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

[Dave Hansen]

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 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.

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:

> 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.