Re: [PATCH v1 06/14] mm, x86: support the access bit on non-leaf PMD entries

[Dave Hansen]

On 3/12/21 11:57 PM, Yu Zhao wrote:
> Some architectures support the accessed bit on non-leaf PMD entries
> (parents) in addition to leaf PTE entries (children) where pages are
> mapped, e.g., x86_64 sets the accessed bit on a parent when using it
> as part of linear-address translation [1]. Page table walkers who are
> interested in the accessed bit on children can take advantage of this:
> they do not need to search the children when the accessed bit is not
> set on a parent, given that they have previously cleared the accessed
> bit on this parent in addition to its children.

I'd like to hear a *LOT* more about how this is going to be used.

The one part of this which is entirely missing is the interaction with
the TLB and mid-level paging structure caches.  The CPU is pretty
aggressive about setting no-leaf accessed bits when TLB entries are
created.  This *looks* to be depending on that behavior, but it would be
nice to spell it out explicitly.