Re: [PATCH 1/1] mm/thp: fix MTE tag mismatch when replacing zero-filled subpages

[Lance Yang]

On 2025/9/23 20:00, David Hildenbrand wrote:
> On 23.09.25 13:52, Catalin Marinas wrote:
> > On Mon, Sep 22, 2025 at 07:59:00PM +0200, David Hildenbrand wrote:
> > > On 22.09.25 19:24, Catalin Marinas wrote:
> > > > On Mon, Sep 22, 2025 at 10:14:58AM +0800, Lance Yang wrote:
> > > > > 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.
> > >
> > > We discussed something similar in the other thread (I suggested
> > > page_is_mergable()). I'd prefer to use pages_identical() for now, so 
> > > we have
> > > the same logic here and in ksm code.
> > >
> > > (this patch here almost looks like a cleanup :) )
> > >
> > > If this becomes a problem, what we could do is in pages_identical() 
> > > would be
> > > simply doing the memchr_inv() in case is_zero_pfn(). KSM might 
> > > benefit from
> > > that as well when merging with the shared zeropage through
> > > try_to_merge_with_zero_page().
> >
> > Yes, we can always optimise it later.
> >
> > I just realised that on arm64 with MTE we won't get any merging with the
> > zero page even if the user page isn't mapped with PROT_MTE. In
> > cpu_enable_mte() we zero the tags in the zero page and set
> > PG_mte_tagged. The reason is that we want to use the zero page with
> > PROT_MTE mappings (until tag setting causes CoW). Hmm, the arm64
> > memcmp_pages() messed up KSM merging with the zero page even before this
> > patch.
> >
> > The MTE tag setting evolved a bit over time with some locking using PG_*
> > flags to avoid a set_pte_at() race trying to initialise the tags on the
> > same page. We also moved the swap restoring to arch_swap_restore()
> > rather than the set_pte_at() path. So it is safe now to merge with the
> > zero page if the other page isn't tagged. A subsequent set_pte_at()
> > attempting to clear the tags would notice that the zero page is already
> > tagged.
> >
> > We could go a step further and add tag comparison (I had some code
> > around) but I think the quick fix is to just not treat the zero page as
> > tagged.
>
> I assume any tag changes would result in CoW.

Right, I did a test and confirmed that any attempt to change
tags on the shared zero page results in a Copy-on-Write, as
expected ;)

> It would be interesting to know if there are use cases with VMs or other 
> workloads where that could be beneficial with KSM.
>
> > Not fully tested yet:
> >
> > diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> > index e5e773844889..72a1dfc54659 100644
> > --- a/arch/arm64/kernel/mte.c
> > +++ b/arch/arm64/kernel/mte.c
> > @@ -73,6 +73,8 @@ int memcmp_pages(struct page *page1, struct page 
> > *page2)
> >   {
> >       char *addr1, *addr2;
> >       int ret;
> > +    bool page1_tagged = page_mte_tagged(page1) && !is_zero_page(page1);
> > +    bool page2_tagged = page_mte_tagged(page2) && !is_zero_page(page2);
> >       addr1 = page_address(page1);
> >       addr2 = page_address(page2);
> > @@ -83,11 +85,10 @@ int memcmp_pages(struct page *page1, struct page 
> > *page2)
> >       /*
> >        * If the page content is identical but at least one of the 
> > pages is
> > -     * tagged, return non-zero to avoid KSM merging. If only one of the
> > -     * pages is tagged, __set_ptes() may zero or change the tags of the
> > -     * other page via mte_sync_tags().
> > +     * tagged, return non-zero to avoid KSM merging. Ignore the zero 
> > page
> > +     * since it is always tagged with the tags cleared.
> >        */
> > -    if (page_mte_tagged(page1) || page_mte_tagged(page2))
> > +    if (page1_tagged || page2_tagged)
> >           return addr1 != addr2;
>
> That looks reasonable to me.
>
> @Lance as you had a test setup, could you give this a try as well with 
> KSM shared zeropage deduplication enabled whether it now works as 
> expected as well?

This works as expected. Both KSM (with use_zero_pages enabled) and
the THP shrinker are now able to successfully merge zero-filled
pages with the shared zero page, as long as those pages are not
mapped with PROT_MTE.

> Then, this should likely be an independent fix.

@Catalin could you send a real patch?