Commit 340ef390 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
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mm: numa: cleanup flow of transhuge page migration 



When correcting commit 04fa5d6a ("mm: migrate: check page_count of
THP before migrating") Hugh Dickins noted that the control flow for
transhuge migration was difficult to follow.  Unconditionally calling
put_page() in numamigrate_isolate_page() made the failure paths of both
migrate_misplaced_transhuge_page() and migrate_misplaced_page() more
complex that they should be.  Further, he was extremely wary that an
unlock_page() should ever happen after a put_page() even if the
put_page() should never be the final put_page.

Hugh implemented the following cleanup to simplify the path by calling
putback_lru_page() inside numamigrate_isolate_page() if it failed to
isolate and always calling unlock_page() within
migrate_misplaced_transhuge_page().

There is no functional change after this patch is applied but the code
is easier to follow and unlock_page() always happens before put_page().

[mgorman@suse.de: changelog only]
Signed-off-by: default avatarMel Gorman <mgorman@suse.de>
Signed-off-by: default avatarHugh Dickins <hughd@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Simon Jeons <simon.jeons@gmail.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 75980e97

mm/huge_memory.c

+9 −19
Original line number Diff line number Diff line
@@ -1296,7 +1296,6 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma, 
	int target_nid;

	int current_nid = -1;

	bool migrated;

	bool page_locked = false;



	spin_lock(&mm->page_table_lock);

	if (unlikely(!pmd_same(pmd, *pmdp)))
@@ -1318,7 +1317,6 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma, 
	/* Acquire the page lock to serialise THP migrations */

	spin_unlock(&mm->page_table_lock);

	lock_page(page);

	page_locked = true;



	/* Confirm the PTE did not while locked */

	spin_lock(&mm->page_table_lock);
@@ -1331,34 +1329,26 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma, 


	/* Migrate the THP to the requested node */

	migrated = migrate_misplaced_transhuge_page(mm, vma,

				pmdp, pmd, addr,

				page, target_nid);

	if (migrated)

		current_nid = target_nid;

	else {

		spin_lock(&mm->page_table_lock);

		if (unlikely(!pmd_same(pmd, *pmdp))) {

			unlock_page(page);

			goto out_unlock;

		}

		goto clear_pmdnuma;

	}

				pmdp, pmd, addr, page, target_nid);

	if (!migrated)

		goto check_same;



	task_numa_fault(current_nid, HPAGE_PMD_NR, migrated);

	task_numa_fault(target_nid, HPAGE_PMD_NR, true);

	return 0;



check_same:

	spin_lock(&mm->page_table_lock);

	if (unlikely(!pmd_same(pmd, *pmdp)))

		goto out_unlock;

clear_pmdnuma:

	pmd = pmd_mknonnuma(pmd);

	set_pmd_at(mm, haddr, pmdp, pmd);

	VM_BUG_ON(pmd_numa(*pmdp));

	update_mmu_cache_pmd(vma, addr, pmdp);

	if (page_locked)

		unlock_page(page);



out_unlock:

	spin_unlock(&mm->page_table_lock);

	if (current_nid != -1)

		task_numa_fault(current_nid, HPAGE_PMD_NR, migrated);

		task_numa_fault(current_nid, HPAGE_PMD_NR, false);

	return 0;

}

mm/migrate.c

+43 −52
Original line number Diff line number Diff line
@@ -1557,41 +1557,40 @@ bool numamigrate_update_ratelimit(pg_data_t *pgdat, unsigned long nr_pages) 


int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)

{

	int ret = 0;

	int page_lru;



	VM_BUG_ON(compound_order(page) && !PageTransHuge(page));



	/* Avoid migrating to a node that is nearly full */

	if (migrate_balanced_pgdat(pgdat, 1UL << compound_order(page))) {

		int page_lru;

	if (!migrate_balanced_pgdat(pgdat, 1UL << compound_order(page)))

		return 0;



		if (isolate_lru_page(page)) {

			put_page(page);

	if (isolate_lru_page(page))

		return 0;



	/*

	 * migrate_misplaced_transhuge_page() skips page migration's usual

	 * check on page_count(), so we must do it here, now that the page

	 * has been isolated: a GUP pin, or any other pin, prevents migration.

	 * The expected page count is 3: 1 for page's mapcount and 1 for the

	 * caller's pin and 1 for the reference taken by isolate_lru_page().

	 */

	if (PageTransHuge(page) && page_count(page) != 3) {

		putback_lru_page(page);

		return 0;

	}



		/* Page is isolated */

		ret = 1;

	page_lru = page_is_file_cache(page);

		if (!PageTransHuge(page))

			inc_zone_page_state(page, NR_ISOLATED_ANON + page_lru);

		else

			mod_zone_page_state(page_zone(page),

					NR_ISOLATED_ANON + page_lru,

					HPAGE_PMD_NR);

	}

	mod_zone_page_state(page_zone(page), NR_ISOLATED_ANON + page_lru,

				hpage_nr_pages(page));



	/*

	 * Page is either isolated or there is not enough space on the target

	 * node. If isolated, then it has taken a reference count and the

	 * callers reference can be safely dropped without the page

	 * disappearing underneath us during migration. Otherwise the page is

	 * not to be migrated but the callers reference should still be

	 * dropped so it does not leak.

	 * Isolating the page has taken another reference, so the

	 * caller's reference can be safely dropped without the page

	 * disappearing underneath us during migration.

	 */

	put_page(page);



	return ret;

	return 1;

}



/*
@@ -1602,7 +1601,7 @@ int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page) 
int migrate_misplaced_page(struct page *page, int node)

{

	pg_data_t *pgdat = NODE_DATA(node);

	int isolated = 0;

	int isolated;

	int nr_remaining;

	LIST_HEAD(migratepages);
@@ -1610,20 +1609,16 @@ int migrate_misplaced_page(struct page *page, int node) 
	 * Don't migrate pages that are mapped in multiple processes.

	 * TODO: Handle false sharing detection instead of this hammer

	 */

	if (page_mapcount(page) != 1) {

		put_page(page);

	if (page_mapcount(page) != 1)

		goto out;

	}



	/*

	 * Rate-limit the amount of data that is being migrated to a node.

	 * Optimal placement is no good if the memory bus is saturated and

	 * all the time is being spent migrating!

	 */

	if (numamigrate_update_ratelimit(pgdat, 1)) {

		put_page(page);

	if (numamigrate_update_ratelimit(pgdat, 1))

		goto out;

	}



	isolated = numamigrate_isolate_page(pgdat, page);

	if (!isolated)
@@ -1640,12 +1635,19 @@ int migrate_misplaced_page(struct page *page, int node) 
	} else

		count_vm_numa_event(NUMA_PAGE_MIGRATE);

	BUG_ON(!list_empty(&migratepages));

out:

	return isolated;



out:

	put_page(page);

	return 0;

}

#endif /* CONFIG_NUMA_BALANCING */



#if defined(CONFIG_NUMA_BALANCING) && defined(CONFIG_TRANSPARENT_HUGEPAGE)

/*

 * Migrates a THP to a given target node. page must be locked and is unlocked

 * before returning.

 */

int migrate_misplaced_transhuge_page(struct mm_struct *mm,

				struct vm_area_struct *vma,

				pmd_t *pmd, pmd_t entry,
@@ -1676,29 +1678,15 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm, 


	new_page = alloc_pages_node(node,

		(GFP_TRANSHUGE | GFP_THISNODE) & ~__GFP_WAIT, HPAGE_PMD_ORDER);

	if (!new_page) {

		count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);

		goto out_dropref;

	}

	if (!new_page)

		goto out_fail;



	page_xchg_last_nid(new_page, page_last_nid(page));



	isolated = numamigrate_isolate_page(pgdat, page);



	/*

	 * Failing to isolate or a GUP pin prevents migration. The expected

	 * page count is 2. 1 for anonymous pages without a mapping and 1

	 * for the callers pin. If the page was isolated, the page will

	 * need to be put back on the LRU.

	 */

	if (!isolated || page_count(page) != 2) {

		count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);

	if (!isolated) {

		put_page(new_page);

		if (isolated) {

			putback_lru_page(page);

			isolated = 0;

			goto out;

		}

		goto out_keep_locked;

		goto out_fail;

	}



	/* Prepare a page as a migration target */
@@ -1730,6 +1718,7 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm, 
		putback_lru_page(page);



		count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);

		isolated = 0;

		goto out;

	}
@@ -1774,9 +1763,11 @@ out: 
			-HPAGE_PMD_NR);

	return isolated;



out_fail:

	count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);

out_dropref:

	unlock_page(page);

	put_page(page);

out_keep_locked:

	return 0;

}

#endif /* CONFIG_NUMA_BALANCING */

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