linux-ext4 - Re: [PATCH v4 03/10] mm/gup: fail get_user_pages for LONGTERM dev coherent type

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Date:   Fri, 28 Jan 2022 13:36:10 +1100
From:   Alistair Popple <apopple@...dia.com>
To:     <akpm@...ux-foundation.org>, <Felix.Kuehling@....com>,
        <linux-mm@...ck.org>, <rcampbell@...dia.com>,
        <linux-ext4@...r.kernel.org>, <linux-xfs@...r.kernel.org>,
        Alex Sierra <alex.sierra@....com>
CC:     <amd-gfx@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>,
        <hch@....de>, <jgg@...dia.com>, <jglisse@...hat.com>,
        <willy@...radead.org>
Subject: Re: [PATCH v4 03/10] mm/gup: fail get_user_pages for LONGTERM dev coherent type

On Thursday, 27 January 2022 2:09:42 PM AEDT Alex Sierra wrote:
> Avoid long term pinning for Coherent device type pages. This could
> interfere with their own device memory manager. For now, we are just
> returning error for PIN_LONGTERM Coherent device type pages. Eventually,
> these type of pages will get migrated to system memory, once the device
> migration pages support is added.
> 
> Signed-off-by: Alex Sierra <alex.sierra@....com>
> ---
>  mm/gup.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 886d6148d3d0..5291d7221826 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1720,6 +1720,12 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
>  		 * If we get a movable page, since we are going to be pinning
>  		 * these entries, try to move them out if possible.
>  		 */
> +		if (is_dev_private_or_coherent_page(head)) {
> +			WARN_ON_ONCE(is_device_private_page(head));
> +			ret = -EFAULT;
> +			goto unpin_pages;
> +		}
> +
>  		if (!is_pinnable_page(head)) {
>  			if (PageHuge(head)) {
>  				if (!isolate_huge_page(head, &movable_page_list))
> @@ -1750,6 +1756,7 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
>  	if (list_empty(&movable_page_list) && !isolation_error_count)
>  		return nr_pages;
>  
> +unpin_pages:
>  	if (gup_flags & FOLL_PIN) {
>  		unpin_user_pages(pages, nr_pages);
>  	} else {
 
If there is a mix of ZONE_MOVABLE and device pages the return value (ret) will
be subsequently lost here:

        if (!list_empty(&movable_page_list)) {
                ret = migrate_pages(&movable_page_list, alloc_migration_target,
                                    NULL, (unsigned long)&mtc, MIGRATE_SYNC,
                                    MR_LONGTERM_PIN, NULL);
                if (ret && !list_empty(&movable_page_list))
                        putback_movable_pages(&movable_page_list);
        }

Which won't actually cause any problems, but it will lead to the GUP getting
retried unnecessarily. I do still intend to address this with a series to
migrate pages instead though, so I think this is ok for now as it's an unlikely
corner case anyway. Therefore feel tree to add the below when you repost:

Reviewed-by: Alistair Poppple <apopple@...dia.com>