mm/gup: fix hugepd handling in hugetlb rework

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.

Link: https://lkml.kernel.org/r/20240430131303.264331-1-peterx@redhat.com
Fixes: a12083d721d7 ("mm/gup: handle hugepd for follow_page()")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reported-by: David Hildenbrand <david@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/gup.c b/mm/gup.c
index 2f7baf96f655936d5085f17557d8ea14e6f24634..ca0f5cedce9b26c0b68efdb636ad129e304acb83 100644 (file)
--- 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))