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SubjectRe: [Experimental][PATCH] putback_lru_page rework
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On Wed, 2008-06-18 at 18:40 +0900, KAMEZAWA Hiroyuki wrote:
> Lee-san, how about this ?
> Tested on x86-64 and tried Nisimura-san's test at el. works good now.

I have been testing with my work load on both ia64 and x86_64 and it
seems to be working well. I'll let them run for a day or so.

> -Kame
> ==
> putback_lru_page()/unevictable page handling rework.
>
> Now, putback_lru_page() requires that the page is locked.
> And in some special case, implicitly unlock it.
>
> This patch tries to make putback_lru_pages() to be lock_page() free.
> (Of course, some callers must take the lock.)
>
> The main reason that putback_lru_page() assumes that page is locked
> is to avoid the change in page's status among Mlocked/Not-Mlocked.
>
> Once it is added to unevictable list, the page is removed from
> unevictable list only when page is munlocked. (there are other special
> case. but we ignore the special case.)
> So, status change during putback_lru_page() is fatal and page should
> be locked.
>
> putback_lru_page() in this patch has a new concepts.
> When it adds page to unevictable list, it checks the status is
> changed or not again. if changed, retry to putback.

Given that the race that would activate this retry is likely quite rare,
this approach makes sense.

>
> This patche changes also caller side and cleaning up lock/unlock_page().
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroy@jp.fujitsu.com>

A couple of minor comments below, but:

Acked-by: Lee Schermerhorn <lee.schermerhorn@hp.com>

>
> ---
> mm/internal.h | 2 -
> mm/migrate.c | 23 +++----------
> mm/mlock.c | 24 +++++++-------
> mm/vmscan.c | 96 +++++++++++++++++++++++++---------------------------------
> 4 files changed, 61 insertions(+), 84 deletions(-)
>
> Index: test-2.6.26-rc5-mm3/mm/vmscan.c
> ===================================================================
> --- test-2.6.26-rc5-mm3.orig/mm/vmscan.c
> +++ test-2.6.26-rc5-mm3/mm/vmscan.c
> @@ -486,73 +486,63 @@ int remove_mapping(struct address_space
> * Page may still be unevictable for other reasons.
> *
> * lru_lock must not be held, interrupts must be enabled.
> - * Must be called with page locked.
> - *
> - * return 1 if page still locked [not truncated], else 0
> */
> -int putback_lru_page(struct page *page)
> +#ifdef CONFIG_UNEVICTABLE_LRU
> +void putback_lru_page(struct page *page)
> {
> int lru;
> - int ret = 1;
> int was_unevictable;
>
> - VM_BUG_ON(!PageLocked(page));
> VM_BUG_ON(PageLRU(page));
>
> - lru = !!TestClearPageActive(page);
> was_unevictable = TestClearPageUnevictable(page); /* for page_evictable() */
>
> - if (unlikely(!page->mapping)) {
> - /*
> - * page truncated. drop lock as put_page() will
> - * free the page.
> - */
> - VM_BUG_ON(page_count(page) != 1);
> - unlock_page(page);
> - ret = 0;
> - } else if (page_evictable(page, NULL)) {
> - /*
> - * For evictable pages, we can use the cache.
> - * In event of a race, worst case is we end up with an
> - * unevictable page on [in]active list.
> - * We know how to handle that.
> - */
> +redo:
> + lru = !!TestClearPageActive(page);
> + if (page_evictable(page, NULL)) {
> lru += page_is_file_cache(page);
> lru_cache_add_lru(page, lru);
> - mem_cgroup_move_lists(page, lru);
> -#ifdef CONFIG_UNEVICTABLE_LRU
> - if (was_unevictable)
> - count_vm_event(NORECL_PGRESCUED);
> -#endif
> } else {
> - /*
> - * Put unevictable pages directly on zone's unevictable
> - * list.
> - */
> + lru = LRU_UNEVICTABLE;
> add_page_to_unevictable_list(page);
> - mem_cgroup_move_lists(page, LRU_UNEVICTABLE);
> -#ifdef CONFIG_UNEVICTABLE_LRU
> - if (!was_unevictable)
> - count_vm_event(NORECL_PGCULLED);
> -#endif
> }
> + mem_cgroup_move_lists(page, lru);
> +
> + /*
> + * page's status can change while we move it among lru. If an evictable
> + * page is on unevictable list, it never be freed. To avoid that,
> + * check after we added it to the list, again.
> + */
> + if (lru == LRU_UNEVICTABLE && page_evictable(page, NULL)) {
> + if (!isolate_lru_page(page)) {
> + put_page(page);
> + goto redo;
> + }
> + /* This means someone else dropped this page from LRU
> + * So, it will be freed or putback to LRU again. There is
> + * nothing to do here.
> + */
> + }
> +
> + if (was_unevictable && lru != LRU_UNEVICTABLE)
> + count_vm_event(NORECL_PGRESCUED);
> + else if (!was_unevictable && lru == LRU_UNEVICTABLE)
> + count_vm_event(NORECL_PGCULLED);
>
> put_page(page); /* drop ref from isolate */
> - return ret; /* ret => "page still locked" */
> }
> -
> -/*
> - * Cull page that shrink_*_list() has detected to be unevictable
> - * under page lock to close races with other tasks that might be making
> - * the page evictable. Avoid stranding an evictable page on the
> - * unevictable list.
> - */
> -static void cull_unevictable_page(struct page *page)
> +#else
> +void putback_lru_page(struct page *page)
> {
> - lock_page(page);
> - if (putback_lru_page(page))
> - unlock_page(page);
> + int lru;
> + VM_BUG_ON(PageLRU(page));
> +
> + lru = !!TestClearPageActive(page) + page_is_file_cache(page);
> + lru_cache_add_lru(page, lru);
> + mem_cgroup_move_lists(page, lru);
> + put_page(page);
> }
> +#endif
>
> /*
> * shrink_page_list() returns the number of reclaimed pages
> @@ -746,8 +736,8 @@ free_it:
> continue;
>
> cull_mlocked:
> - if (putback_lru_page(page))
> - unlock_page(page);
> + unlock_page(page);
> + putback_lru_page(page);
> continue;
>
> activate_locked:
> @@ -1127,7 +1117,7 @@ static unsigned long shrink_inactive_lis
> list_del(&page->lru);
> if (unlikely(!page_evictable(page, NULL))) {
> spin_unlock_irq(&zone->lru_lock);
> - cull_unevictable_page(page);
> + putback_lru_page(page);
> spin_lock_irq(&zone->lru_lock);
> continue;
> }
> @@ -1231,7 +1221,7 @@ static void shrink_active_list(unsigned
> list_del(&page->lru);
>
> if (unlikely(!page_evictable(page, NULL))) {
> - cull_unevictable_page(page);
> + putback_lru_page(page);
> continue;
> }
>
> @@ -2393,8 +2383,6 @@ int zone_reclaim(struct zone *zone, gfp_
> int page_evictable(struct page *page, struct vm_area_struct *vma)
> {
>
> - VM_BUG_ON(PageUnevictable(page));
> -
> if (mapping_unevictable(page_mapping(page)))
> return 0;
>
> Index: test-2.6.26-rc5-mm3/mm/mlock.c
> ===================================================================
> --- test-2.6.26-rc5-mm3.orig/mm/mlock.c
> +++ test-2.6.26-rc5-mm3/mm/mlock.c
> @@ -55,7 +55,6 @@ EXPORT_SYMBOL(can_do_mlock);
> */
> void __clear_page_mlock(struct page *page)
> {
> - VM_BUG_ON(!PageLocked(page)); /* for LRU isolate/putback */
>
> dec_zone_page_state(page, NR_MLOCK);
> count_vm_event(NORECL_PGCLEARED);
> @@ -79,7 +78,6 @@ void __clear_page_mlock(struct page *pag
> */
> void mlock_vma_page(struct page *page)
> {
> - BUG_ON(!PageLocked(page));
>
> if (!TestSetPageMlocked(page)) {
> inc_zone_page_state(page, NR_MLOCK);
> @@ -109,7 +107,6 @@ void mlock_vma_page(struct page *page)
> */
> static void munlock_vma_page(struct page *page)
> {
> - BUG_ON(!PageLocked(page));
>
> if (TestClearPageMlocked(page)) {
> dec_zone_page_state(page, NR_MLOCK);
> @@ -169,7 +166,8 @@ static int __mlock_vma_pages_range(struc
>
> /*
> * get_user_pages makes pages present if we are
> - * setting mlock.
> + * setting mlock. and this extra reference count will
> + * disable migration of this page.
> */
> ret = get_user_pages(current, mm, addr,
> min_t(int, nr_pages, ARRAY_SIZE(pages)),
> @@ -197,14 +195,8 @@ static int __mlock_vma_pages_range(struc
> for (i = 0; i < ret; i++) {
> struct page *page = pages[i];
>
> - /*
> - * page might be truncated or migrated out from under
> - * us. Check after acquiring page lock.
> - */
> - lock_page(page);

Hmmm. Still thinking about this. No need to protect against in flight
truncation or migration?

> - if (page->mapping)
> + if (page_mapcount(page))
> mlock_vma_page(page);
> - unlock_page(page);
> put_page(page); /* ref from get_user_pages() */
>
> /*
> @@ -240,6 +232,9 @@ static int __munlock_pte_handler(pte_t *
> struct page *page;
> pte_t pte;
>
> + /*
> + * page is never be unmapped by page-reclaim. we lock this page now.
> + */

I don't understand what you're trying to say here. That is, what the
point of this comment is...

> retry:
> pte = *ptep;
> /*
> @@ -261,7 +256,15 @@ retry:
> goto out;
>
> lock_page(page);
> - if (!page->mapping) {
> + /*
> + * Because we lock page here, we have to check 2 cases.
> + * - the page is migrated.
> + * - the page is truncated (file-cache only)
> + * Note: Anonymous page doesn't clear page->mapping even if it
> + * is removed from rmap.
> + */
> + if (!page->mapping ||
> + (PageAnon(page) && !page_mapcount(page))) {
> unlock_page(page);
> goto retry;
> }
> Index: test-2.6.26-rc5-mm3/mm/migrate.c
> ===================================================================
> --- test-2.6.26-rc5-mm3.orig/mm/migrate.c
> +++ test-2.6.26-rc5-mm3/mm/migrate.c
> @@ -67,9 +67,7 @@ int putback_lru_pages(struct list_head *
>
> list_for_each_entry_safe(page, page2, l, lru) {
> list_del(&page->lru);
> - lock_page(page);
> - if (putback_lru_page(page))
> - unlock_page(page);
> + putback_lru_page(page);
> count++;
> }
> return count;
> @@ -571,7 +569,6 @@ static int fallback_migrate_page(struct
> static int move_to_new_page(struct page *newpage, struct page *page)
> {
> struct address_space *mapping;
> - int unlock = 1;
> int rc;
>
> /*
> @@ -610,12 +607,11 @@ static int move_to_new_page(struct page
> * Put back on LRU while holding page locked to
> * handle potential race with, e.g., munlock()
> */
> - unlock = putback_lru_page(newpage);
> + putback_lru_page(newpage);
> } else
> newpage->mapping = NULL;
>
> - if (unlock)
> - unlock_page(newpage);
> + unlock_page(newpage);
>
> return rc;
> }
> @@ -632,7 +628,6 @@ static int unmap_and_move(new_page_t get
> struct page *newpage = get_new_page(page, private, &result);
> int rcu_locked = 0;
> int charge = 0;
> - int unlock = 1;
>
> if (!newpage)
> return -ENOMEM;
> @@ -713,6 +708,7 @@ rcu_unlock:
> rcu_read_unlock();
>
> unlock:
> + unlock_page(page);
>
> if (rc != -EAGAIN) {
> /*
> @@ -722,18 +718,9 @@ unlock:
> * restored.
> */
> list_del(&page->lru);
> - if (!page->mapping) {
> - VM_BUG_ON(page_count(page) != 1);
> - unlock_page(page);
> - put_page(page); /* just free the old page */
> - goto end_migration;
> - } else
> - unlock = putback_lru_page(page);
> + putback_lru_page(page);
> }
>
> - if (unlock)
> - unlock_page(page);
> -
> end_migration:
> if (!charge)
> mem_cgroup_end_migration(newpage);
> Index: test-2.6.26-rc5-mm3/mm/internal.h
> ===================================================================
> --- test-2.6.26-rc5-mm3.orig/mm/internal.h
> +++ test-2.6.26-rc5-mm3/mm/internal.h
> @@ -43,7 +43,7 @@ static inline void __put_page(struct pag
> * in mm/vmscan.c:
> */
> extern int isolate_lru_page(struct page *page);
> -extern int putback_lru_page(struct page *page);
> +extern void putback_lru_page(struct page *page);
>
> /*
> * in mm/page_alloc.c
>
> --
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