Re: [RFC] pin_user_pages_fast failure count increased

[John Hubbard <jhubbard@nvidia.com>]

On 4/28/25 8:17 AM, Jaewon Kim wrote:
> Hi
> 
> If pin_user_pages_fast does not pin all the requested number of pages,
> then drivers calling to pin_user_pages_fast should retry until the gup
> pins all?
>

Approaches vary, for handling partial success of pin_user_pages().

* Many drivers unpin everything and either bail out entirely, or retry
pinning the entire original range.

* A few drivers try to pin the remain pages, in a retry loop (I think
the gpu/drm drivers IIRC).

It's really up to the driver author, how to respond to the inability
(which may be temporary) to pin the entire range all at once.

 
> Our GPU driver uses pin_user_pages_fast.
> After the recent kernel update, pin_user_pages_fast sometimes does not
> pin all the pages, nr_pinned < nr_pages.
> 
> I think the following patch affected the pin_user_pages_fast behavior.
> 1aaf8c122918 mm: gup: fix infinite loop within __get_longterm_locked
> 

OK. If you see a bug or a problem, please elaborate.

thanks,
-- 
John Hubbard