Messages in this thread | | | Date | Tue, 31 Oct 2023 13:03:53 +0100 | Subject | Re: [PATCH v6 0/9] variable-order, large folios for anonymous memory | From | David Hildenbrand <> |
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On 31.10.23 12:55, Ryan Roberts wrote: > On 31/10/2023 11:50, Ryan Roberts wrote: >> On 06/10/2023 21:06, David Hildenbrand wrote: >> [...] >>> >>> Change 2: sysfs interface. >>> >>> If we call it THP, it shall go under "/sys/kernel/mm/transparent_hugepage/", I >>> agree. >>> >>> What we expose there and how, is TBD. Again, not a friend of "orders" and >>> bitmaps at all. We can do better if we want to go down that path. >>> >>> Maybe we should take a look at hugetlb, and how they added support for multiple >>> sizes. What *might* make sense could be (depending on which values we actually >>> support!) >>> >>> >>> /sys/kernel/mm/transparent_hugepage/hugepages-64kB/ >>> /sys/kernel/mm/transparent_hugepage/hugepages-128kB/ >>> /sys/kernel/mm/transparent_hugepage/hugepages-256kB/ >>> /sys/kernel/mm/transparent_hugepage/hugepages-512kB/ >>> /sys/kernel/mm/transparent_hugepage/hugepages-1024kB/ >>> /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/ >>> >>> Each one would contain an "enabled" and "defrag" file. We want something minimal >>> first? Start with the "enabled" option. >>> >>> >>> enabled: always [global] madvise never >>> >>> Initially, we would set it for PMD-sized THP to "global" and for everything else >>> to "never". >> >> Hi David, >> >> I've just started coding this, and it occurs to me that I might need a small >> clarification here; the existing global "enabled" control is used to drive >> decisions for both anonymous memory and (non-shmem) file-backed memory. But the >> proposed new per-size "enabled" is implicitly only controlling anon memory (for >> now). >> >> 1) Is this potentially confusing for the user? Should we rename the per-size >> controls to "anon_enabled"? Or is it preferable to jsut keep it vague for now so >> we can reuse the same control for file-backed memory in future? >> >> 2) The global control will continue to drive the file-backed memory decision >> (for now), even when hugepages-2048kB/enabled != "global"; agreed? >> >> Thanks, >> Ryan >> > > Also, an implementation question: > > hugepage_vma_check() doesn't currently care whether enabled="never" for DAX VMAs > (although it does honour MADV_NOHUGEPAGE and the prctl); It will return true > regardless. Is that by design? It couldn't fathom any reasoning from the commit log:
The whole DAX "hugepage" and THP mixup is just plain confusing. We're simply using PUD/PMD mappings of DAX memory, and PMD/PTE- remap when required (VMA split I assume, COW).
It doesn't result in any memory waste, so who really cares how it's mapped? Apparently we want individual processes to just disable PMD/PUD mappings of DAX using the prctl and madvise. Maybe there are good reasons.
Looks like a design decision, probably some legacy leftovers.
-- Cheers,
David / dhildenb
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