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

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Message-ID: <dc8fd102-ba30-d980-bdbb-11f39326103d@nvidia.com>
Date:   Fri, 12 Aug 2022 00:05:18 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Cc:     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>,
        Jason Gunthorpe <jgg@...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 8/11/22 19:13, 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>

OK, I've finally convinced myself that this is a correct transformation.
This cleanup does help clarify things, definitely.

I've got two documentation additions (and changes) to suggest, below, and a
couple of too-long lines, but the code itself looks good, so with those
tweaks or something approximating them, please feel free to add:

Reviewed-by: John Hubbard <jhubbard@...dia.com>

...

> +/*
> + * 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.
> + */

Instead of the above, I'd like to suggest this:

/*
  * Check whether all pages are *allowed* to be pinned. Rather confusingly, all
  * pages in the range are required to be pinned via FOLL_PIN, before calling
  * this routine.
  *
  * If any pages in the range are not allowed to be pinned, then this routine
  * will migrate those pages away, unpin all the pages in the range and return
  * -EAGAIN. The caller should re-pin the entire range with FOLL_PIN and then
  * call this routine again.
  *
  * If an error other than -EAGAIN occurs, this indicates a migration failure.
  * The caller should give up, and propagate the error back up the call stack.
  *
  * If everything is OK and all pages in the range are allowed to be pinned, then
  * this routine leaves all pages pinned and returns zero for success.
  */

> +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);

There is no reason to exceed 80 cols here.

> +	if (!collected)
> +		return 0;
> +
> +	ret = migrate_longterm_unpinnable_pages(&movable_page_list, nr_pages, pages);

Nor here.

...

> @@ -2051,10 +2079,10 @@ static long __gup_longterm_locked(struct mm_struct *mm,
>   			break;

...and in this routine, let's fortify the comment like so:

@@ -2068,7 +2078,15 @@ static long __gup_longterm_locked(struct mm_struct *mm,
  	if (!(gup_flags & FOLL_LONGTERM))
  		return __get_user_pages_locked(mm, start, nr_pages, pages, vmas,
  					       NULL, gup_flags);
-	/* check_and_migrate_movable_pages() assumes pages have been pinned. */
+	/*
+	 * If we get to this point then FOLL_LONGTERM is set. And FOLL_LONGTERM
+	 * implies FOLL_PIN (although the reverse is not true). And that, in
+	 * turn, makes it correct to unconditionally call
+	 * check_and_migrate_movable_pages(), which assumes pages have been
+	 * pinned via FOLL_PIN.
+	 *
+	 * Enforce the above reasoning, by asserting that FOLL_PIN is set:
+	 */
  	if (WARN_ON(!(gup_flags & FOLL_PIN)))
  		return -EINVAL;
  	flags = memalloc_pin_save();


...and with that, it's actually possible for the reader to work their way
through this story, I think.


thanks,
-- 
John Hubbard
NVIDIA