Re: [patch][v2] swap: virtual swap readahead

Andi Kleen
2009-06-02T23:27:57Z

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@linux.intel.com -- Speaking for myself only.