John Hubbard jhubbard@nvidia.com 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,