Re: [RFC PATCH 0/3] Introduce new huge_ptep_get_access_flags() interface

[Muchun Song <songmuchun@bytedance.com>]

On Sun, May 08, 2022 at 06:08:18PM +0100, Matthew Wilcox wrote:
> On Sun, May 08, 2022 at 04:58:51PM +0800, Baolin Wang wrote:
> > As Mike pointed out [1], the huge_ptep_get() will only return one specific
> > pte value for the CONT-PTE or CONT-PMD size hugetlb on ARM64 system, which
> > will not take into account the subpages' dirty or young bits of a CONT-PTE/PMD
> > size hugetlb page. That will make us miss dirty or young flags of a CONT-PTE/PMD
> > size hugetlb page for those functions that want to check the dirty or
> > young flags of a hugetlb page. For example, the gather_hugetlb_stats() will
> > get inaccurate dirty hugetlb page statistics, and the DAMON for hugetlb monitoring
> > will also get inaccurate access statistics.
> > 
> > To fix this issue, one approach is that we can define an ARM64 specific huge_ptep_get()
> > implementation, which will take into account any subpages' dirty or young bits.
> > However we should add a new parameter for ARM64 specific huge_ptep_get() to check
> > how many continuous PTEs or PMDs in this CONT-PTE/PMD size hugetlb, that means we
> > should convert all the places using huge_ptep_get(), meanwhile most places using
> > huge_ptep_get() did not care about the dirty or young flags at all.
> > 
> > So instead of changing the prototype of huge_ptep_get(), this patch set introduces
> > a new huge_ptep_get_access_flags() interface and define an ARM64 specific implementation,
> > that will take into account any subpages' dirty or young bits for CONT-PTE/PMD size
> > hugetlb page. And we can only change to use huge_ptep_get_access_flags() for those
> > functions that care about the dirty or young flags of a hugetlb page.
> 
> I question whether this is the right approach.  I understand that
> different hardware implementations have different requirements here,
> but at least one that I'm aware of (AMD Zen 2/3) requires that all
> PTEs that are part of a contig PTE must have identical A/D bits.  Now,
> you could say that's irrelevant because it's x86 and we don't currently
> support contPTE on x86, but I wouldn't be surprised to see that other
> hardware has the same requirement.
> 
> So what if we make that a Linux requirement?  Setting a contPTE dirty or
> accessed becomes a bit more expensive (although still one/two cachelines,
> so not really much more expensive than a single write).  Then there's no
> need to change the "get" side of things because they're always identical.
> 
> It does mean that we can't take advantage of hardware setting A/D bits,
> unless hardware can be persuaded to behave this way.  I don't have any
> ARM specs in front of me to check.
>

I have looked at the comments in get_clear_flush() (in arch/arm64/mm/hugetlbpage.c).
That says:

	/*
	 * If HW_AFDBM is enabled, then the HW could turn on
	 * the dirty or accessed bit for any page in the set,
	 * so check them all.
	 */

Unfortunately, the AD bits are not identical in all subpages.

Thanks.