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    SubjectRe: [RFC PATCH 2/3] mm: Drop use of test_and_set_skip in favor of just setting skip
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    在 2020/8/13 下午12:02, Alexander Duyck 写道:
    > From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
    >
    > The only user of test_and_set_skip was isolate_migratepages_block and it
    > was using it after a call that was testing and clearing the LRU flag. As
    > such it really didn't need to be behind the LRU lock anymore as it wasn't
    > really fulfilling its purpose.
    >
    > With that being the case we can simply drop the bit and instead directly
    > just call the set_pageblock_skip function if the page we are working on is
    > the valid_page at the start of the pageblock. It shouldn't be possible for
    > us to encounter the bit being set since we obtained the LRU flag for the
    > first page in the pageblock which means we would have exclusive access to
    > setting the skip bit. As such we don't need to worry about the abort case
    > since no other thread will be able to call what used to be
    > test_and_set_skip.
    >
    > Since we have dropped the late abort case we can drop the code that was
    > clearing the LRU flag and calling page_put since the abort case will now
    > not be holding a reference to a page.
    >
    > Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>

    After my false sharing remove on pageblock_flags, this patch looks fine with
    a minor change

    > ---
    > mm/compaction.c | 50 +++++++-------------------------------------------
    > 1 file changed, 7 insertions(+), 43 deletions(-)
    >
    > diff --git a/mm/compaction.c b/mm/compaction.c
    > index 5021a18ef722..c1e9918f9dd4 100644
    > --- a/mm/compaction.c
    > +++ b/mm/compaction.c
    > @@ -399,29 +399,6 @@ void reset_isolation_suitable(pg_data_t *pgdat)
    > }
    > }
    >
    > -/*
    > - * Sets the pageblock skip bit if it was clear. Note that this is a hint as
    > - * locks are not required for read/writers. Returns true if it was already set.
    > - */
    > -static bool test_and_set_skip(struct compact_control *cc, struct page *page,
    > - unsigned long pfn)
    > -{
    > - bool skip;
    > -
    > - /* Do no update if skip hint is being ignored */
    > - if (cc->ignore_skip_hint)
    > - return false;
    > -
    > - if (!IS_ALIGNED(pfn, pageblock_nr_pages))
    > - return false;
    > -
    > - skip = get_pageblock_skip(page);
    > - if (!skip && !cc->no_set_skip_hint)
    > - skip = !set_pageblock_skip(page);
    > -
    > - return skip;
    > -}
    > -
    > static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
    > {
    > struct zone *zone = cc->zone;
    > @@ -480,12 +457,6 @@ static inline void update_pageblock_skip(struct compact_control *cc,
    > static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
    > {
    > }
    > -
    > -static bool test_and_set_skip(struct compact_control *cc, struct page *page,
    > - unsigned long pfn)
    > -{
    > - return false;
    > -}
    > #endif /* CONFIG_COMPACTION */
    >
    > /*
    > @@ -895,7 +866,6 @@ static bool too_many_isolated(pg_data_t *pgdat)
    > if (!valid_page && IS_ALIGNED(low_pfn, pageblock_nr_pages)) {
    > if (!cc->ignore_skip_hint && get_pageblock_skip(page)) {
    > low_pfn = end_pfn;
    > - page = NULL;
    > goto isolate_abort;
    > }
    > valid_page = page;
    > @@ -991,6 +961,13 @@ static bool too_many_isolated(pg_data_t *pgdat)
    > if (!TestClearPageLRU(page))
    > goto isolate_fail_put;
    >
    > + /* Indicate that we want exclusive access to this pageblock */
    > + if (page == valid_page) {
    > + skip_updated = true;
    > + if (!cc->ignore_skip_hint)

    if (!cc->ignore_skip_hint && !cc->no_set_skip_hint)
    no_set_skip_hint needs to add here.

    Thanks
    Alex

    > + set_pageblock_skip(page);
    > + }
    > +
    > /* If we already hold the lock, we can skip some rechecking */
    > if (!lruvec || !lruvec_holds_page_lru_lock(page, lruvec)) {
    > if (lruvec)
    > @@ -1002,13 +979,6 @@ static bool too_many_isolated(pg_data_t *pgdat)
    >
    > lruvec_memcg_debug(lruvec, page);
    >
    > - /* Try get exclusive access under lock */
    > - if (!skip_updated) {
    > - skip_updated = true;
    > - if (test_and_set_skip(cc, page, low_pfn))
    > - goto isolate_abort;
    > - }
    > -
    > /*
    > * Page become compound since the non-locked check,
    > * and it's on LRU. It can only be a THP so the order
    > @@ -1094,15 +1064,9 @@ static bool too_many_isolated(pg_data_t *pgdat)
    > if (unlikely(low_pfn > end_pfn))
    > low_pfn = end_pfn;
    >
    > - page = NULL;
    > -
    > isolate_abort:
    > if (lruvec)
    > unlock_page_lruvec_irqrestore(lruvec, flags);
    > - if (page) {
    > - SetPageLRU(page);
    > - put_page(page);
    > - }
    >
    > /*
    > * Updated the cached scanner pfn once the pageblock has been scanned
    >

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