Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory

[David Hildenbrand]

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