linux-kernel - Re: [PATCH v2 2/2] mm/gup.c: Refactor check_and_migrate_movable

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Date:   Fri, 5 Aug 2022 10:12:52 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org
Cc:     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>,
        Jason Gunthorpe <jgg@...dia.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>
Subject: Re: [PATCH v2 2/2] mm/gup.c: Refactor
 check_and_migrate_movable_pages()

On 05.08.22 08:29, 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>
> 
> ---
> 
> Changes for v2:
> 
>  - Split into different functions as suggested by John.
>  - Made error handling more conventional as requested by Jason.
> 
> Personally I'm not entirely convinced the conventional approach to error
> handling is easier to follow here but have left it in for feedback as I
> might be getting too familiar with the code.
> 
> Originally posted as "mm/gup.c: Simplify and fix
> check_and_migrate_movable_pages() return codes"[1].
> 
> Changes from that version:
> 
>  - Restore the original isolation failure behaviour and don't fail the
>    pup. Instead retry indefinitely.
>  - Unpin all pages on retry or failure rather than just failure.
> 
> [1] https://lore.kernel.org/linux-mm/814dee5d3aadd38c3370eaaf438ba7eee9bf9d2b.1659399696.git-series.apopple@nvidia.com/
> ---
>  mm/gup.c | 159 ++++++++++++++++++++++++++++++--------------------------
>  1 file changed, 88 insertions(+), 71 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index e26ccc0..60cb30a 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1900,20 +1900,14 @@ struct page *get_dump_page(unsigned long addr)
>  #endif /* CONFIG_ELF_CORE */
>  
>  #ifdef CONFIG_MIGRATION
> -/*
> - * Check whether all pages are pinnable, if so return number of pages.  If some
> - * pages are not pinnable, migrate them, and unpin all pages. Return zero if
> - * pages were migrated, or if some pages were not successfully isolated.
> - * Return negative error if migration fails.
> - */
> -static long check_and_migrate_movable_pages(unsigned long nr_pages,
> -					    struct page **pages)
> +static int collect_unpinnable_pages(struct list_head *movable_page_list,


If we're already renaming stuff, can we make it clearer, that we are
dealing with *longterm* unpinnable pages?


-- 
Thanks,

David / dhildenb