On 23.06.23 16:29, Peter Xu wrote:
follow_page() doesn't use FOLL_PIN, meanwhile hugetlb seems to not be the
target of FOLL_WRITE either. However add the checks.
Namely, either the need to CoW due to missing write bit, or proper
unsharing on !AnonExclusive pages over R/O pins to reject the follow page.
That brings this function closer to follow_hugetlb_page().
So we don't care before, and also for now. But we'll care if we switch
over slow-gup to use hugetlb_follow_page_mask(). We'll also care when to
return -EMLINK properly, as that's the gup internal api to mean "we should
unshare". Not really needed for follow page path, though.
When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be
clear that it just should never fail. When error happens, instead of
setting page==NULL, capture the errno instead.
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
mm/hugetlb.c | 31 ++++++++++++++++++++-----------
1 file changed, 20 insertions(+), 11 deletions(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f75f5e78ff0b..27367edf5c72 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6462,13 +6462,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
struct page *page = NULL;
spinlock_t *ptl;
pte_t *pte, entry;
-
- /*
- * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
- * follow_hugetlb_page().
- */
- if (WARN_ON_ONCE(flags & FOLL_PIN))
- return NULL;
+ int ret;
hugetlb_vma_lock_read(vma);
pte = hugetlb_walk(vma, haddr, huge_page_size(h));
@@ -6478,8 +6472,21 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
ptl = huge_pte_lock(h, mm, pte);
entry = huge_ptep_get(pte);
if (pte_present(entry)) {
- page = pte_page(entry) +
- ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
+ page = pte_page(entry);
+
+ if ((flags & FOLL_WRITE) && !huge_pte_write(entry)) {
+ page = NULL;
+ goto out;
+ }
+
+ if (gup_must_unshare(vma, flags, page)) {
+ /* Tell the caller to do unsharing */
+ page = ERR_PTR(-EMLINK);
+ goto out;
+ }
No need to check if the page is writable (like all other callers and as
gup_must_unshare() documents -- "for which pages that are
write-protected in the page table")
if (!huge_pte_write(entry) && gup_must_unshare(vma, flags, page)) {
With that
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
--
Cheers,
David / dhildenb
