Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory | Date | Wed, 17 Feb 2021 18:06:39 +0100 |
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On 17.02.21 17:46, Dave Hansen wrote: > On 2/17/21 7:48 AM, David Hildenbrand wrote: >> While MADV_DONTNEED and FALLOC_FL_PUNCH_HOLE provide us ways to reliably >> discard memory, there is no generic approach to populate ("preallocate") >> memory. >> >> Although mmap() supports MAP_POPULATE, it is not applicable to the concept >> of sparse memory mappings, where we want to do populate/discard >> dynamically and avoid expensive/problematic remappings. In addition, >> we never actually report error during the final populate phase - it is >> best-effort only. > > Seems pretty sane to me. > > But, I was surprised that MADV_WILLNEED was no mentioned. It might be > nice to touch on on why MADV_WILLNEED is a bad choice for this > functionality? We could theoretically have it populate anonymous > mappings instead of just swapping in.
I stumbled over it, but it ended up looking like mixing in different semantics.
"Expect access in the near future." and "might be a good idea to read some pages" vs. "Definitely populate/preallocate all memory and definitely fail.".
> > I guess it's possible that folks are using MADV_WILLNEED on sparse > mappings that they don't want to populate, but it would be nice to get > that in the changelog.
Indeed: prime example is virtio-balloon in QEMU when deflating. Just because we are deflating the balloon doesn't mean that the guest is going to use all memory immediately - and that we want to actually consume memory immediately. ... we call MADV_WILLNEED unconditionally on any memory backing when deflating ...
I'll definitely add that to the changelog - thanks.
> > I was also a bit bummed to see the broad VM_IO/PFNMAP restriction show > up again. I was just looking at implementing pre-faulting for the new > SGX driver:
I added that because __mm_populate() similarly skips over VM_IO | VM_PFNMAP. So it mimics existing "populate semantics" we have.
> >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/cpu/sgx/driver.c > > It has a vm_ops->fault handler, but the VMAs are VM_IO. It obviously > don't work with gup, though. Not a deal breaker, and something we could > certainly add to this later.
I assume you would then also want to support MAP_POPULATE, right? Because it ends up using __mm_populate() and would not work.
Thanks!
-- Thanks,
David / dhildenb
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