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

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On 02.08.23 17:16, Mel Gorman wrote:
On Tue, Aug 01, 2023 at 02:48:38PM +0200, David Hildenbrand wrote:
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@xxxxxxxxxx>

Maybe move the follow_trans_huge_pmd() declaration from linux/huge_mm.h
to mm/internal.h to discourage future mistakes? Otherwise


Makes sense.

Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Thanks!

--
Cheers,

David / dhildenb




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