Messages in this thread | | | Subject | Re: [PATCH v21 07/19] mm: page_idle_get_page() does not need lru_lock | From | Vlastimil Babka <> | Date | Wed, 11 Nov 2020 13:52:01 +0100 |
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On 11/11/20 9:17 AM, huang ying wrote: > On Thu, Nov 5, 2020 at 4:56 PM Alex Shi <alex.shi@linux.alibaba.com> wrote: >> >> From: Hugh Dickins <hughd@google.com> >> >> It is necessary for page_idle_get_page() to recheck PageLRU() after >> get_page_unless_zero(), but holding lru_lock around that serves no >> useful purpose, and adds to lru_lock contention: delete it. >> >> See https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop for the >> discussion that led to lru_lock there; but __page_set_anon_rmap() now >> uses WRITE_ONCE(), and I see no other risk in page_idle_clear_pte_refs() >> using rmap_walk() (beyond the risk of racing PageAnon->PageKsm, mostly >> but not entirely prevented by page_count() check in ksm.c's >> write_protect_page(): that risk being shared with page_referenced() and >> not helped by lru_lock). >> >> Signed-off-by: Hugh Dickins <hughd@google.com> >> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com> >> Cc: Andrew Morton <akpm@linux-foundation.org> >> Cc: Vladimir Davydov <vdavydov.dev@gmail.com> >> Cc: Vlastimil Babka <vbabka@suse.cz> >> Cc: Minchan Kim <minchan@kernel.org> >> Cc: Alex Shi <alex.shi@linux.alibaba.com> >> Cc: linux-mm@kvack.org >> Cc: linux-kernel@vger.kernel.org >> --- >> mm/page_idle.c | 4 ---- >> 1 file changed, 4 deletions(-) >> >> diff --git a/mm/page_idle.c b/mm/page_idle.c >> index 057c61df12db..64e5344a992c 100644 >> --- a/mm/page_idle.c >> +++ b/mm/page_idle.c >> @@ -32,19 +32,15 @@ >> static struct page *page_idle_get_page(unsigned long pfn) >> { >> struct page *page = pfn_to_online_page(pfn); >> - pg_data_t *pgdat; >> >> if (!page || !PageLRU(page) || >> !get_page_unless_zero(page)) >> return NULL; >> >> - pgdat = page_pgdat(page); >> - spin_lock_irq(&pgdat->lru_lock); > > get_page_unless_zero() is a full memory barrier. But do we need a > compiler barrier here to prevent the compiler to cache PageLRU() > results here? Otherwise looks OK to me,
I think the compiler barrier is also implied by the full memory barrier and prevents the caching.
Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: "Huang, Ying" <ying.huang@intel.com> > > Best Regards, > Huang, Ying > >> if (unlikely(!PageLRU(page))) { >> put_page(page); >> page = NULL; >> } >> - spin_unlock_irq(&pgdat->lru_lock); >> return page; >> } >> >> -- >> 1.8.3.1 >> >> >
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