Re: [PATCHv2 3/3] mm: Use updated pmdp_invalidate() inteface to track dirty/accessed bits

On Fri, Jun 16, 2017 at 05:01:30PM +0530, Aneesh Kumar K.V wrote:

"Kirill A. Shutemov" [off-list ref] writes:

quoted

This patch uses modifed pmdp_invalidate(), that return previous value of pmd,
to transfer dirty and accessed bits.

Signed-off-by: Kirill A. Shutemov <redacted>
---
 fs/proc/task_mmu.c |  8 ++++----
 mm/huge_memory.c   | 29 ++++++++++++-----------------
 2 files changed, 16 insertions(+), 21 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index f0c8b33d99b1..f2fc1ef5bba2 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c

.....

quoted

@@ -1965,7 +1955,6 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 	page_ref_add(page, HPAGE_PMD_NR - 1);
 	write = pmd_write(*pmd);
 	young = pmd_young(*pmd);
-	dirty = pmd_dirty(*pmd);
 	soft_dirty = pmd_soft_dirty(*pmd);

 	pmdp_huge_split_prepare(vma, haddr, pmd);

@@ -1995,8 +1984,6 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 			if (soft_dirty)
 				entry = pte_mksoft_dirty(entry);
 		}
-		if (dirty)
-			SetPageDirty(page + i);
 		pte = pte_offset_map(&_pmd, addr);
 		BUG_ON(!pte_none(*pte));
 		set_pte_at(mm, addr, pte, entry);

@@ -2045,7 +2032,15 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 	 * and finally we write the non-huge version of the pmd entry with
 	 * pmd_populate.
 	 */
-	pmdp_invalidate(vma, haddr, pmd);
+	old = pmdp_invalidate(vma, haddr, pmd);
+
+	/*
+	 * Transfer dirty bit using value returned by pmd_invalidate() to be
+	 * sure we don't race with CPU that can set the bit under us.
+	 */
+	if (pmd_dirty(old))
+		SetPageDirty(page);
+
 	pmd_populate(mm, pmd, pgtable);

 	if (freeze) {

Can we invalidate the pmd early here ? ie, do pmdp_invalidate instead of
pmdp_huge_split_prepare() ?

I think we can. But it means we would block access to the page for longer
than it's necessary on most architectures. I guess it's not a bit deal.

Maybe as separate patch on top of this patchet? Aneesh, would you take
care of this?

-- 
 Kirill A. Shutemov