From 930668c34408ba983049322e04f13f03b6f1fafa Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Sat, 30 Nov 2019 17:56:49 -0800 Subject: [PATCH] hugetlbfs: take read_lock on i_mmap for PMD sharing A customer with large SMP systems (up to 16 sockets) with application that uses large amount of static hugepages (~500-1500GB) are experiencing random multisecond delays. These delays were caused by the long time it took to scan the VMA interval tree with mmap_sem held. The sharing of huge PMD does not require changes to the i_mmap at all. Therefore, we can just take the read lock and let other threads searching for the right VMA share it in parallel. Once the right VMA is found, either the PMD lock (2M huge page for x86-64) or the mm->page_table_lock will be acquired to perform the actual PMD sharing. Lock contention, if present, will happen in the spinlock. That is much better than contention in the rwsem where the time needed to scan the the interval tree is indeterminate. With this patch applied, the customer is seeing significant performance improvement over the unpatched kernel. Link: http://lkml.kernel.org/r/20191107211809.9539-1-longman@redhat.com Signed-off-by: Waiman Long Suggested-by: Mike Kravetz Reviewed-by: Mike Kravetz Cc: Davidlohr Bueso Cc: Peter Zijlstra Cc: Ingo Molnar Cc: Will Deacon Cc: Matthew Wilcox Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/hugetlb.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 39579f98d6f3..18c92cb9bf43 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -4769,7 +4769,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud) if (!vma_shareable(vma, addr)) return (pte_t *)pmd_alloc(mm, pud, addr); - i_mmap_lock_write(mapping); + i_mmap_lock_read(mapping); vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) { if (svma == vma) continue; @@ -4799,7 +4799,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud) spin_unlock(ptl); out: pte = (pte_t *)pmd_alloc(mm, pud, addr); - i_mmap_unlock_write(mapping); + i_mmap_unlock_read(mapping); return pte; } -- 2.34.1