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    SubjectRe: [PATCH] mm: add free_hot_cold_page_list helper
    Andrew Morton wrote:
    > On Fri, 29 Jul 2011 11:58:37 +0400
    > Konstantin Khlebnikov<khlebnikov@openvz.org> wrote:
    >
    >> This patch adds helper free_hot_cold_page_list() to free list of 0-order pages.
    >> It frees pages directly from list without temporary page-vector.
    >> It also calls trace_mm_pagevec_free() to simulate pagevec_free() behaviour.
    >>
    >> bloat-o-meter:
    >>
    >> add/remove: 1/1 grow/shrink: 1/3 up/down: 267/-295 (-28)
    >> function old new delta
    >> free_hot_cold_page_list - 264 +264
    >> get_page_from_freelist 2129 2132 +3
    >> __pagevec_free 243 239 -4
    >> split_free_page 380 373 -7
    >> release_pages 606 510 -96
    >> free_page_list 188 - -188
    >>
    >
    > It saves a total of 150 bytes for me.
    >
    >> index cb40892..dd7b9cc 100644
    >> --- a/include/linux/gfp.h
    >> +++ b/include/linux/gfp.h
    >> @@ -358,6 +358,7 @@ void *alloc_pages_exact_nid(int nid, size_t size, gfp_t gfp_mask);
    >> extern void __free_pages(struct page *page, unsigned int order);
    >> extern void free_pages(unsigned long addr, unsigned int order);
    >> extern void free_hot_cold_page(struct page *page, int cold);
    >> +extern void free_hot_cold_page_list(struct list_head *list, int cold);
    >>
    >> #define __free_page(page) __free_pages((page), 0)
    >> #define free_page(addr) free_pages((addr), 0)
    >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    >> index 1dbcf88..af486e4 100644
    >> --- a/mm/page_alloc.c
    >> +++ b/mm/page_alloc.c
    >> @@ -1209,6 +1209,18 @@ out:
    >> local_irq_restore(flags);
    >> }
    >>
    >> +void free_hot_cold_page_list(struct list_head *list, int cold)
    >> +{
    >> + struct page *page, *next;
    >> +
    >> + list_for_each_entry_safe(page, next, list, lru) {
    >> + trace_mm_pagevec_free(page, cold);
    >> + free_hot_cold_page(page, cold);
    >> + }
    >> +
    >> + INIT_LIST_HEAD(list);
    >> +}
    >> +
    >> /*
    >> * split_page takes a non-compound higher-order page, and splits it into
    >> * n (1<<order) sub-pages: page[0..n]
    >> diff --git a/mm/swap.c b/mm/swap.c
    >> index 3a442f1..b9138c7 100644
    >> --- a/mm/swap.c
    >> +++ b/mm/swap.c
    >> @@ -562,11 +562,10 @@ int lru_add_drain_all(void)
    >> void release_pages(struct page **pages, int nr, int cold)
    >> {
    >> int i;
    >> - struct pagevec pages_to_free;
    >> + LIST_HEAD(pages_to_free);
    >> struct zone *zone = NULL;
    >> unsigned long uninitialized_var(flags);
    >>
    >> - pagevec_init(&pages_to_free, cold);
    >> for (i = 0; i< nr; i++) {
    >> struct page *page = pages[i];
    >>
    >> @@ -597,19 +596,12 @@ void release_pages(struct page **pages, int nr, int cold)
    >> del_page_from_lru(zone, page);
    >> }
    >>
    >> - if (!pagevec_add(&pages_to_free, page)) {
    >> - if (zone) {
    >> - spin_unlock_irqrestore(&zone->lru_lock, flags);
    >> - zone = NULL;
    >> - }
    >> - __pagevec_free(&pages_to_free);
    >> - pagevec_reinit(&pages_to_free);
    >> - }
    >> + list_add_tail(&page->lru,&pages_to_free);
    >
    > There's a potential problem here with cache longevity. If
    > release_pages() is called with a large number of pages then the current
    > code's approach of freeing pages 16-at-a-time will hopefully cause
    > those pageframes to still be in CPU cache when we get to actually
    > freeing them.
    >
    > But after this change, we free all the pages in a single operation
    > right at the end, which adds risk that we'll have to reload all their
    > pageframes into CPU cache again.
    >
    > That'll only be a problem if release_pages() _is_ called with a large
    > number of pages. And manipulating large numbers of pages represents a
    > lot of work, so the additional work from one cachemiss per page will
    > presumably be tiny.
    >
    >

    all release_pages() callers (except fuse) call it for pages array not bigger than PAGEVEC_SIZE (=14).
    while for fuse it put likely not last page reference, so we didn't free them on this path.


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