Messages in this thread | | | Date | Mon, 22 Feb 2021 13:56:35 +0100 | From | Michal Hocko <> | Subject | Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory |
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On Sat 20-02-21 10:12:26, David Hildenbrand wrote: [...] > Thinking about MADV_POPULATE vs. MADV_POPULATE_WRITE I wonder if it would be > more versatile to break with existing MAP_POPULATE semantics and directly go > with > > MADV_POPULATE_READ: simulate user space read access without actually > reading. Trigger a read fault if required. > > MADV_POPULATE_WRITE: simulate user space write access without actually > writing. Trigger a write fault if required. > > For my use case, I could use MADV_POPULATE_WRITE on anonymous memory and > RAM-backed files (shmem/hugetlb) - I would not have a minor fault when the > guest inside the VM first initializes memory. This mimics how QEMU currently > preallocates memory. > > However, I would use MADV_POPULATE_READ on any !RAM-backed files where we > actually have to write-back to a (slow?) device. Dirtying everything > although the guest might not actually consume it in the near future might be > undesired.
Isn't what the current mm_populate does? if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE) gup_flags |= FOLL_WRITE;
So it will write fault to shared memory mappings but it will touch others.
-- Michal Hocko SUSE Labs
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