[PATCH v2 2/8] smaps: use vm_normal_page_pmd() instead of follow_trans_huge_pmd()

[David Hildenbrand <david@redhat.com>]

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.

Fixes: ff9f47f6f00c ("mm: proc: smaps_rollup: do not stall write attempts on mmap_lock")
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 fs/proc/task_mmu.c | 3 +--
 1 file changed, 1 insertion(+), 2 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);
 
-- 
2.41.0