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SubjectRe: [patch 3/3] vmscan: detect mapped file pages used only once
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On Mon, 2010-02-22 at 20:49 +0100, Johannes Weiner wrote:
> The VM currently assumes that an inactive, mapped and referenced file
> page is in use and promotes it to the active list.
>
> However, every mapped file page starts out like this and thus a problem
> arises when workloads create a stream of such pages that are used only
> for a short time. By flooding the active list with those pages, the VM
> quickly gets into trouble finding eligible reclaim canditates. The
> result is long allocation latencies and eviction of the wrong pages.
>
> This patch reuses the PG_referenced page flag (used for unmapped file
> pages) to implement a usage detection that scales with the speed of
> LRU list cycling (i.e. memory pressure).
>
> If the scanner encounters those pages, the flag is set and the page
> cycled again on the inactive list. Only if it returns with another
> page table reference it is activated. Otherwise it is reclaimed as
> 'not recently used cache'.
>
> This effectively changes the minimum lifetime of a used-once mapped
> file page from a full memory cycle to an inactive list cycle, which
> allows it to occur in linear streams without affecting the stable
> working set of the system.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
> include/linux/rmap.h | 2 +-
> mm/rmap.c | 3 ---
> mm/vmscan.c | 45 +++++++++++++++++++++++++++++++++++----------
> 3 files changed, 36 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index b019ae6..f4accb5 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -181,7 +181,7 @@ static inline int page_referenced(struct page *page, int is_locked,
> unsigned long *vm_flags)
> {
> *vm_flags = 0;
> - return TestClearPageReferenced(page);
> + return 0;
> }
>
> #define try_to_unmap(page, refs) SWAP_FAIL
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 278cd27..5a48bda 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -511,9 +511,6 @@ int page_referenced(struct page *page,
> int referenced = 0;
> int we_locked = 0;
>
> - if (TestClearPageReferenced(page))
> - referenced++;
> -

>From now on, page_referenced see only page table for reference.
So let's comment it on function description.
like "This function checks reference from only pte"

> *vm_flags = 0;
> if (page_mapped(page) && page_rmapping(page)) {
> if (!is_locked && (!PageAnon(page) || PageKsm(page))) {
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a8e4cbe..674a78b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -561,18 +561,18 @@ redo:
> enum page_references {
> PAGEREF_RECLAIM,
> PAGEREF_RECLAIM_CLEAN,
> + PAGEREF_KEEP,
> PAGEREF_ACTIVATE,
> };
>
> static enum page_references page_check_references(struct page *page,
> struct scan_control *sc)
> {
> + int referenced_ptes, referenced_page;
> unsigned long vm_flags;
> - int referenced;
>
> - referenced = page_referenced(page, 1, sc->mem_cgroup, &vm_flags);
> - if (!referenced)
> - return PAGEREF_RECLAIM;
> + referenced_ptes = page_referenced(page, 1, sc->mem_cgroup, &vm_flags);
> + referenced_page = TestClearPageReferenced(page);
>
> /* Lumpy reclaim - ignore references */
> if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
> @@ -582,11 +582,36 @@ static enum page_references page_check_references(struct page *page,
> if (vm_flags & VM_LOCKED)
> return PAGEREF_RECLAIM;
>
> - if (page_mapped(page))
> - return PAGEREF_ACTIVATE;
> + if (referenced_ptes) {
> + if (PageAnon(page))
> + return PAGEREF_ACTIVATE;
> + /*
> + * All mapped pages start out with page table
> + * references from the instantiating fault, so we need
> + * to look twice if a mapped file page is used more
> + * than once.
> + *
> + * Mark it and spare it for another trip around the
> + * inactive list. Another page table reference will
> + * lead to its activation.
> + *
> + * Note: the mark is set for activated pages as well
> + * so that recently deactivated but used pages are
> + * quickly recovered.
> + */
> + SetPageReferenced(page);
> +
> + if (referenced_page)
> + return PAGEREF_ACTIVATE;
> +
> + return PAGEREF_KEEP;
> + }
>
> /* Reclaim if clean, defer dirty pages to writeback */
> - return PAGEREF_RECLAIM_CLEAN;
> + if (referenced_page)
> + return PAGEREF_RECLAIM_CLEAN;
> +
> + return PAGEREF_RECLAIM;
> }
>
> /*
> @@ -654,6 +679,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> switch (references) {
> case PAGEREF_ACTIVATE:
> goto activate_locked;
> + case PAGEREF_KEEP:
> + goto keep_locked;
> case PAGEREF_RECLAIM:
> case PAGEREF_RECLAIM_CLEAN:
> ; /* try to reclaim the page below */
> @@ -1356,9 +1383,7 @@ static void shrink_active_list(unsigned long nr_pages, struct zone *zone,
> continue;
> }
>
> - /* page_referenced clears PageReferenced */
> - if (page_mapped(page) &&
> - page_referenced(page, 0, sc->mem_cgroup, &vm_flags)) {
> + if (page_referenced(page, 0, sc->mem_cgroup, &vm_flags)) {
> nr_rotated++;
> /*
> * Identify referenced, file-backed active pages and

It looks good to me except PAGEREF_RECLAIM_CLEAN.

I am glad to meet your this effort, again, Hannes. :)

--
Kind regards,
Minchan Kim




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