Messages in this thread |  | | Date | Mon, 22 Sep 2025 18:24:33 +0100 | | From | Catalin Marinas <> | | Subject | Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing zero-filled subpages |
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On Mon, Sep 22, 2025 at 10:14:58AM +0800, Lance Yang wrote: > From: Lance Yang <lance.yang@linux.dev> > > When both THP and MTE are enabled, splitting a THP and replacing its > zero-filled subpages with the shared zeropage can cause MTE tag mismatch > faults in userspace. > > Remapping zero-filled subpages to the shared zeropage is unsafe, as the > zeropage has a fixed tag of zero, which may not match the tag expected by > the userspace pointer. > > KSM already avoids this problem by using memcmp_pages(), which on arm64 > intentionally reports MTE-tagged pages as non-identical to prevent unsafe > merging. > > As suggested by David[1], this patch adopts the same pattern, replacing the > memchr_inv() byte-level check with a call to pages_identical(). This > leverages existing architecture-specific logic to determine if a page is > truly identical to the shared zeropage. > > Having both the THP shrinker and KSM rely on pages_identical() makes the > design more future-proof, IMO. Instead of handling quirks in generic code, > we just let the architecture decide what makes two pages identical. > > [1] https://lore.kernel.org/all/ca2106a3-4bb2-4457-81af-301fd99fbef4@redhat.com > > Cc: <stable@vger.kernel.org> > Reported-by: Qun-wei Lin <Qun-wei.Lin@mediatek.com> > Closes: https://lore.kernel.org/all/a7944523fcc3634607691c35311a5d59d1a3f8d4.camel@mediatek.com > Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") > Suggested-by: David Hildenbrand <david@redhat.com> > Signed-off-by: Lance Yang <lance.yang@linux.dev>
Functionally, the patch looks fine, both with and without MTE.
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 32e0ec2dde36..28d4b02a1aa5 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -4104,29 +4104,20 @@ static unsigned long deferred_split_count(struct shrinker *shrink, > static bool thp_underused(struct folio *folio) > { > int num_zero_pages = 0, num_filled_pages = 0; > - void *kaddr; > int i; > > for (i = 0; i < folio_nr_pages(folio); i++) { > - kaddr = kmap_local_folio(folio, i * PAGE_SIZE); > - if (!memchr_inv(kaddr, 0, PAGE_SIZE)) { > - num_zero_pages++; > - if (num_zero_pages > khugepaged_max_ptes_none) { > - kunmap_local(kaddr); > + if (pages_identical(folio_page(folio, i), ZERO_PAGE(0))) { > + if (++num_zero_pages > khugepaged_max_ptes_none) > return true;
I wonder what the overhead of doing a memcmp() vs memchr_inv() is. The former will need to read from two places. If it's noticeable, it would affect architectures that don't have an MTE equivalent.
Alternatively we could introduce something like folio_has_metadata() which on arm64 simply checks PG_mte_tagged.
-- Catalin
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