Re: [PATCH RFC] userfaultfd: introduce UFFDIO_COPY_MODE_YOUNG

[David Hildenbrand <david@redhat.com>]

On 13.06.22 22:40, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> As we know, using a PTE on x86 with cleared access-bit (aka young-bit)
> takes ~600 cycles more than when the access-bit is set. At the same
> time, setting the access-bit for memory that is not used (e.g.,
> prefetched) can introduce greater overheads, as the prefetched memory is
> reclaimed later than it should be.
> 
> Userfaultfd currently does not set the access-bit (excluding the
> huge-pages case). Arguably, it is best to let the uffd monitor control
> whether the access-bit should be set or not. The expected use is for the
> monitor to request userfaultfd to set the access-bit when the copy
> operation is done to resolve a page-fault, and not to set the young-bit
> when the memory is prefetched.

Thinking out loud about existing users: postcopy live migration in QEMU
has two usage for placement of pages

a) Resolving a fault. E.g., a VCPU might be waiting for resolution to
   make progress.
b) Background migration to converge without faults on all relevant
   pages.

I guess in a) we'd want UFFDIO_COPY_MODE_YOUNG in b) we don't want it.


I wonder, however, instead of calling this "young", which implies what
the OS should or shouldn't do, to define this as a hint that the placed
page is very likely to be accessed next.

I'm bad at naming, UFFDIO_COPY_MODE_ACCESS_LIKELY would express what I
have in mind.

> 
> Introduce UFFDIO_COPY_MODE_YOUNG to enable userspace to request the
> young bit to be set. For UFFDIO_CONTINUE and UFFDIO_ZEROPAGE set the bit
> unconditionally since the former is only used to resolve page-faults and
> the latter would not benefit from not setting the access-bit.
> 
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Mike Rapoport <rppt@linux.ibm.com>
> Signed-off-by: Nadav Amit <namit@vmware.com>
> 
> ---
> 
> There are 2 possible enhancements:
> 
> 1. Use the flag to decide on whether to mark the PTE as dirty (for
> writable PTEs). I guess that setting the dirty-bit is as expensive as
> setting the access-bit, and setting it introduces similar tradeoffs,
> as mentioned above.
> 
> 2. Introduce a similar mode for write-protect and use this information
> for setting both the young and dirty bits. Makes one wonder whether
> mprotect() should also set the bit in certain cases...

I wonder if UFFDIO_COPY_MODE_READ_ACCESS_LIKELY vs.
UFFDIO_COPY_WRITE_ACCESS_LIKELY could evenmake sense. I feel like it could.

For example, QEMU knows if a page fault it's resolving was due to a read
or a write fault and could use that information accordingly. Of course,
we don't completely know if we currently have a read fault, if we could
get a write fault immediately after.

Especially in the context of UFFDIO_ZEROPAGE,
UFFDIO_ZEROPAGE_WRITE_ACCESS_LIKELY could ... not place the zeropage but
instead populate an actual page and mark it accessed+dirty. I even have
a use case for that ;)


The kernel could decide how to treat these hints -- for example, if it
doesn't want user space to mess with access/dirty bits, it could just
mostly ignore the hints.

-- 
Thanks,

David / dhildenb