On 2024/8/21 17:53, David Hildenbrand wrote:

On 21.08.24 11:51, Qi Zheng wrote:

quoted

On 2024/8/21 17:41, David Hildenbrand wrote:

quoted

On 21.08.24 11:24, Qi Zheng wrote:

quoted

On 2024/8/21 17:17, LEROY Christophe wrote:

quoted

Le 21/08/2024 à 10:18, Qi Zheng a écrit :

quoted

In handle_pte_fault(), we may modify the vmf->pte after acquiring the
vmf->ptl, so convert it to using pte_offset_map_maywrite_nolock(). 
But
since we already do the pte_same() check, so there is no need to get
pmdval to do pmd_same() check, just pass NULL to pmdvalp parameter.

Signed-off-by: Qi Zheng <redacted>
---
     mm/memory.c | 9 +++++++--
     1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 93c0c25433d02..d3378e98faf13 100644
--- a/mm/memory.c
+++ b/mm/memory.c

@@ -5504,9 +5504,14 @@ static vm_fault_t handle_pte_fault(struct

vm_fault *vmf)
              * pmd by anon khugepaged, since that takes mmap_lock in
write
              * mode; but shmem or file collapse to THP could still
morph
              * it into a huge pmd: just retry later if so.
+         *
+         * Use the maywrite version to indicate that vmf->pte 
will be
+         * modified, but since we will use pte_same() to detect the
+         * change of the pte entry, there is no need to get pmdval.
              */
-        vmf->pte = pte_offset_map_nolock(vmf->vma->vm_mm, vmf->pmd,
-                         vmf->address, &vmf->ptl);
+        vmf->pte = pte_offset_map_maywrite_nolock(vmf->vma->vm_mm,
+                              vmf->pmd, vmf->address,
+                              NULL, &vmf->ptl);

I think we discussed that passing NULL should be forbidden for that
function.

Yes, but for some maywrite case, there is no need to get pmdval to
do pmd_same() check. So I passed NULL and added a comment to
explain this.

I wonder if it's better to pass a dummy variable instead. One has to 
think harder why that is required compared to blindly passing "NULL" :)

You are afraid that subsequent caller will abuse this function, right?
My initial concern was that this would add a useless local vaiable, but
perhaps that is not a big deal.

Both are fine for me. ;)