linux-ext4 - Re: [PATCH v3 03/10] mm/gup: fail get_user_pages for LONGTERM dev coherent type

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Date:   Thu, 20 Jan 2022 12:36:21 +0000
From:   Joao Martins <joao.m.martins@...cle.com>
To:     Alex Sierra <alex.sierra@....com>
Cc:     amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        hch@....de, jglisse@...hat.com, apopple@...dia.com,
        willy@...radead.org, akpm@...ux-foundation.org,
        Felix.Kuehling@....com, linux-mm@...ck.org, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        jgg@...dia.com
Subject: Re: [PATCH v3 03/10] mm/gup: fail get_user_pages for LONGTERM dev
 coherent type

On 1/10/22 22:31, 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..9c8a075d862d 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_device_page(head)) {
> +			WARN_ON_ONCE(is_device_private_page(head));
> +			ret = -EFAULT;
> +			goto unpin_pages;
> +		}
> +

Wouldn't be more efficient for you failing earlier instead of after all the pages are pinned?

Filesystem DAX suffers from a somewhat similar issue[0] -- albeit it's more related to
blocking FOLL_LONGTERM in gup-fast while gup-slow can still do it. Coherent devmap appears
to want to block it in all gup.

On another thread Jason was suggesting about having different pgmap::flags to capture
these special cases[1] instead of selecting what different pgmap types can do in various
different places.

[0] https://lore.kernel.org/linux-mm/6a18179e-65f7-367d-89a9-d5162f10fef0@oracle.com/
[1] https://lore.kernel.org/linux-mm/20211019160136.GH3686969@ziepe.ca/