On 12/29/20 1:20 PM, Mike Kravetz wrote:
> On 12/28/20 8:21 PM, Li Xinhai wrote:
>> The current code would unnecessarily expand the address range. Consider
>> one example, (start, end) = (1G-2M, 3G+2M), and (vm_start, vm_end) =
>> (1G-4M, 3G+4M), the expected adjustment should be keep (1G-2M, 3G+2M)
>> without expand. But the current result will be (1G-4M, 3G+4M). Actually,
>> the range (1G-4M, 1G) and (3G, 3G+4M) would never been involved in pmd
>> sharing.
>>
>> After this patch, if pud aligned *start across vm_start, then we know the
>> *start and vm_start are in same pud_index, and vm_start is not pud
>> aligned, so don't adjust *start. Same logic applied to *end.
>>
>> Fixes: commit 75802ca66354 ("mm/hugetlb: fix calculation of adjust_range_if_pmd_sharing_possible")
>> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>> Cc: Peter Xu <peterx@xxxxxxxxxx>
>> Signed-off-by: Li Xinhai <lixinhai.lxh@xxxxxxxxx>
>
> Thank you. That does indeed fix an issue in the current code.
>
> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Upon further thought, this patch also expands the passed range when not
necessary. Consider the example (start, end) = (1G-6M, 1G-4M), and
(vm_start, vm_end) = (1G, 1G-2M). This patch would adjust the range to
(1G, 1G-4M). However, no adjustment should be performed as no sharing
is possible.
Below is proposed code to address the issue. I'm not sending a formal
patch yet as I would like comments on the code first. It is not a critical
issue and any fix can wait a bit.
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 7e89f31d7ef8..41ccec617f74 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5264,16 +5264,19 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
if (!(vma->vm_flags & VM_MAYSHARE))
return;
- /* Extend the range to be PUD aligned for a worst case scenario */
- a_start = ALIGN_DOWN(*start, PUD_SIZE);
- a_end = ALIGN(*end, PUD_SIZE);
-
/*
- * Intersect the range with the vma range, since pmd sharing won't be
- * across vma after all
+ * Check if start and end are within a PUD aligned range of the
+ * vma. If they are, then adjust to PUD alignment.
*/
- *start = max(vma->vm_start, a_start);
- *end = min(vma->vm_end, a_end);
+ a_start = ALIGN_DOWN(*start, PUD_SIZE);
+ a_end = ALIGN(*start, PUD_SIZE);
+ if (range_in_vma(vma, a_start, a_end))
+ *start = a_start;
+
+ a_start = ALIGN_DOWN(*end, PUD_SIZE);
+ a_end = ALIGN(*end, PUD_SIZE);
+ if (range_in_vma(vma, a_start, a_end))
+ *end = a_end;
}
/*
