linux-kernel - Re: [patch][v2] swap: virtual swap readahead

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Date:	Wed, 3 Jun 2009 01:34:57 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andi Kleen <andi@...stfloor.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch][v2] swap: virtual swap readahead

On Wed, Jun 03, 2009 at 12:37:39AM +0200, Johannes Weiner wrote:
> + *
> + * Caller must hold down_read on the vma->vm_mm if vma is not NULL.
> + */
> +struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> +			struct vm_area_struct *vma, unsigned long addr)
> +{
> +	unsigned long start, pos, end;
> +	unsigned long pmin, pmax;
> +	int cluster, window;
> +
> +	if (!vma || !vma->vm_mm)	/* XXX: shmem case */
> +		return swapin_readahead_phys(entry, gfp_mask, vma, addr);
> +
> +	cluster = 1 << page_cluster;
> +	window = cluster << PAGE_SHIFT;
> +
> +	/* Physical range to read from */
> +	pmin = swp_offset(entry) & ~(cluster - 1);

Is cluster really properly sign extended on 64bit? Looks a little
dubious. long from the start would be safer

> +
> +	/* Virtual range to read from */
> +	start = addr & ~(window - 1);

Same.

> +		pgd = pgd_offset(vma->vm_mm, pos);
> +		if (!pgd_present(*pgd))
> +			continue;
> +		pud = pud_offset(pgd, pos);
> +		if (!pud_present(*pud))
> +			continue;
> +		pmd = pmd_offset(pud, pos);
> +		if (!pmd_present(*pmd))
> +			continue;
> +		pte = pte_offset_map_lock(vma->vm_mm, pmd, pos, &ptl);

You could be more efficient here by using the standard mm/* nested loop
pattern that avoids relookup of everything in each iteration. I suppose
it would mainly make a difference with 32bit highpte where mapping a pte
can be somewhat costly. And you would take less locks this way.

> +		page = read_swap_cache_async(swp, gfp_mask, vma, pos);
> +		if (!page)
> +			continue;

That's out of memory, break would be better here because prefetch
while oom is usually harmful.

> +		page_cache_release(page);
> +	}
> +	lru_add_drain();	/* Push any new pages onto the LRU now */
> +	return read_swap_cache_async(entry, gfp_mask, vma, addr);

Shouldn't that page be already handled in the loop earlier? Why doing that
again? It would be better to remember it from there.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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