

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Re: [PATCH] slab: cleanup kmem_getpages

[David Chinner]

On Fri, Apr 14, 2006 at 08:36:18PM +0200, Christoph Hellwig wrote:
> The last ifdef addition hit the ugliness treshold on this functions, so:
> 
>  - rename the varibale i to nr_pages so it's somewhat descriptive
>  - remove the addr variable and do the page_address call at the very end
>  - instead of ifdef'ing the whole alloc_pages_node call just make the
>    __GFP_COMP addition to flags conditional
>  - rewrite the __GFP_COMP comment to make sense
....
> +	page = alloc_pages_node(nodeid, flags, cachep->gfporder);
>  	if (!page)
>  		return NULL;
> -	addr = page_address(page);
.....
> +	while (nr_pages--) {
>  		__SetPageSlab(page);
>  		page++;
>  	}
> -	return addr;
> +	return page_address(page);

I think that's a bug - you return the address of the page after the
allocation, not the first page of the allocation.

Cheers,

Dave.
-- 
Dave Chinner
R&D Software Enginner
SGI Australian Software Group

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Re: [PATCH] slab: cleanup kmem_getpages

[Christoph Hellwig]

On Wed, Apr 19, 2006 at 09:20:00AM +1000, David Chinner wrote:
> On Fri, Apr 14, 2006 at 08:36:18PM +0200, Christoph Hellwig wrote:
> > The last ifdef addition hit the ugliness treshold on this functions, so:
> > 
> >  - rename the varibale i to nr_pages so it's somewhat descriptive
> >  - remove the addr variable and do the page_address call at the very end
> >  - instead of ifdef'ing the whole alloc_pages_node call just make the
> >    __GFP_COMP addition to flags conditional
> >  - rewrite the __GFP_COMP comment to make sense
> ....
> > +	page = alloc_pages_node(nodeid, flags, cachep->gfporder);
> >  	if (!page)
> >  		return NULL;
> > -	addr = page_address(page);
> .....
> > +	while (nr_pages--) {
> >  		__SetPageSlab(page);
> >  		page++;
> >  	}
> > -	return addr;
> > +	return page_address(page);
> 
> I think that's a bug - you return the address of the page after the
> allocation, not the first page of the allocation.

You're right.  I wonder why this didn't show up in my testing.  Looks
like slab will never allocate any high-order pages if your page size
is big enough..

Andrew, please drop this for now.  I'll redo it without that bit once
I'll get some time.

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Re: [PATCH] slab: cleanup kmem_getpages

[Andrew Morton]

Christoph Hellwig <hch@lst.de> wrote:
>
> > > +	page = alloc_pages_node(nodeid, flags, cachep->gfporder);
> > >  	if (!page)
> > >  		return NULL;
> > > -	addr = page_address(page);
> > .....
> > > +	while (nr_pages--) {
> > >  		__SetPageSlab(page);
> > >  		page++;
> > >  	}
> > > -	return addr;
> > > +	return page_address(page);
> > 
> > I think that's a bug - you return the address of the page after the
> > allocation, not the first page of the allocation.
> 
> You're right.  I wonder why this didn't show up in my testing.  Looks
> like slab will never allocate any high-order pages if your page size
> is big enough..
> 
> Andrew, please drop this for now.  I'll redo it without that bit once
> I'll get some time.

I already fixed it - it was giving me instantaneous oopses.

--- devel/mm/slab.c~slab-cleanup-kmem_getpages-fix	2006-04-15 01:00:53.000000000 -0700
+++ devel-akpm/mm/slab.c	2006-04-15 01:01:49.000000000 -0700
@@ -1492,6 +1492,7 @@ static void *kmem_getpages(struct kmem_c
 {
 	struct page *page;
 	int nr_pages;
+	int i;
 
 #ifndef CONFIG_MMU
 	/*
@@ -1510,10 +1511,8 @@ static void *kmem_getpages(struct kmem_c
 	if (cachep->flags & SLAB_RECLAIM_ACCOUNT)
 		atomic_add(nr_pages, &slab_reclaim_pages);
 	add_page_state(nr_slab, nr_pages);
-	while (nr_pages--) {
-		__SetPageSlab(page);
-		page++;
-	}
+	for (i = 0; i < nr_pages; i++)
+		__SetPageSlab(page + i);
 	return page_address(page);
 }
 
_

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