Re: [PATCH 4/5] teach smaps_pte_range() about THP pmds
From: Andrea Arcangeli <hidden>
Date: 2011-02-10 18:08:20
Also in:
lkml
On Wed, Feb 09, 2011 at 11:54:11AM -0800, Dave Hansen wrote:
quoted hunk ↗ jump to hunk
@@ -385,8 +387,16 @@ static int smaps_pte_range(pmd_t *pmd, u pte_t *pte; spinlock_t *ptl; - split_huge_page_pmd(walk->mm, pmd); - + if (pmd_trans_huge(*pmd)) { + if (pmd_trans_splitting(*pmd)) { + spin_unlock(&walk->mm->page_table_lock); + wait_split_huge_page(vma->anon_vma, pmd); + spin_lock(&walk->mm->page_table_lock);
the locking looks wrong, who is taking the &walk->mm->page_table_lock,
and isn't this going to deadlock on the pte_offset_map_lock for
NR_CPUS < 4, and where is it released? This spin_lock don't seem
necessary to me.
The right locking would be:
spin_lock(&walk->mm->page_table_lock);
if (pmd_trans_huge(*pmd)) {
if (pmd_trans_splitting(*pmd)) {
spin_unlock(&walk->mm->page_table_lock);
wait_split_huge_page(vma->anon_vma, pmd);
} else {
smaps_pte_entry(*(pte_t *)pmd, addr, HPAGE_SIZE, walk);
spin_unlock(&walk->mm->page_table_lock);
return 0;
}
I think it worked because you never run into a pmd_trans_splitting pmd
yet, and you were running smaps_pte_entry lockless which could race
against split_huge_page (but it normally doesn't).
+ } else {
+ smaps_pte_entry(*(pte_t *)pmd, addr, HPAGE_SIZE, walk);
+ return 0;
+ }
+ }
pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
for (; addr != end; pte++, addr += PAGE_SIZE)
smaps_pte_entry(*pte, addr, PAGE_SIZE, walk);
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