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
