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Date:   Wed, 17 Aug 2022 13:35:12 -0700
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Subject: Re: [PATCH v4 2/2] mm/gup.c: Refactor
 check_and_migrate_movable_pages()
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To:     Jason Gunthorpe <jgg@nvidia.com>,
        Alistair Popple <apopple@nvidia.com>
Cc:     linux-mm@kvack.org, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@amd.com>,
        Chaitanya Kulkarni <kch@nvidia.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        David Hildenbrand <david@redhat.com>,
        Matthew Wilcox <willy@infradead.org>
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From:   John Hubbard <jhubbard@nvidia.com>
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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