On Tue, Aug 13, 2019 at 06:24:51PM +0200, Oleg Nesterov wrote:
> > Let me see first that my explanation makes sense :P
>
> It does ;)
Does it look fine to you? It's on top of Song's patchset.
>From 58834d6c1e63321af742b208558a6b5cb86fc7ec Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Date: Fri, 16 Aug 2019 17:50:41 +0300
Subject: [PATCH] khugepaged: Add comments for retract_page_tables()
Oleg Nesterov pointed that logic behind checks in retract_page_tables()
are not obvious.
Add comments to clarify the reasoning for the checks and why they are
safe.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
mm/khugepaged.c | 28 +++++++++++++++++++++++-----
1 file changed, 23 insertions(+), 5 deletions(-)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 5c0a5f0826b2..00cec6a127aa 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1421,7 +1421,22 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
i_mmap_lock_write(mapping);
vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
- /* probably overkill */
+ /*
+ * Check vma->anon_vma to exclude MAP_PRIVATE mappings that
+ * got written to. These VMAs are likely not worth investing
+ * down_write(mmap_sem) as PMD-mapping is likely to be split
+ * later.
+ *
+ * Not that vma->anon_vma check is racy: it can be set up after
+ * the check but before we took mmap_sem by the fault path.
+ * But page lock would prevent establishing any new ptes of the
+ * page, so we are safe.
+ *
+ * An alternative would be drop the check, but check that page
+ * table is clear before calling pmdp_collapse_flush() under
+ * ptl. It has higher chance to recover THP for the VMA, but
+ * has higher cost too.
+ */
if (vma->anon_vma)
continue;
addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
@@ -1434,9 +1449,10 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
continue;
/*
* We need exclusive mmap_sem to retract page table.
- * If trylock fails we would end up with pte-mapped THP after
- * re-fault. Not ideal, but it's more important to not disturb
- * the system too much.
+ *
+ * We use trylock due to lock inversion: we need to acquire
+ * mmap_sem while holding page lock. Fault path does it in
+ * reverse order. Trylock is a way to avoid deadlock.
*/
if (down_write_trylock(&vma->vm_mm->mmap_sem)) {
spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd);
@@ -1446,8 +1462,10 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
up_write(&vma->vm_mm->mmap_sem);
mm_dec_nr_ptes(vma->vm_mm);
pte_free(vma->vm_mm, pmd_pgtable(_pmd));
- } else
+ } else {
+ /* Try again later */
khugepaged_add_pte_mapped_thp(vma->vm_mm, addr);
+ }
}
i_mmap_unlock_write(mapping);
}
--
Kirill A. Shutemov
