linux-kernel - Re: [PATCH v4 2/2] mm/gup.c: Refactor check_and_migrate_movable

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Message-ID: <YvpxOyrDBUHagpC6@nvidia.com>
Date:   Mon, 15 Aug 2022 13:15:55 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Muchun Song <songmuchun@...edance.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v4 2/2] mm/gup.c: Refactor
 check_and_migrate_movable_pages()

On Fri, Aug 12, 2022 at 12:13:09PM +1000, Alistair Popple wrote:
> When pinning pages with FOLL_LONGTERM check_and_migrate_movable_pages()
> is called to migrate pages out of zones which should not contain any
> longterm pinned pages.
> 
> When migration succeeds all pages will have been unpinned so pinning
> needs to be retried. Migration can also fail, in which case the pages
> will also have been unpinned but the operation should not be retried. If
> all pages are in the correct zone nothing will be unpinned and no retry
> is required.
> 
> The logic in check_and_migrate_movable_pages() tracks unnecessary state
> and the return codes for each case are difficult to follow. Refactor the
> code to clean this up. No behaviour change is intended.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> 
> ---

This seems like the cleanest version yet!

> +static long check_and_migrate_movable_pages(unsigned long nr_pages,
> +					    struct page **pages)
> +{
> +	int ret;
> +	unsigned long collected;
> +	LIST_HEAD(movable_page_list);
> +
> +	collected = collect_longterm_unpinnable_pages(&movable_page_list, nr_pages, pages);
> +	if (!collected)
> +		return 0;
> +
> +	ret = migrate_longterm_unpinnable_pages(&movable_page_list, nr_pages, pages);
> +	if (!ret)
> +		return -EAGAIN;
> +	else
> +		return ret;

I would drop the else path and just return zero

Arguably migrate_longterm_unpinnable_pages() should do the same?

> @@ -2051,10 +2079,10 @@ static long __gup_longterm_locked(struct mm_struct *mm,
>  			break;
>  
>  		rc = check_and_migrate_movable_pages(rc, pages);
> -	} while (!rc);
> +	} while (rc == -EAGAIN);

Since the only reader only cares about errno or not errno..

But no biggie either way

Thanks,
Jason