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

[Matthew Wilcox]

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 don't have a hard objection to your approach, I just want to discuss
other possibilities.