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

[Aneesh Kumar K.V]

On Friday 16 June 2017 06:51 PM, Kirill A. Shutemov wrote:

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

"Kirill A. Shutemov" <kirill.shute...@linux.intel.com> writes:

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

Signed-off-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
---
  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

.....

@@ -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?

Yes, I cam do that.

-aneesh