We shouldn't be using a GUP-internal helper if it can be avoided.
Similar to smaps_pte_entry() that uses vm_normal_page(), let's use
vm_normal_page_pmd() that similarly refuses to return the huge zeropage.
In contrast to follow_trans_huge_pmd(), vm_normal_page_pmd():
(1) Will always return the head page, not a tail page of a THP.
If we'd ever call smaps_account with a tail page while setting "compound
= true", we could be in trouble, because smaps_account() would look at
the memmap of unrelated pages.
If we're unlucky, that memmap does not exist at all. Before we removed
PG_doublemap, we could have triggered something similar as in
commit 24d7275ce279 ("fs/proc: task_mmu.c: don't read mapcount for
migration entry").
This can theoretically happen ever since commit ff9f47f6f00c ("mm: proc:
smaps_rollup: do not stall write attempts on mmap_lock"):
(a) We're in show_smaps_rollup() and processed a VMA
(b) We release the mmap lock in show_smaps_rollup() because it is
contended
(c) We merged that VMA with another VMA
(d) We collapsed a THP in that merged VMA at that position
If the end address of the original VMA falls into the middle of a THP
area, we would call smap_gather_stats() with a start address that falls
into a PMD-mapped THP. It's probably very rare to trigger when not
really forced.
(2) Will succeed on a is_pci_p2pdma_page(), like vm_normal_page()
Treat such PMDs here just like smaps_pte_entry() would treat such PTEs.
If such pages would be anonymous, we most certainly would want to
account them.
(3) Will skip over pmd_devmap(), like vm_normal_page() for pte_devmap()
As noted in vm_normal_page(), that is only for handling legacy ZONE_DEVICE
pages. So just like smaps_pte_entry(), we'll now also ignore such PMD
entries.
Especially, follow_pmd_mask() never ends up calling
follow_trans_huge_pmd() on pmd_devmap(). Instead it calls
follow_devmap_pmd() -- which will fail if neither FOLL_GET nor FOLL_PIN
is set.
So skipping pmd_devmap() pages seems to be the right thing to do.
(4) Will properly handle VM_MIXEDMAP/VM_PFNMAP, like vm_normal_page()
We won't be returning a memmap that should be ignored by core-mm, or
worse, a memmap that does not even exist. Note that while
walk_page_range() will skip VM_PFNMAP mappings, walk_page_vma() won't.
Most probably this case doesn't currently really happen on the PMD level,
otherwise we'd already be able to trigger kernel crashes when reading
smaps / smaps_rollup.
So most probably only (1) is relevant in practice as of now, but could only
cause trouble in extreme corner cases.
Let's move follow_trans_huge_pmd() to mm/internal.h to discourage future
reuse in wrong context.
Fixes: ff9f47f6f00c ("mm: proc: smaps_rollup: do not stall write attempts on mmap_lock")
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
fs/proc/task_mmu.c | 3 +--
include/linux/huge_mm.h | 3 ---
mm/internal.h | 7 +++++++
3 files changed, 8 insertions(+), 5 deletions(-)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index bf25178ae66a..7a7d6e2e6a14 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -571,8 +571,7 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
bool migration = false;
if (pmd_present(*pmd)) {
- /* FOLL_DUMP will return -EFAULT on huge zero page */
- page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
+ page = vm_normal_page_pmd(vma, addr, *pmd);
} else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
swp_entry_t entry = pmd_to_swp_entry(*pmd);
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 20284387b841..e718dbe928ba 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -25,9 +25,6 @@ static inline void huge_pud_set_accessed(struct vm_fault *vmf, pud_t orig_pud)
#endif
vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf);
-struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
- unsigned long addr, pmd_t *pmd,
- unsigned int flags);
bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
pmd_t *pmd, unsigned long addr, unsigned long next);
int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, pmd_t *pmd,
diff --git a/mm/internal.h b/mm/internal.h
index 5a03bc4782a2..c94eda536c4c 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -941,6 +941,13 @@ int migrate_device_coherent_page(struct page *page);
struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags);
int __must_check try_grab_page(struct page *page, unsigned int flags);
+/*
+ * mm/huge_memory.c
+ */
+struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
+ unsigned long addr, pmd_t *pmd,
+ unsigned int flags);
+
enum {
/* mark page accessed */
FOLL_TOUCH = 1 << 16,
--
2.41.0
