John Hubbard <jhubbard@xxxxxxxxxx> writes: > On 2/11/22 18:51, Alistair Popple wrote: […] >>> See below… >>> >>>> + } >>>> + >>>> + pages[i] = migrate_device_page(head, gup_flags); >> migrate_device_page() will return a new page that has been correctly pinned >> with gup_flags by try_grab_page(). Therefore this page can still be released >> with unpin_user_page() or put_page() as appropriate for the given gup_flags. >> The reference we had on the source page (head) always gets dropped in >> migrate_vma_finalize(). > > OK. Good. > > The above would be good to have in a comment, right around here, imho. > Because we have this marvelous mix of references for migration (get_page()) > and other, and it’s a bit hard to see that it’s all correct without a > hint or two. Ok, will do. > > … >> Which unless I’ve missed something is still the correct thing to do. >> >>> This reminds me: out of the many things to monitor, the FOLL_PIN counts >>> in /proc/vmstat are especially helpful, whenever making changes to code >>> that deals with this: >>> >>> nr_foll_pin_acquired >>> nr_foll_pin_released >>> >>> …and those should normally be equal to each other when “at rest”. >>> > > I hope this is/was run, just to be sure? Thanks for the suggestion, these remain equal to each other after running hmm-tests which confirms everything is working as expected. > > thanks,
