mm: Question about the use of 'accessed' flags and pte_young() helper

[Vlastimil Babka]

On 10/8/20 11:49 AM, Christophe Leroy wrote:
> In a 10 years old commit
> (https://github.com/linuxppc/linux/commit/d069cb4373fe0d451357c4d3769623a7564dfa9f), powerpc 8xx has
> made the handling of PTE accessed bit conditional to CONFIG_SWAP.
> Since then, this has been extended to some other powerpc variants.
> 
> That commit means that when CONFIG_SWAP is not selected, the accessed bit is not set by SW TLB miss
> handlers, leading to pte_young() returning garbage, or should I say possibly returning false
> allthough a page has been accessed since its access flag was reset.
> 
> Looking at various mm/ places, pte_young() is used independent of CONFIG_SWAP
> 
> Is it still valid the not manage accessed flags when CONFIG_SWAP is not selected ?

AFAIK it's wrong, reclaim needs it to detect accessed pages on inactive list, 
via page_referenced(), including file pages (page cache) where CONFIG_SWAP plays 
no role. Maybe it was different 10 years ago.

> If yes, should pte_young() always return true in that case ?

It should best work as intended. If not possible, true is maybe better, as false 
will lead to inactive file list thrashing.

> While we are at it, I'm wondering whether powerpc should redefine arch_faults_on_old_pte()
> On some variants of powerpc, accessed flag is managed by HW. On others, it is managed by SW TLB miss
> handlers via page fault handling.
> 
> Thanks
> Christophe
>