Commit a12083d721d7 added hugepd handling for gup-slow, reusing gup-fast
functions. follow_hugepd() correctly took the vma pointer in, however
didn't pass it over into the lower functions, which was overlooked.
The issue is gup_fast_hugepte() uses the vma pointer to make the correct
decision on whether an unshare is needed for a FOLL_PIN|FOLL_LONGTERM. Now
without vma ponter it will constantly return "true" (needs an unshare) for
a page cache, even though in the SHARED case it will be wrong to unshare.
The other problem is, even if an unshare is needed, it now returns 0 rather
than -EMLINK, which will not trigger a follow up FAULT_FLAG_UNSHARE fault.
That will need to be fixed too when the unshare is wanted.
gup_longterm test didn't expose this issue in the past because it didn't
yet test R/O unshare in this case, another separate patch will enable that
in future tests.
Fix it by passing vma correctly to the bottom, rename gup_fast_hugepte()
back to gup_hugepte() as it is shared between the fast/slow paths, and also
allow -EMLINK to be returned properly by gup_hugepte() even though gup-fast
will take it the same as zero.
Reported-by: David Hildenbrand <david@xxxxxxxxxx>
Fixes: a12083d721d7 ("mm/gup: handle hugepd for follow_page()")
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
v1: https://lore.kernel.org/r/20240428190151.201002-1-peterx@xxxxxxxxxx
This is v2 and dropped the 2nd test patch as a better one can come later,
this patch alone is kept untouched, added David's R-b. Should apply to
both mm-stable and mm-unstable. The target commit to be fixed should just
been moved into mm-stable, so no need to cc stable.
---
mm/gup.c | 64 ++++++++++++++++++++++++++++++++++----------------------
1 file changed, 39 insertions(+), 25 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c
index 2f7baf96f655..ca0f5cedce9b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -525,9 +525,17 @@ static unsigned long hugepte_addr_end(unsigned long addr, unsigned long end,
return (__boundary - 1 < end - 1) ? __boundary : end;
}
-static int gup_fast_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
- unsigned long end, unsigned int flags, struct page **pages,
- int *nr)
+/*
+ * Returns 1 if succeeded, 0 if failed, -EMLINK if unshare needed.
+ *
+ * NOTE: for the same entry, gup-fast and gup-slow can return different
+ * results (0 v.s. -EMLINK) depending on whether vma is available. This is
+ * the expected behavior, where we simply want gup-fast to fallback to
+ * gup-slow to take the vma reference first.
+ */
+static int gup_hugepte(struct vm_area_struct *vma, pte_t *ptep, unsigned long sz,
+ unsigned long addr, unsigned long end, unsigned int flags,
+ struct page **pages, int *nr)
{
unsigned long pte_end;
struct page *page;
@@ -559,9 +567,9 @@ static int gup_fast_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
return 0;
}
- if (!pte_write(pte) && gup_must_unshare(NULL, flags, &folio->page)) {
+ if (!pte_write(pte) && gup_must_unshare(vma, flags, &folio->page)) {
gup_put_folio(folio, refs, flags);
- return 0;
+ return -EMLINK;
}
*nr += refs;
@@ -577,19 +585,22 @@ static int gup_fast_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
* of the other folios. See writable_file_mapping_allowed() and
* gup_fast_folio_allowed() for more information.
*/
-static int gup_fast_hugepd(hugepd_t hugepd, unsigned long addr,
- unsigned int pdshift, unsigned long end, unsigned int flags,
- struct page **pages, int *nr)
+static int gup_hugepd(struct vm_area_struct *vma, hugepd_t hugepd,
+ unsigned long addr, unsigned int pdshift,
+ unsigned long end, unsigned int flags,
+ struct page **pages, int *nr)
{
pte_t *ptep;
unsigned long sz = 1UL << hugepd_shift(hugepd);
unsigned long next;
+ int ret;
ptep = hugepte_offset(hugepd, addr, pdshift);
do {
next = hugepte_addr_end(addr, end, sz);
- if (!gup_fast_hugepte(ptep, sz, addr, end, flags, pages, nr))
- return 0;
+ ret = gup_hugepte(vma, ptep, sz, addr, end, flags, pages, nr);
+ if (ret != 1)
+ return ret;
} while (ptep++, addr = next, addr != end);
return 1;
@@ -613,22 +624,25 @@ static struct page *follow_hugepd(struct vm_area_struct *vma, hugepd_t hugepd,
h = hstate_vma(vma);
ptep = hugepte_offset(hugepd, addr, pdshift);
ptl = huge_pte_lock(h, vma->vm_mm, ptep);
- ret = gup_fast_hugepd(hugepd, addr, pdshift, addr + PAGE_SIZE,
- flags, &page, &nr);
+ ret = gup_hugepd(vma, hugepd, addr, pdshift, addr + PAGE_SIZE,
+ flags, &page, &nr);
spin_unlock(ptl);
- if (ret) {
+ if (ret == 1) {
+ /* GUP succeeded */
WARN_ON_ONCE(nr != 1);
ctx->page_mask = (1U << huge_page_order(h)) - 1;
return page;
}
- return NULL;
+ /* ret can be either 0 (translates to NULL) or negative */
+ return ERR_PTR(ret);
}
#else /* CONFIG_ARCH_HAS_HUGEPD */
-static inline int gup_fast_hugepd(hugepd_t hugepd, unsigned long addr,
- unsigned int pdshift, unsigned long end, unsigned int flags,
- struct page **pages, int *nr)
+static inline int gup_hugepd(struct vm_area_struct *vma, hugepd_t hugepd,
+ unsigned long addr, unsigned int pdshift,
+ unsigned long end, unsigned int flags,
+ struct page **pages, int *nr)
{
return 0;
}
@@ -3261,8 +3275,8 @@ static int gup_fast_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr,
* architecture have different format for hugetlbfs
* pmd format and THP pmd format
*/
- if (!gup_fast_hugepd(__hugepd(pmd_val(pmd)), addr,
- PMD_SHIFT, next, flags, pages, nr))
+ if (gup_hugepd(NULL, __hugepd(pmd_val(pmd)), addr,
+ PMD_SHIFT, next, flags, pages, nr) != 1)
return 0;
} else if (!gup_fast_pte_range(pmd, pmdp, addr, next, flags,
pages, nr))
@@ -3291,8 +3305,8 @@ static int gup_fast_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr,
pages, nr))
return 0;
} else if (unlikely(is_hugepd(__hugepd(pud_val(pud))))) {
- if (!gup_fast_hugepd(__hugepd(pud_val(pud)), addr,
- PUD_SHIFT, next, flags, pages, nr))
+ if (gup_hugepd(NULL, __hugepd(pud_val(pud)), addr,
+ PUD_SHIFT, next, flags, pages, nr) != 1)
return 0;
} else if (!gup_fast_pmd_range(pudp, pud, addr, next, flags,
pages, nr))
@@ -3318,8 +3332,8 @@ static int gup_fast_p4d_range(pgd_t *pgdp, pgd_t pgd, unsigned long addr,
return 0;
BUILD_BUG_ON(p4d_leaf(p4d));
if (unlikely(is_hugepd(__hugepd(p4d_val(p4d))))) {
- if (!gup_fast_hugepd(__hugepd(p4d_val(p4d)), addr,
- P4D_SHIFT, next, flags, pages, nr))
+ if (gup_hugepd(NULL, __hugepd(p4d_val(p4d)), addr,
+ P4D_SHIFT, next, flags, pages, nr) != 1)
return 0;
} else if (!gup_fast_pud_range(p4dp, p4d, addr, next, flags,
pages, nr))
@@ -3347,8 +3361,8 @@ static void gup_fast_pgd_range(unsigned long addr, unsigned long end,
pages, nr))
return;
} else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
- if (!gup_fast_hugepd(__hugepd(pgd_val(pgd)), addr,
- PGDIR_SHIFT, next, flags, pages, nr))
+ if (gup_hugepd(NULL, __hugepd(pgd_val(pgd)), addr,
+ PGDIR_SHIFT, next, flags, pages, nr) != 1)
return;
} else if (!gup_fast_p4d_range(pgdp, pgd, addr, next, flags,
pages, nr))
--
2.44.0
