| Message ID | 20240103091423.400294-10-peterx@redhat.com (mailing list archive) |
|---|---|
| State | Handled Elsewhere |
| Headers | show |
| Series | mm/gup: Unify hugetlb, part 2 | expand |
| Context | Check | Description |
|---|---|---|
| conchuod/vmtest-fixes-PR | fail | merge-conflict |
On Wed, Jan 03, 2024 at 05:14:19PM +0800, peterx@redhat.com wrote: > From: Peter Xu <peterx@redhat.com> > > Introduce "pud_t pud" in the function, so the code won't dereference *pudp > multiple time. Not only because that looks less straightforward, but also > because if the dereference really happened, it's not clear whether there > can be race to see different *pudp values if it's being modified at the > same time. > > Acked-by: James Houghton <jthoughton@google.com> > Signed-off-by: Peter Xu <peterx@redhat.com> > --- > mm/gup.c | 17 +++++++++-------- > 1 file changed, 9 insertions(+), 8 deletions(-) I think we have several more case like this, and I ceratinly agree code should not access a READ_ONCE variable more than once :( Reviewed-by: Jason Gunthorpe <jgg@nvidia.com> Jason
diff --git a/mm/gup.c b/mm/gup.c index b8a80e2bfe08..63845b3ec44f 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -753,26 +753,27 @@ static struct page *follow_pud_mask(struct vm_area_struct *vma, unsigned int flags, struct follow_page_context *ctx) { - pud_t *pud; + pud_t *pudp, pud; spinlock_t *ptl; struct page *page; struct mm_struct *mm = vma->vm_mm; - pud = pud_offset(p4dp, address); - if (pud_none(*pud)) + pudp = pud_offset(p4dp, address); + pud = READ_ONCE(*pudp); + if (pud_none(pud)) return no_page_table(vma, flags, address); - if (pud_devmap(*pud)) { - ptl = pud_lock(mm, pud); - page = follow_devmap_pud(vma, address, pud, flags, &ctx->pgmap); + if (pud_devmap(pud)) { + ptl = pud_lock(mm, pudp); + page = follow_devmap_pud(vma, address, pudp, flags, &ctx->pgmap); spin_unlock(ptl); if (page) return page; return no_page_table(vma, flags, address); } - if (unlikely(pud_bad(*pud))) + if (unlikely(pud_bad(pud))) return no_page_table(vma, flags, address); - return follow_pmd_mask(vma, address, pud, flags, ctx); + return follow_pmd_mask(vma, address, pudp, flags, ctx); } static struct page *follow_p4d_mask(struct vm_area_struct *vma,