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SubjectRe: [PATCH mmotm] mm: __isolate_lru_page_prepare() in isolate_migratepages_block()
On Sat, Mar 5, 2022 at 1:01 PM Hugh Dickins <hughd@google.com> wrote:
>
> __isolate_lru_page_prepare() conflates two unrelated functions, with
> the flags to one disjoint from the flags to the other; and hides some
> of the important checks outside of isolate_migratepages_block(), where
> the sequence is better to be visible. It comes from the days of lumpy
> reclaim, before compaction, when the combination made more sense.
>
> Move what's needed by mm/compaction.c isolate_migratepages_block() inline
> there, and what's needed by mm/vmscan.c isolate_lru_pages() inline there.
>
> Shorten "isolate_mode" to "mode", so the sequence of conditions is easier
> to read. Declare a "mapping" variable, to save one call to page_mapping()
> (but not another: calling again after page is locked is necessary).
> Simplify isolate_lru_pages() with a "move_to" list pointer.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>

Reviewed-by: Alex Shi <alexs@kernel.org>

> ---
>
> include/linux/swap.h | 1 -
> mm/compaction.c | 51 ++++++++++++++++++++++----
> mm/vmscan.c | 101 +++++++++------------------------------------------
> 3 files changed, 62 insertions(+), 91 deletions(-)
>
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -386,7 +386,6 @@ extern void lru_cache_add_inactive_or_unevictable(struct page *page,
> extern unsigned long zone_reclaimable_pages(struct zone *zone);
> extern unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
> gfp_t gfp_mask, nodemask_t *mask);
> -extern bool __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode);
> extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
> unsigned long nr_pages,
> gfp_t gfp_mask,
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -785,7 +785,7 @@ static bool too_many_isolated(pg_data_t *pgdat)
> * @cc: Compaction control structure.
> * @low_pfn: The first PFN to isolate
> * @end_pfn: The one-past-the-last PFN to isolate, within same pageblock
> - * @isolate_mode: Isolation mode to be used.
> + * @mode: Isolation mode to be used.
> *
> * Isolate all pages that can be migrated from the range specified by
> * [low_pfn, end_pfn). The range is expected to be within same pageblock.
> @@ -798,7 +798,7 @@ static bool too_many_isolated(pg_data_t *pgdat)
> */
> static int
> isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> - unsigned long end_pfn, isolate_mode_t isolate_mode)
> + unsigned long end_pfn, isolate_mode_t mode)
> {
> pg_data_t *pgdat = cc->zone->zone_pgdat;
> unsigned long nr_scanned = 0, nr_isolated = 0;
> @@ -806,6 +806,7 @@ static bool too_many_isolated(pg_data_t *pgdat)
> unsigned long flags = 0;
> struct lruvec *locked = NULL;
> struct page *page = NULL, *valid_page = NULL;
> + struct address_space *mapping;
> unsigned long start_pfn = low_pfn;
> bool skip_on_failure = false;
> unsigned long next_skip_pfn = 0;
> @@ -990,7 +991,7 @@ static bool too_many_isolated(pg_data_t *pgdat)
> locked = NULL;
> }
>
> - if (!isolate_movable_page(page, isolate_mode))
> + if (!isolate_movable_page(page, mode))
> goto isolate_success;
> }
>
> @@ -1002,15 +1003,15 @@ static bool too_many_isolated(pg_data_t *pgdat)
> * so avoid taking lru_lock and isolating it unnecessarily in an
> * admittedly racy check.
> */
> - if (!page_mapping(page) &&
> - page_count(page) > page_mapcount(page))
> + mapping = page_mapping(page);
> + if (!mapping && page_count(page) > page_mapcount(page))
> goto isolate_fail;
>
> /*
> * Only allow to migrate anonymous pages in GFP_NOFS context
> * because those do not depend on fs locks.
> */
> - if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page))
> + if (!(cc->gfp_mask & __GFP_FS) && mapping)
> goto isolate_fail;
>
> /*
> @@ -1021,9 +1022,45 @@ static bool too_many_isolated(pg_data_t *pgdat)
> if (unlikely(!get_page_unless_zero(page)))
> goto isolate_fail;
>
> - if (!__isolate_lru_page_prepare(page, isolate_mode))
> + /* Only take pages on LRU: a check now makes later tests safe */
> + if (!PageLRU(page))
> + goto isolate_fail_put;
> +
> + /* Compaction might skip unevictable pages but CMA takes them */
> + if (!(mode & ISOLATE_UNEVICTABLE) && PageUnevictable(page))
> + goto isolate_fail_put;
> +
> + /*
> + * To minimise LRU disruption, the caller can indicate with
> + * ISOLATE_ASYNC_MIGRATE that it only wants to isolate pages
> + * it will be able to migrate without blocking - clean pages
> + * for the most part. PageWriteback would require blocking.
> + */
> + if ((mode & ISOLATE_ASYNC_MIGRATE) && PageWriteback(page))
> goto isolate_fail_put;
>
> + if ((mode & ISOLATE_ASYNC_MIGRATE) && PageDirty(page)) {
> + bool migrate_dirty;
> +
> + /*
> + * Only pages without mappings or that have a
> + * ->migratepage callback are possible to migrate
> + * without blocking. However, we can be racing with
> + * truncation so it's necessary to lock the page
> + * to stabilise the mapping as truncation holds
> + * the page lock until after the page is removed
> + * from the page cache.
> + */
> + if (!trylock_page(page))
> + goto isolate_fail_put;
> +
> + mapping = page_mapping(page);
> + migrate_dirty = !mapping || mapping->a_ops->migratepage;
> + unlock_page(page);
> + if (!migrate_dirty)
> + goto isolate_fail_put;
> + }
> +
> /* Try isolate the page */
> if (!TestClearPageLRU(page))
> goto isolate_fail_put;
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2009,69 +2009,6 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
> }
>
> /*
> - * Attempt to remove the specified page from its LRU. Only take this page
> - * if it is of the appropriate PageActive status. Pages which are being
> - * freed elsewhere are also ignored.
> - *
> - * page: page to consider
> - * mode: one of the LRU isolation modes defined above
> - *
> - * returns true on success, false on failure.
> - */
> -bool __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode)
> -{
> - /* Only take pages on the LRU. */
> - if (!PageLRU(page))
> - return false;
> -
> - /* Compaction should not handle unevictable pages but CMA can do so */
> - if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE))
> - return false;
> -
> - /*
> - * To minimise LRU disruption, the caller can indicate that it only
> - * wants to isolate pages it will be able to operate on without
> - * blocking - clean pages for the most part.
> - *
> - * ISOLATE_ASYNC_MIGRATE is used to indicate that it only wants to pages
> - * that it is possible to migrate without blocking
> - */
> - if (mode & ISOLATE_ASYNC_MIGRATE) {
> - /* All the caller can do on PageWriteback is block */
> - if (PageWriteback(page))
> - return false;
> -
> - if (PageDirty(page)) {
> - struct address_space *mapping;
> - bool migrate_dirty;
> -
> - /*
> - * Only pages without mappings or that have a
> - * ->migratepage callback are possible to migrate
> - * without blocking. However, we can be racing with
> - * truncation so it's necessary to lock the page
> - * to stabilise the mapping as truncation holds
> - * the page lock until after the page is removed
> - * from the page cache.
> - */
> - if (!trylock_page(page))
> - return false;
> -
> - mapping = page_mapping(page);
> - migrate_dirty = !mapping || mapping->a_ops->migratepage;
> - unlock_page(page);
> - if (!migrate_dirty)
> - return false;
> - }
> - }
> -
> - if ((mode & ISOLATE_UNMAPPED) && page_mapped(page))
> - return false;
> -
> - return true;
> -}
> -
> -/*
> * Update LRU sizes after isolating pages. The LRU size updates must
> * be complete before mem_cgroup_update_lru_size due to a sanity check.
> */
> @@ -2122,11 +2059,11 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> unsigned long skipped = 0;
> unsigned long scan, total_scan, nr_pages;
> LIST_HEAD(pages_skipped);
> - isolate_mode_t mode = (sc->may_unmap ? 0 : ISOLATE_UNMAPPED);
>
> total_scan = 0;
> scan = 0;
> while (scan < nr_to_scan && !list_empty(src)) {
> + struct list_head *move_to = src;
> struct page *page;
>
> page = lru_to_page(src);
> @@ -2136,9 +2073,9 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> total_scan += nr_pages;
>
> if (page_zonenum(page) > sc->reclaim_idx) {
> - list_move(&page->lru, &pages_skipped);
> nr_skipped[page_zonenum(page)] += nr_pages;
> - continue;
> + move_to = &pages_skipped;
> + goto move;
> }
>
> /*
> @@ -2146,37 +2083,34 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> * return with no isolated pages if the LRU mostly contains
> * ineligible pages. This causes the VM to not reclaim any
> * pages, triggering a premature OOM.
> - *
> - * Account all tail pages of THP. This would not cause
> - * premature OOM since __isolate_lru_page() returns -EBUSY
> - * only when the page is being freed somewhere else.
> + * Account all tail pages of THP.
> */
> scan += nr_pages;
> - if (!__isolate_lru_page_prepare(page, mode)) {
> - /* It is being freed elsewhere */
> - list_move(&page->lru, src);
> - continue;
> - }
> +
> + if (!PageLRU(page))
> + goto move;
> + if (!sc->may_unmap && page_mapped(page))
> + goto move;
> +
> /*
> * Be careful not to clear PageLRU until after we're
> * sure the page is not being freed elsewhere -- the
> * page release code relies on it.
> */
> - if (unlikely(!get_page_unless_zero(page))) {
> - list_move(&page->lru, src);
> - continue;
> - }
> + if (unlikely(!get_page_unless_zero(page)))
> + goto move;
>
> if (!TestClearPageLRU(page)) {
> /* Another thread is already isolating this page */
> put_page(page);
> - list_move(&page->lru, src);
> - continue;
> + goto move;
> }
>
> nr_taken += nr_pages;
> nr_zone_taken[page_zonenum(page)] += nr_pages;
> - list_move(&page->lru, dst);
> + move_to = dst;
> +move:
> + list_move(&page->lru, move_to);
> }
>
> /*
> @@ -2200,7 +2134,8 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> }
> *nr_scanned = total_scan;
> trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan,
> - total_scan, skipped, nr_taken, mode, lru);
> + total_scan, skipped, nr_taken,
> + sc->may_unmap ? 0 : ISOLATE_UNMAPPED, lru);
> update_lru_sizes(lruvec, lru, nr_zone_taken);
> return nr_taken;
> }

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