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

[Yu Zhao]

On Sun, Mar 14, 2021 at 10:51:03PM +0000, Matthew Wilcox wrote:
> On Sun, Mar 14, 2021 at 06:12:42PM -0400, Zi Yan wrote:
> > On 13 Mar 2021, at 2:57, 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.
> > >
> > > [1]: Intel 64 and IA-32 Architectures Software Developer's Manual
> > >      Volume 3 (October 2019), section 4.8
> > 
> > Just curious. Does this also apply to non-leaf PUD entries? Do you
> > mind sharing which sentence from the manual gives the information?
> 
> The first few sentences from 4.8:
> 
> : For any paging-structure entry that is used during linear-address
> : translation, bit 5 is the accessed flag. For paging-structure
> : entries that map a page (as opposed to referencing another paging
> : structure), bit 6 is the dirty flag. These flags are provided for
> : use by memory-management software to manage the transfer of pages and
> : paging structures into and out of physical memory.
> 
> : Whenever the processor uses a paging-structure entry as part of
> : linear-address translation, it sets the accessed flag in that entry
> : (if it is not already set).

As far as I know x86 is the one that supports this.

> The way they differentiate between the A and D bits makes it clear to
> me that the A bit is set at each level of the tree, but the D bit is
> only set on leaf entries.

And the difference makes perfect sense (to me). Kudos to Intel.