Re: [PATCH RFC] userfaultfd: introduce UFFDIO_COPY_MODE_YOUNG

[Nadav Amit <nadav.amit@gmail.com>]

On Jun 14, 2022, at 8:22 AM, David Hildenbrand <david@redhat.com> wrote:

> 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.

How about UFFDIO_COPY_MODE_WILLNEED_READ ?

> 
>> 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.

I can do that. I think users can do the zero page-copy themselves today, but
whatever you prefer.

But, I cannot take it anymore: the list of arguments for uffd stuff is
crazy. I would like to collect all the possible arguments that are used for
uffd operation into some “struct uffd_op”.

Any objection?