On 08.10.24 05:53, Qi Zheng wrote:
Hi Jann,
On 2024/10/8 05:42, Jann Horn wrote:
[...]
diff --git a/mm/mremap.c b/mm/mremap.c
index 24712f8dbb6b..dda09e957a5d 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -238,6 +238,7 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
{
spinlock_t *old_ptl, *new_ptl;
struct mm_struct *mm = vma->vm_mm;
+ bool res = false;
pmd_t pmd;
if (!arch_supports_page_table_move())
@@ -277,19 +278,25 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
if (new_ptl != old_ptl)
spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
- /* Clear the pmd */
pmd = *old_pmd;
+
+ /* Racing with collapse? */
+ if (unlikely(!pmd_present(pmd) || pmd_leaf(pmd)))
Since we already hold the exclusive mmap lock, after a racing
with collapse occurs, the pmd entry cannot be refilled with
new content by page fault. So maybe we only need to recheck
pmd_none(pmd) here?
My thinking was that it is cheap and more future proof to check that we
really still have a page table here. For example, what if collapse code
is ever changed to replace the page table by the collapsed PMD?
So unless there is a good reason not to have this check here, I would
keep it like that.
Thanks!
--
Cheers,
David / dhildenb
