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.
