Re: [GIT PULL] arm64 updates for 6.13-rc1

[David Hildenbrand]

On 04.12.24 16:50, Catalin Marinas wrote:
> On Wed, Dec 04, 2024 at 04:32:11PM +0100, David Hildenbrand wrote:
> > On 04.12.24 16:29, Catalin Marinas wrote:
> > > On Mon, Dec 02, 2024 at 08:22:57AM -0800, Yang Shi wrote:
> > > > On 11/28/24 1:56 AM, David Hildenbrand wrote:
> > > > > On 28.11.24 02:21, Yang Shi wrote:
> > > > > > > > diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
> > > > > > > > index 87b3f1a25535..ef303a2262c5 100644
> > > > > > > > --- a/arch/arm64/mm/copypage.c
> > > > > > > > +++ b/arch/arm64/mm/copypage.c
> > > > > > > > @@ -30,9 +30,9 @@ void copy_highpage(struct page *to, struct
> > > > > > > > page *from)
> > > > > > > >           if (!system_supports_mte())
> > > > > > > >               return;
> > > > > > > > -    if (folio_test_hugetlb(src) &&
> > > > > > > > -        folio_test_hugetlb_mte_tagged(src)) {
> > > > > > > > -        if (!folio_try_hugetlb_mte_tagging(dst))
> > > > > > > > +    if (folio_test_hugetlb(src)) {
> > > > > > > > +        if (!folio_test_hugetlb_mte_tagged(src) ||
> > > > > > > > +            !folio_try_hugetlb_mte_tagging(dst))
> > > > > > > >                   return;
> > > > > > > >               /*
> > > > > > > I wonder why we had a 'return' here originally rather than a
> > > > > > > WARN_ON_ONCE() as we do further down for the page case. Do you seen any
> > > > > > > issue with the hunk below? Destination should be a new folio and not
> > > > > > > tagged yet:
> > > > > >
> > > > > > Yes, I did see problem. Because we copy tags for all sub pages then set
> > > > > > folio mte tagged when copying the data for the first subpage. The
> > > > > > warning will be triggered when we copy the second subpage.
> > > > >
> > > > > It's rather weird, though. We're instructed to copy a single page, yet
> > > > > copy tags for all pages.
> > > > >
> > > > > This really only makes sense when called from folio_copy(), where we are
> > > > > guaranteed to copy all pages.
> > > > >
> > > > > I'm starting to wonder if we should be able to hook into / overload
> > > > > folio_copy() instead, to just handle the complete hugetlb copy ourselves
> > > > > in one shot, and assume that copy_highpage() will never be called for
> > > > > hugetlb pages (WARN and don't copy tags).
> > > >
> > > > Actually folio_copy() is just called by migration. Copy huge page in CoW is
> > > > more complicated and uses copy_user_highpage()->copy_highpage() instead of
> > > > folio_copy(). It may start the page copy from any subpage. For example, if
> > > > the CoW is triggered by accessing to the address in the middle of 2M. Kernel
> > > > may copy the second half first then the first half to guarantee the accessed
> > > > data in cache.
> > >
> > > Still trying to understand the possible call paths here. If we get a
> > > write fault on a large folio, does the core code allocate a folio of the
> > > same size for CoW or it starts with smaller ones? wp_page_copy()
> > > allocates order 0 AFAICT, though if it was a pmd fault, it takes a
> > > different path in handle_mm_fault(). But we also have huge pages using
> > > contiguous ptes.
> > >
> > > Unless the source and destinations folios are exactly the same size, it
> > > will break many assumptions in the code above. Going the other way
> > > around is also wrong, dst larger than src, we are not initialising the
> > > whole dst folio.
> > >
> > > Maybe going back to per-page PG_mte_tagged flag rather than per-folio
> > > would keep things simple, less risk of wrong assumptions.
> >
> > I think the magic bit here is that for hugetlb, we only get hugetlb folios
> > of the same size, and no mixtures.
>
> Ah, ok, we do check for this and only do the advance copy for hugetlb
> folios. I'd add a check for folio size just in case, something like
> below (I'll add some description and post it properly):
>
> diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
> index 87b3f1a25535..c3a83db46ec6 100644
> --- a/arch/arm64/mm/copypage.c
> +++ b/arch/arm64/mm/copypage.c
> @@ -30,11 +30,14 @@ void copy_highpage(struct page *to, struct page *from)
>   	if (!system_supports_mte())
>   		return;
>   
> -	if (folio_test_hugetlb(src) &&
> -	    folio_test_hugetlb_mte_tagged(src)) {
> -		if (!folio_try_hugetlb_mte_tagging(dst))
> +	if (folio_test_hugetlb(src)) {

To be safe, maybe also test that dst is a hugetlb folio? But maybe the 
implicit checks we added in folio_try_hugetlb_mte_tagging() will already 
check that.

> +		if (!folio_test_hugetlb_mte_tagged(src) ||
> +		    from != folio_page(src, 0) ||
> +		    WARN_ON_ONCE(folio_nr_pages(src) != folio_nr_pages(dst)))
>   			return;
>   
> +		WARN_ON_ONCE(!folio_try_hugetlb_mte_tagging(dst));
> +
>   		/*
>   		 * Populate tags for all subpages.
>   		 *


--
Cheers,

David / dhildenb