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 <longman@redhat.com>
Suggested-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 39579f98d6f3ca3e1e93918195b0f271d31ffe55..18c92cb9bf43b4b9ff9613759316cadd523c1cba 100644 (file)
--- 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;
 }