On Jun 14, 2022, at 8:22 AM, David Hildenbrand <david@xxxxxxxxxx> wrote: > On 13.06.22 22:40, Nadav Amit wrote: >> From: Nadav Amit <namit@xxxxxxxxxx> >> >> 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@xxxxxxxxxx> >> Cc: Hugh Dickins <hughd@xxxxxxxxxx> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> >> Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx> >> Cc: Peter Xu <peterx@xxxxxxxxxx> >> Cc: David Hildenbrand <david@xxxxxxxxxx> >> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx> >> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx> >> >> --- >> >> 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?
