linux-kernel - Re: [patch 3/3] arch_rebalance

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Message-Id: <1195040986.9446.23.camel@localhost>
Date:	Wed, 14 Nov 2007 12:49:46 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	benh@...nel.crashing.org
Cc:	Nick Piggin <nickpiggin@...oo.com.au>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [patch 3/3] arch_rebalance_pgtables call

On Wed, 2007-11-14 at 21:06 +1100, Benjamin Herrenschmidt wrote:
> On Wed, 2007-11-14 at 10:26 +0100, Martin Schwidefsky wrote:
> > That patch allows processes to have different number of page table
> > levels, 31 bit processes have 2 levels (2GB), normal 64 bit processes
> > have 3 levels (4TB) and really big 64 bit processes can have 4 levels
> > (8PB). The downgrade of a page table to use less levels than the
> > parent
> > process is done in arch_pick_mmap_layout. The upgrade is done by using
> > the arch_rebalance_pgtables call. I've considered using the
> > arch_get_unmapped_area but got scared by the indirection in
> > get_unmapped_area:
> > 
> >         get_area = current->mm->get_unmapped_area;
> >         if (file && file->f_op && file->f_op->get_unmapped_area)
> >                 get_area = file->f_op->get_unmapped_area;
> >         addr = get_area(file, addr, len, pgoff, flags);
> 
> Don't be, it's really only hugetlb and other arch specific stuff that
> hook in here on platforms with an MMU (It's also used by /dev/mem etc...
> for mmu-less platforms but you don't care).

I find 8 places where a get_unmapped_area function pointer is used:
ipc/shm.c: shm_get_unmapped_area / shm_file_operations
drivers/char/mem.c: get_unmapped_area_mem / mem_fops & kmem_fops
drivers/video/fbmem.c: get_fb_unmapped_area / fb_fops
drivers/pci/proc.c: get_pci_unmapped_area / proc_bus_pci_operations
fs/hugetlbfs/inode.c: hugetlb_get_unmapped_area / hugetlbfs_file_operations
fs/bad_inode.c: bad_file_get_unmapped_area / bad_file_ops
fs/ramfs/file-nommu.c: ramfs_nommu_get_unmapped_area / ramfs_file_operations
arch/powerpc/platforms/cell/spufs/file.c:
				spufs_get_unmapped_area / spufs_mem_fops

They all either have an arch override, call get_unmapped_area again or
are not relevant. So it should be possible to do the upgrade in
arch_get_unmapped_area. I still have my doubts though, all future uses
of the get_unmapped_area pointer have to be checked and I feel it is
easier to understand to do the upgrade / rebalance of the page table at
the end of get_unmapped_area where every caller of mmap is guaranteed to
pass through.

-- 
blue skies,
  Martin.

"Reality continues to ruin my life." - Calvin.


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