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@redhat.com
Maybe move the follow_trans_huge_pmd() declaration from linux/huge_mm.h to mm/internal.h to discourage future mistakes? Otherwise
Acked-by: Mel Gorman mgorman@techsingularity.net