linux-kernel - Re: [PATCH 05/13] mm/frame-vector: Use FOLL

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Message-ID: <fc0ac3fb-2758-bef1-76b4-8ac2449f5743@nvidia.com>
Date:   Wed, 7 Oct 2020 14:13:42 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Daniel Vetter <daniel.vetter@...ll.ch>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>
CC:     <kvm@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-samsung-soc@...r.kernel.org>, <linux-media@...r.kernel.org>,
        <linux-s390@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Pawel Osciak <pawel@...iak.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        "Kyungmin Park" <kyungmin.park@...sung.com>,
        Tomasz Figa <tfiga@...omium.org>,
        "Mauro Carvalho Chehab" <mchehab@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 05/13] mm/frame-vector: Use FOLL_LONGTERM

On 10/7/20 9:44 AM, Daniel Vetter wrote:
> This is used by media/videbuf2 for persistent dma mappings, not just
> for a single dma operation and then freed again, so needs
> FOLL_LONGTERM.
> 
> Unfortunately current pup_locked doesn't support FOLL_LONGTERM due to
> locking issues. Rework the code to pull the pup path out from the
> mmap_sem critical section as suggested by Jason.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> Cc: Jason Gunthorpe <jgg@...pe.ca>
> Cc: Pawel Osciak <pawel@...iak.com>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Kyungmin Park <kyungmin.park@...sung.com>
> Cc: Tomasz Figa <tfiga@...omium.org>
> Cc: Mauro Carvalho Chehab <mchehab@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: John Hubbard <jhubbard@...dia.com>
> Cc: Jérôme Glisse <jglisse@...hat.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: linux-mm@...ck.org
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-samsung-soc@...r.kernel.org
> Cc: linux-media@...r.kernel.org
> ---
>   mm/frame_vector.c | 36 +++++++++++-------------------------
>   1 file changed, 11 insertions(+), 25 deletions(-)
> 
> diff --git a/mm/frame_vector.c b/mm/frame_vector.c
> index 10f82d5643b6..39db520a51dc 100644
> --- a/mm/frame_vector.c
> +++ b/mm/frame_vector.c
> @@ -38,7 +38,6 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
>   	struct vm_area_struct *vma;
>   	int ret = 0;
>   	int err;
> -	int locked;
>   
>   	if (nr_frames == 0)
>   		return 0;
> @@ -48,35 +47,22 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
>   
>   	start = untagged_addr(start);
>   
> +	ret = pin_user_pages_fast(start, nr_frames,
> +				  FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM,
> +				  (struct page **)(vec->ptrs));
> +	if (ret > 0) {
> +		vec->got_ref = true;
> +		vec->is_pfns = false;
> +		goto out_unlocked;
> +	}

This part looks good, and changing to _fast is a potential performance improvement,
too.

> +
>   	mmap_read_lock(mm);
> -	locked = 1;
>   	vma = find_vma_intersection(mm, start, start + 1);
>   	if (!vma) {
>   		ret = -EFAULT;
>   		goto out;
>   	}
>   
> -	/*
> -	 * While get_vaddr_frames() could be used for transient (kernel
> -	 * controlled lifetime) pinning of memory pages all current
> -	 * users establish long term (userspace controlled lifetime)
> -	 * page pinning. Treat get_vaddr_frames() like
> -	 * get_user_pages_longterm() and disallow it for filesystem-dax
> -	 * mappings.
> -	 */
> -	if (vma_is_fsdax(vma)) {
> -		ret = -EOPNOTSUPP;
> -		goto out;
> -	}

Are you sure we don't need to check vma_is_fsdax() anymore?

> -
> -	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
> -		vec->got_ref = true;
> -		vec->is_pfns = false;
> -		ret = pin_user_pages_locked(start, nr_frames,
> -			gup_flags, (struct page **)(vec->ptrs), &locked);
> -		goto out;
> -	}
> -
>   	vec->got_ref = false;
>   	vec->is_pfns = true;
>   	do {
> @@ -101,8 +87,8 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
>   		vma = find_vma_intersection(mm, start, start + 1);
>   	} while (vma && vma->vm_flags & (VM_IO | VM_PFNMAP));
>   out:
> -	if (locked)
> -		mmap_read_unlock(mm);
> +	mmap_read_unlock(mm);
> +out_unlocked:
>   	if (!ret)
>   		ret = -EFAULT;
>   	if (ret > 0)
> 

All of the error handling still looks accurate there.

thanks,
-- 
John Hubbard
NVIDIA