linux-kernel - Re: [patch] mm/vmalloc: remove guard page from between vmap blocks

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Date:	Tue, 19 Apr 2011 09:30:59 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Dave Chinner <david@...morbit.com>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] mm/vmalloc: remove guard page from between vmap blocks

On Thu, Apr 14, 2011 at 05:14:41PM -0400, Johannes Weiner wrote:
> The vmap allocator is used to, among other things, allocate per-cpu
> vmap blocks, where each vmap block is naturally aligned to its own
> size.  Obviously, leaving a guard page after each vmap area forbids
> packing vmap blocks efficiently and can make the kernel run out of
> possible vmap blocks long before overall vmap space is exhausted.
> 
> The new interface to map a user-supplied page array into linear
> vmalloc space (vm_map_ram) insists on allocating from a vmap block
> (instead of falling back to a custom area) when the area size is below
> a certain threshold.  With heavy users of this interface (e.g. XFS)
> and limited vmalloc space on 32-bit, vmap block exhaustion is a real
> problem.
> 
> Remove the guard page from the core vmap allocator.  vmalloc and the
> old vmap interface enforce a guard page on their own at a higher
> level.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Cc: Nick Piggin <npiggin@...nel.dk>
> Cc: Dave Chinner <david@...morbit.com>
> Cc: Mel Gorman <mel@....ul.ie>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Christoph Hellwig <hch@...radead.org>

If necessary, the guard page could be reintroduced as a debugging-only
option (CONFIG_DEBUG_PAGEALLOC?). Otherwise it seems reasonable.

Acked-by: Mel Gorman <mel@....ul.ie>

-- 
Mel Gorman
SUSE Labs
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