On 05/08/2024 00:04, Usama Arif wrote: > > > On 01/08/2024 07:36, David Hildenbrand wrote: >>>> I just added a bunch of quick printfs to QEMU and ran a precopy+postcopy live migration. Looks like my assumption was right: >>>> >>>> On the destination: >>>> >>>> Writing received pages during precopy # ram_load_precopy() >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Writing received pages during precopy >>>> Disabling THP: MADV_NOHUGEPAGE # postcopy_ram_prepare_discard() >>>> Discarding pages # loadvm_postcopy_ram_handle_discard() >>>> Discarding pages >>>> Discarding pages >>>> Discarding pages >>>> Discarding pages >>>> Discarding pages >>>> Discarding pages >>>> Registering UFFD # postcopy_ram_incoming_setup() >>>> >>> >>> Thanks for this, yes it makes sense after you mentioned postcopy_ram_incoming_setup. >>> postcopy_ram_incoming_setup happens in the Listen phase, which is after the discard phase, so I was able to follow in code in qemu the same sequence of events that the above prints show. >> >> >> I just added another printf to postcopy_ram_supported_by_host(), where we temporarily do a UFFDIO_REGISTER on some test area. >> >> Sensing UFFD support # postcopy_ram_supported_by_host() >> Sensing UFFD support >> Writing received pages during precopy # ram_load_precopy() >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Writing received pages during precopy >> Disabling THP: MADV_NOHUGEPAGE # postcopy_ram_prepare_discard() >> Discarding pages # loadvm_postcopy_ram_handle_discard() >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Discarding pages >> Registering UFFD # postcopy_ram_incoming_setup() >> >> We could think about using this "ever user uffd" to avoid the shared zeropage in most processes. >> >> Of course, there might be other applications where that wouldn't work, but I think this behavior (write to area before enabling uffd) might be fairly QEMU specific already. >> >> Avoiding the shared zeropage has the benefit that a later write fault won't have to do a TLB flush and can simply install a fresh anon page. >> > > I checked CRIU and that does a check at the start as well before attempting to use uffd: https://github.com/checkpoint-restore/criu/blob/criu-dev/criu/kerndat.c#L1349 > > If writing to an area before enabling uffd is likely to be QEMU specific, then you make a good point to clear pte instead of using shared zeropage to avoid the TLB flush if uffd is ever used. > > I think "ever used uffd" would need to be tracked using mm_struct. This also won't cause an issue if the check is done in a parent process and the actual use is in a forked process, as copy_mm should take care of it. > The possibilities would then be: > 1) Have a new bit in mm->flags, set it in new_userfaultfd and test it in try_to_unmap_unused, but unfortunately all the bits in mm->flags are taken. > 2) We could use mm->def_flags as it looks like there is an unused bit (0x800) just before VM_UFFD_WP. But that makes the code confusing as its used to initialize the default flags for VMAs and is not supposed to be used as a "mm flag". > 3) Introducing mm->flags2 and set/test as 1. This would introduce a 8 byte overhead for all mm_structs. > > I am not sure either 2 or 3 are acceptable upstream, unless there is a need for more flags in the near future and the 8 byte overhead starts to make sense. Maybe we go with shared zeropage? There is an another option to use bit 32 of mm->flags for 64 bit kernel only for ever_used_uffd, but considering the 2 reasons below, I will send a v2 of the series (in a few days incase any more comments come up) with shared zeropage in all circumstances (and addressing the comments in the other patches). - "ever used uffd" is not a 100% safe, i.e. someone might not check uffd support before using it and do the same sequence of events as qemu precopy + postcopy (+ some bitmap to track and check whether to request a page from uffd handler). Its very unlikely that anyone else does this, but we have to cater for all current and future usecases. - If THP shrinker is splitting and pointing pages to a shared zeropage, then the page was considered "unused" and unlikely to have a write fault at some point in the near future, hence the probability of incurring that TLB flush on write fault is low.
