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Commit 2e441889 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
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swapoff: scan ptes preemptibly 



Provided that CONFIG_HIGHPTE is not set, unuse_pte_range can reduce latency
in swapoff by scanning the page table preemptibly: so long as unuse_pte is
careful to recheck that entry under pte lock.

(To tell the truth, this patch was not inspired by any cries for lower
latency here: rather, this restructuring permits a future memory controller
patch to allocate with GFP_KERNEL in unuse_pte, where before it could not.
But it would be wrong to tuck this change away inside a memcgroup patch.)

Signed-off-by: default avatarHugh Dickins <hugh@veritas.com>
Acked-by: default avatarBalbir Singh <balbir@linux.vnet.ibm.com>
Tested-by: default avatarBalbir Singh <balbir@linux.vnet.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 8952898b

mm/swapfile.c

+31 −7
Original line number Diff line number Diff line
@@ -506,9 +506,19 @@ unsigned int count_swap_pages(int type, int free) 
 * just let do_wp_page work it out if a write is requested later - to

 * force COW, vm_page_prot omits write permission from any private vma.

 */

static void unuse_pte(struct vm_area_struct *vma, pte_t *pte,

static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,

		unsigned long addr, swp_entry_t entry, struct page *page)

{

	spinlock_t *ptl;

	pte_t *pte;

	int found = 1;



	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);

	if (unlikely(!pte_same(*pte, swp_entry_to_pte(entry)))) {

		found = 0;

		goto out;

	}



	inc_mm_counter(vma->vm_mm, anon_rss);

	get_page(page);

	set_pte_at(vma->vm_mm, addr, pte,
@@ -520,6 +530,9 @@ static void unuse_pte(struct vm_area_struct *vma, pte_t *pte, 
	 * immediately swapped out again after swapon.

	 */

	activate_page(page);

out:

	pte_unmap_unlock(pte, ptl);

	return found;

}



static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
@@ -528,22 +541,33 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd, 
{

	pte_t swp_pte = swp_entry_to_pte(entry);

	pte_t *pte;

	spinlock_t *ptl;

	int found = 0;



	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);

	/*

	 * We don't actually need pte lock while scanning for swp_pte: since

	 * we hold page lock and mmap_sem, swp_pte cannot be inserted into the

	 * page table while we're scanning; though it could get zapped, and on

	 * some architectures (e.g. x86_32 with PAE) we might catch a glimpse

	 * of unmatched parts which look like swp_pte, so unuse_pte must

	 * recheck under pte lock.  Scanning without pte lock lets it be

	 * preemptible whenever CONFIG_PREEMPT but not CONFIG_HIGHPTE.

	 */

	pte = pte_offset_map(pmd, addr);

	do {

		/*

		 * swapoff spends a _lot_ of time in this loop!

		 * Test inline before going to call unuse_pte.

		 */

		if (unlikely(pte_same(*pte, swp_pte))) {

			unuse_pte(vma, pte++, addr, entry, page);

			found = 1;

			break;

			pte_unmap(pte);

			found = unuse_pte(vma, pmd, addr, entry, page);

			if (found)

				goto out;

			pte = pte_offset_map(pmd, addr);

		}

	} while (pte++, addr += PAGE_SIZE, addr != end);

	pte_unmap_unlock(pte - 1, ptl);

	pte_unmap(pte - 1);

out:

	return found;

}