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SubjectRe: [PATCH 4/8] vmscan: Do not writeback filesystem pages in direct reclaim
On Wed, 21 Jul 2010 12:52:50 +0100
Mel Gorman <mel@csn.ul.ie> wrote:


> ==== CUT HERE ====
> [PATCH] vmscan: Do not writeback filesystem pages in direct reclaim
>
> When memory is under enough pressure, a process may enter direct
> reclaim to free pages in the same manner kswapd does. If a dirty page is
> encountered during the scan, this page is written to backing storage using
> mapping->writepage. This can result in very deep call stacks, particularly
> if the target storage or filesystem are complex. It has already been observed
> on XFS that the stack overflows but the problem is not XFS-specific.
>
> This patch prevents direct reclaim writing back filesystem pages by checking
> if current is kswapd or the page is anonymous before writing back. If the
> dirty pages cannot be written back, they are placed back on the LRU lists
> for either background writing by the BDI threads or kswapd. If in direct
> lumpy reclaim and dirty pages are encountered, the process will stall for
> the background flusher before trying to reclaim the pages again.
>
> As the call-chain for writing anonymous pages is not expected to be deep
> and they are not cleaned by flusher threads, anonymous pages are still
> written back in direct reclaim.
>
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Acked-by: Rik van Riel <riel@redhat.com>
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 6587155..e3a5816 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -323,6 +323,51 @@ typedef enum {
> PAGE_CLEAN,
> } pageout_t;
>
> +int write_reclaim_page(struct page *page, struct address_space *mapping,
> + enum pageout_io sync_writeback)
> +{
> + int res;
> + struct writeback_control wbc = {
> + .sync_mode = WB_SYNC_NONE,
> + .nr_to_write = SWAP_CLUSTER_MAX,
> + .range_start = 0,
> + .range_end = LLONG_MAX,
> + .nonblocking = 1,
> + .for_reclaim = 1,
> + };
> +
> + if (!clear_page_dirty_for_io(page))
> + return PAGE_CLEAN;
> +
> + SetPageReclaim(page);
> + res = mapping->a_ops->writepage(page, &wbc);
> + if (res < 0)
> + handle_write_error(mapping, page, res);
> + if (res == AOP_WRITEPAGE_ACTIVATE) {
> + ClearPageReclaim(page);
> + return PAGE_ACTIVATE;
> + }
> +
> + /*
> + * Wait on writeback if requested to. This happens when
> + * direct reclaiming a large contiguous area and the
> + * first attempt to free a range of pages fails.
> + */
> + if (PageWriteback(page) && sync_writeback == PAGEOUT_IO_SYNC)
> + wait_on_page_writeback(page);
> +
> + if (!PageWriteback(page)) {
> + /* synchronous write or broken a_ops? */
> + ClearPageReclaim(page);
> + }
> + trace_mm_vmscan_writepage(page,
> + page_is_file_cache(page),
> + sync_writeback == PAGEOUT_IO_SYNC);
> + inc_zone_page_state(page, NR_VMSCAN_WRITE);
> +
> + return PAGE_SUCCESS;
> +}
> +
> /*
> * pageout is called by shrink_page_list() for each dirty page.
> * Calls ->writepage().
> @@ -367,46 +412,7 @@ static pageout_t pageout(struct page *page, struct address_space *mapping,
> if (!may_write_to_queue(mapping->backing_dev_info))
> return PAGE_KEEP;
>
> - if (clear_page_dirty_for_io(page)) {
> - int res;
> - struct writeback_control wbc = {
> - .sync_mode = WB_SYNC_NONE,
> - .nr_to_write = SWAP_CLUSTER_MAX,
> - .range_start = 0,
> - .range_end = LLONG_MAX,
> - .nonblocking = 1,
> - .for_reclaim = 1,
> - };
> -
> - SetPageReclaim(page);
> - res = mapping->a_ops->writepage(page, &wbc);
> - if (res < 0)
> - handle_write_error(mapping, page, res);
> - if (res == AOP_WRITEPAGE_ACTIVATE) {
> - ClearPageReclaim(page);
> - return PAGE_ACTIVATE;
> - }
> -
> - /*
> - * Wait on writeback if requested to. This happens when
> - * direct reclaiming a large contiguous area and the
> - * first attempt to free a range of pages fails.
> - */
> - if (PageWriteback(page) && sync_writeback == PAGEOUT_IO_SYNC)
> - wait_on_page_writeback(page);
> -
> - if (!PageWriteback(page)) {
> - /* synchronous write or broken a_ops? */
> - ClearPageReclaim(page);
> - }
> - trace_mm_vmscan_writepage(page,
> - page_is_file_cache(page),
> - sync_writeback == PAGEOUT_IO_SYNC);
> - inc_zone_page_state(page, NR_VMSCAN_WRITE);
> - return PAGE_SUCCESS;
> - }
> -
> - return PAGE_CLEAN;
> + return write_reclaim_page(page, mapping, sync_writeback);
> }
>
> /*
> @@ -639,18 +645,25 @@ static noinline_for_stack void free_page_list(struct list_head *free_pages)
> pagevec_free(&freed_pvec);
> }
>
> +/* Direct lumpy reclaim waits up to 5 seconds for background cleaning */
> +#define MAX_SWAP_CLEAN_WAIT 50
> +
> /*
> * shrink_page_list() returns the number of reclaimed pages
> */
> static unsigned long shrink_page_list(struct list_head *page_list,
> struct scan_control *sc,
> - enum pageout_io sync_writeback)
> + enum pageout_io sync_writeback,
> + unsigned long *nr_still_dirty)
> {
> - LIST_HEAD(ret_pages);
> LIST_HEAD(free_pages);
> - int pgactivate = 0;
> + LIST_HEAD(putback_pages);
> + LIST_HEAD(dirty_pages);
> + int pgactivate;
> + unsigned long nr_dirty = 0;
> unsigned long nr_reclaimed = 0;
>
> + pgactivate = 0;
> cond_resched();
>
> while (!list_empty(page_list)) {
> @@ -741,7 +754,18 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> }
> }
>
> - if (PageDirty(page)) {
> + if (PageDirty(page)) {
> + /*
> + * Only kswapd can writeback filesystem pages to
> + * avoid risk of stack overflow
> + */
> + if (page_is_file_cache(page) && !current_is_kswapd()) {
> + list_add(&page->lru, &dirty_pages);
> + unlock_page(page);
> + nr_dirty++;
> + goto keep_dirty;
> + }
> +
> if (references == PAGEREF_RECLAIM_CLEAN)
> goto keep_locked;
> if (!may_enter_fs)
> @@ -852,13 +876,19 @@ activate_locked:
> keep_locked:
> unlock_page(page);
> keep:
> - list_add(&page->lru, &ret_pages);
> + list_add(&page->lru, &putback_pages);
> +keep_dirty:
> VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
> }
>
> free_page_list(&free_pages);
>
> - list_splice(&ret_pages, page_list);
> + if (nr_dirty) {
> + *nr_still_dirty = nr_dirty;
> + list_splice(&dirty_pages, page_list);
> + }
> + list_splice(&putback_pages, page_list);
> +
> count_vm_events(PGACTIVATE, pgactivate);
> return nr_reclaimed;
> }
> @@ -1245,6 +1275,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
> unsigned long nr_active;
> unsigned long nr_anon;
> unsigned long nr_file;
> + unsigned long nr_dirty;
>
> while (unlikely(too_many_isolated(zone, file, sc))) {
> congestion_wait(BLK_RW_ASYNC, HZ/10);
> @@ -1293,26 +1324,34 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
>
> spin_unlock_irq(&zone->lru_lock);
>
> - nr_reclaimed = shrink_page_list(&page_list, sc, PAGEOUT_IO_ASYNC);
> + nr_reclaimed = shrink_page_list(&page_list, sc, PAGEOUT_IO_ASYNC,
> + &nr_dirty);
>
> /*
> - * If we are direct reclaiming for contiguous pages and we do
> + * If specific pages are needed such as with direct reclaiming
> + * for contiguous pages or for memory containers and we do
> * not reclaim everything in the list, try again and wait
> - * for IO to complete. This will stall high-order allocations
> - * but that should be acceptable to the caller
> + * for IO to complete. This will stall callers that require
> + * specific pages but it should be acceptable to the caller
> */
> - if (nr_reclaimed < nr_taken && !current_is_kswapd() &&
> - sc->lumpy_reclaim_mode) {
> - congestion_wait(BLK_RW_ASYNC, HZ/10);
> + if (sc->may_writepage && !current_is_kswapd() &&
> + (sc->lumpy_reclaim_mode || sc->mem_cgroup)) {
> + int dirty_retry = MAX_SWAP_CLEAN_WAIT;

Hmm, ok. I see what will happen to memcg.
But, hmm, memcg will have to select to enter this rounine based on
the result of 1st memory reclaim.

>
> - /*
> - * The attempt at page out may have made some
> - * of the pages active, mark them inactive again.
> - */
> - nr_active = clear_active_flags(&page_list, NULL);
> - count_vm_events(PGDEACTIVATE, nr_active);
> + while (nr_reclaimed < nr_taken && nr_dirty && dirty_retry--) {
> + wakeup_flusher_threads(laptop_mode ? 0 : nr_dirty);
> + congestion_wait(BLK_RW_ASYNC, HZ/10);
>
Congestion wait is required ?? Where the congestion happens ?
I'm sorry you already have some other trick in other patch.

> - nr_reclaimed += shrink_page_list(&page_list, sc, PAGEOUT_IO_SYNC);
> + /*
> + * The attempt at page out may have made some
> + * of the pages active, mark them inactive again.
> + */
> + nr_active = clear_active_flags(&page_list, NULL);
> + count_vm_events(PGDEACTIVATE, nr_active);
> +
> + nr_reclaimed += shrink_page_list(&page_list, sc,
> + PAGEOUT_IO_SYNC, &nr_dirty);
> + }

Just a question. This PAGEOUT_IO_SYNC has some meanings ?

Thanks,
-Kame



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