Re: [PATCH v4 2/2] mm/gup.c: Refactor check_and_migrate_movable_pages()

[John Hubbard]

On 8/17/22 05:09, Jason Gunthorpe wrote:
> > > if (!ret) == if (ret != 0)
> >
> > Huh? Unless I'm misinterpretting you or am really behind on coffee
> > if (!ret) is equivalent to if (ret == 0), not if (ret != 0).

Although the !ret idiom is a huge, huge part of kernel coding practice,
it also adds a slight bit of unnecessary mental translation (go ahead,
flame me hard, I know I'm asking for it...but...see above, after all.
Pixels have been spilt working through it).

And also...

> Oh Dear, maybe I am still a bit jetlagged
>
> Regardless, it is confusingly written :)

...this is a slightly tricky and unusual error case, so it helps to be
extra clear by comparing against zero, and even adding a few short comments.

How's this look to you:

	collected = collect_longterm_unpinnable_pages(&movable_page_list,
						      nr_pages, pages);
	if (collected == 0)
		return 0;

	ret = migrate_longterm_unpinnable_pages(&movable_page_list, nr_pages,
						pages);

	/* If we got here, we have some unpinnable pages... */

	if (ret == 0) {
		/*
		 * ...and we successfully migrated those pages. Which means that
		 * the caller should retry the operation now.
		 */
		ret = -EAGAIN;
	}

	return ret;


thanks,
--
John Hubbard
NVIDIA