linux-kernel - Re: [PATCH] mm/gup.c: Simplify and fix check_and_migrate_movable

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Date:   Mon, 01 Aug 2022 12:38:05 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>, akpm@...ux-foundation.org,
        minchan@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, pasha.tatashin@...een.com
Subject: Re: [PATCH] mm/gup.c: Simplify and fix
 check_and_migrate_movable_pages() return codes


John Hubbard <jhubbard@...dia.com> writes:

> On 7/29/22 12:44, Jason Gunthorpe wrote:
>
>> I came up with this ontop:
>>
> That cleans it up even more, looks nice. I'd go just a touch further,
> and also (unless there is some odd reason?) stay with -EAGAIN rather
> than -EBUSY, because otherwise both the function's comment header, and
> the caller, should change from -EBUSY to -EAGAIN just for consistency.

We return both:

-EBUSY to indicate that we should fail the entire PUP operation. -EAGAIN
 to indicate to __gup_longterm_locked() that the pages have been
 unpinned and it should repin them and check again.

> And also because the way it's used: the caller is literally "trying
> again".
>
> So on top of the ontop:
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 43c1fc532842..5f04033ee0ed 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1901,10 +1901,12 @@ struct page *get_dump_page(unsigned long addr)
>
>  #ifdef CONFIG_MIGRATION
>  /*
> - * Check whether all pages are pinnable. If some pages are not pinnable migrate
> - * them and unpin all the pages. Returns -EAGAIN if pages were unpinned or zero
> - * if all pages are pinnable and in the right zone. Other errors indicate
> - * migration failure.
> + * Check whether all pages are pinnable. If some pages are not pinnable, migrate
> + * them and unpin all the pages.
> + * Return values:
> + *       0:      all pages are already pinnable and in the right zone
> + *      -EAGAIN: some pages were unpinned or were zero
> + *      -ENOMEM: migration of some pages failed
>   */
>  static long check_and_migrate_movable_pages(unsigned long nr_pages,
>  					    struct page **pages,
> @@ -1914,11 +1916,11 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
>  		.nid = NUMA_NO_NODE,
>  		.gfp_mask = GFP_USER | __GFP_NOWARN,
>  	};
> -	unsigned long i;
> +	unsigned long i, not_migrated;
>  	struct folio *prev_folio = NULL;
>  	LIST_HEAD(movable_page_list);
>  	bool drain_allow = true, coherent_pages = false;
> -	int ret = -EBUSY;
> +	int ret = -EAGAIN;
>
>  	for (i = 0; i < nr_pages; i++) {
>  		struct folio *folio = page_folio(pages[i]);
> @@ -1990,7 +1992,7 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
>
>  	/*
>  	 * Unpin all pages. If device coherent pages were found
> -	 * migrate_deivce_coherent_page() will have dropped the pin and set
> +	 * migrate_device_coherent_page() will have dropped the pin and set
>  	 * pages[i] == NULL.
>  	 */
>  	for (i = 0; i < nr_pages; i++) {
>
>
> thanks,