Messages in this thread | | | From | Vlastimil Babka <> | Subject | Re: mm: Question about the use of 'accessed' flags and pte_young() helper | Date | Tue, 20 Oct 2020 17:52:07 +0200 |
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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 >
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