linux-kernel - Re: [PATCH 8/9] tmpfs: radix_tree

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Message-ID: <20071219065049.GA31872@wotan.suse.de>
Date:	Wed, 19 Dec 2007 07:50:49 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Rohland <hans-christoph.rohland@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/9] tmpfs: radix_tree_preloading

On Tue, Dec 18, 2007 at 10:05:19PM +0000, Hugh Dickins wrote:
> Nick has observed that shmem.c still uses GFP_ATOMIC when adding to page
> cache or swap cache, without any radix tree preload: so tending to deplete
> emergency reserves of memory.
> 
> GFP_ATOMIC remains appropriate in shmem_writepage's add_to_swap_cache:
> it's being called under memory pressure, so must not wait for more memory
> to become available.  But shmem_unuse_inode now has a window in which it
> can and should preload with GFP_KERNEL, and say GFP_NOWAIT instead of
> GFP_ATOMIC in its add_to_page_cache.
> 
> shmem_getpage is not so straightforward: its filepage/swappage integrity
> relies upon exchanging between caches under spinlock, and it would need a
> lot of restructuring to place the preloads correctly.  Instead, follow
> its pattern of retrying on races: use GFP_NOWAIT instead of GFP_ATOMIC in
> add_to_page_cache, and begin each circuit of the repeat loop with a sleeping
> radix_tree_preload, followed immediately by radix_tree_preload_end - that
> won't guarantee success in the next add_to_page_cache, but doesn't need to.
> 
> And we can then remove that bothersome congestion_wait: when needed,
> it'll automatically get done in the course of the radix_tree_preload.
> 
> Signed-off-by: Hugh Dickins <hugh@...itas.com>

Looks good to me. Thanks for this!
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