linux-kernel - Re: [PATCH 3/5] mm/notifier: add migration invalidation type

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Message-ID: <20200710193939.GA2129070@nvidia.com>
Date:   Fri, 10 Jul 2020 16:39:39 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Ralph Campbell <rcampbell@...dia.com>
CC:     <linux-rdma@...r.kernel.org>, <linux-mm@...ck.org>,
        <nouveau@...ts.freedesktop.org>, <kvm-ppc@...r.kernel.org>,
        <linux-kselftest@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Ben Skeggs <bskeggs@...hat.com>,
        Bharata B Rao <bharata@...ux.ibm.com>
Subject: Re: [PATCH 3/5] mm/notifier: add migration invalidation type

On Mon, Jul 06, 2020 at 03:23:45PM -0700, Ralph Campbell wrote:
> Currently migrate_vma_setup() calls mmu_notifier_invalidate_range_start()
> which flushes all device private page mappings whether or not a page
> is being migrated to/from device private memory. In order to not disrupt
> device mappings that are not being migrated, shift the responsibility
> for clearing device private mappings to the device driver and leave
> CPU page table unmapping handled by migrate_vma_setup(). To support
> this, the caller of migrate_vma_setup() should always set struct
> migrate_vma::src_owner to a non NULL value that matches the device
> private page->pgmap->owner. This value is then passed to the struct
> mmu_notifier_range with a new event type which the driver's invalidation
> function can use to avoid device MMU invalidations.
> 
> Signed-off-by: Ralph Campbell <rcampbell@...dia.com>
>  include/linux/mmu_notifier.h | 7 +++++++
>  mm/migrate.c                 | 8 +++++++-
>  2 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> index fc68f3570e19..bd0b34dbe4de 100644
> +++ b/include/linux/mmu_notifier.h
> @@ -38,6 +38,10 @@ struct mmu_interval_notifier;
>   *
>   * @MMU_NOTIFY_RELEASE: used during mmu_interval_notifier invalidate to signal
>   * that the mm refcount is zero and the range is no longer accessible.
> + *
> + * @MMU_NOTIFY_MIGRATE: used during migrate_vma_collect() invalidate to signal
> + * a device driver to possibly ignore the invalidation if the src_own
> + * field matches.
>   */
>  enum mmu_notifier_event {
>  	MMU_NOTIFY_UNMAP = 0,
> @@ -46,6 +50,7 @@ enum mmu_notifier_event {
>  	MMU_NOTIFY_PROTECTION_PAGE,
>  	MMU_NOTIFY_SOFT_DIRTY,
>  	MMU_NOTIFY_RELEASE,
> +	MMU_NOTIFY_MIGRATE,
>  };
>  
>  #define MMU_NOTIFIER_RANGE_BLOCKABLE (1 << 0)
> @@ -264,6 +269,7 @@ struct mmu_notifier_range {
>  	unsigned long end;
>  	unsigned flags;
>  	enum mmu_notifier_event event;
> +	void *data;
>  };

This generic member usually ends up a bit ugly, can we do a tagged
union instead?

union
{
     void *migrate_pgmap_owner;
};

and probably drop the union until we actually need two things here.

Jason