linux-kernel - Re: [PATCH] Fix corruption of memmap on IA64 SPARSEMEM when mem

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Date:	Wed, 25 Jul 2007 13:55:02 +0100
From:	mel@...net.ie (Mel Gorman)
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	bob.picco@...com, apw@...dowen.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix corruption of memmap on IA64 SPARSEMEM when mem_section is not a power of 2

On (24/07/07 22:01), Andrew Morton didst pronounce:
> On Tue, 13 Mar 2007 10:42:02 +0000 mel@...net.ie (Mel Gorman) wrote:
> 
> > There are problems in the use of SPARSEMEM and pageblock flags that causes
> > problems on ia64.
> > 
> > The first part of the problem is that units are incorrect in
> > SECTION_BLOCKFLAGS_BITS computation. This results in a map_section's
> > section_mem_map being treated as part of a bitmap which isn't good. This
> > was evident with an invalid virtual address when mem_init attempted to free
> > bootmem pages while relinquishing control from the bootmem allocator.
> > 
> > The second part of the problem occurs because the pageblock flags bitmap is
> > be located with the mem_section. The SECTIONS_PER_ROOT computation using
> > sizeof (mem_section) may not be a power of 2 depending on the size of the
> > bitmap. This renders masks and other such things not power of 2 base. This
> > issue was seen with SPARSEMEM_EXTREME on ia64. This patch moves the bitmap
> > outside of mem_section and uses a pointer instead in the mem_section. The
> > bitmaps are allocated when the section is being initialised.
> > 
> > Note that sparse_early_usemap_alloc() does not use alloc_remap() like
> > sparse_early_mem_map_alloc(). The allocation required for the bitmap on x86,
> > the only architecture that uses alloc_remap is typically smaller than a cache
> > line. alloc_remap() pads out allocations to the cache size which would be
> > a needless waste.
> > 
> > Credit to Bob Picco for identifying the original problem and effecting a
> > fix for the SECTION_BLOCKFLAGS_BITS calculation. Credit to Andy Whitcroft
> > for devising the best way of allocating the bitmaps only when required for
> > the section.
> 
> SuperH allmodconfig blew up:
> 
> mm/sparse.c: In function `sparse_init':
> mm/sparse.c:482: error: implicit declaration of function `sparse_early_usemap_alloc'
> mm/sparse.c:482: warning: assignment makes pointer from integer without a cast
> mm/sparse.c: In function `sparse_add_one_section':
> mm/sparse.c:553: error: implicit declaration of function `__kmalloc_section_usemap'
> mm/sparse.c:553: warning: assignment makes pointer from integer without a cast

I'm looking at this now and getting a superh cross-compiler built to
build-test any fix. My first impression is that
sparse_early_usemap_alloc() needs to be defined whether
CONFIG_SPARSEMEM_VMEMMAP is set or not. Right now,
sparse_early_usemap_alloc() is only defined when it is set and it's not
clear why although "by accident" is the most likely explanation.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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