linux-kernel - Re: [PATCH mm-unstable v1 13/20] media: videobuf-dma-sg: remove FOLL

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Message-ID: <Y3TAGAUIo/IR+tAa@phenom.ffwll.local>
Date:   Wed, 16 Nov 2022 11:48:56 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-ia64@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        dri-devel@...ts.freedesktop.org, linux-mm@...ck.org,
        Nadav Amit <namit@...are.com>, linux-kselftest@...r.kernel.org,
        sparclinux@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-samsung-soc@...r.kernel.org, linux-rdma@...r.kernel.org,
        x86@...nel.org, Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Vlastimil Babka <vbabka@...e.cz>, linux-media@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        John Hubbard <jhubbard@...dia.com>,
        linux-um@...ts.infradead.org, etnaviv@...ts.freedesktop.org,
        Alex Williamson <alex.williamson@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Muchun Song <songmuchun@...edance.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, Oded Gabbay <ogabbay@...nel.org>,
        linux-mips@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH mm-unstable v1 13/20] media: videobuf-dma-sg: remove
 FOLL_FORCE usage

On Wed, Nov 16, 2022 at 11:26:52AM +0100, David Hildenbrand wrote:
> GUP now supports reliable R/O long-term pinning in COW mappings, such
> that we break COW early. MAP_SHARED VMAs only use the shared zeropage so
> far in one corner case (DAXFS file with holes), which can be ignored
> because GUP does not support long-term pinning in fsdax (see
> check_vma_flags()).
> 
> Consequently, FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM is no longer required
> for reliable R/O long-term pinning: FOLL_LONGTERM is sufficient. So stop
> using FOLL_FORCE, which is really only for ptrace access.
> 
> Cc: Mauro Carvalho Chehab <mchehab@...nel.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>

I looked at this a while ago when going through some of the follow_pfn
stuff, so

Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>

> ---
>  drivers/media/v4l2-core/videobuf-dma-sg.c | 14 +++++---------
>  1 file changed, 5 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/videobuf-dma-sg.c b/drivers/media/v4l2-core/videobuf-dma-sg.c
> index f75e5eedeee0..234e9f647c96 100644
> --- a/drivers/media/v4l2-core/videobuf-dma-sg.c
> +++ b/drivers/media/v4l2-core/videobuf-dma-sg.c
> @@ -151,17 +151,16 @@ static void videobuf_dma_init(struct videobuf_dmabuf *dma)
>  static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
>  			int direction, unsigned long data, unsigned long size)
>  {
> +	unsigned int gup_flags = FOLL_LONGTERM;
>  	unsigned long first, last;
> -	int err, rw = 0;
> -	unsigned int flags = FOLL_FORCE;
> +	int err;
>  
>  	dma->direction = direction;
>  	switch (dma->direction) {
>  	case DMA_FROM_DEVICE:
> -		rw = READ;
> +		gup_flags |= FOLL_WRITE;
>  		break;
>  	case DMA_TO_DEVICE:
> -		rw = WRITE;
>  		break;
>  	default:
>  		BUG();
> @@ -177,14 +176,11 @@ static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
>  	if (NULL == dma->pages)
>  		return -ENOMEM;
>  
> -	if (rw == READ)
> -		flags |= FOLL_WRITE;
> -
>  	dprintk(1, "init user [0x%lx+0x%lx => %lu pages]\n",
>  		data, size, dma->nr_pages);
>  
> -	err = pin_user_pages(data & PAGE_MASK, dma->nr_pages,
> -			     flags | FOLL_LONGTERM, dma->pages, NULL);
> +	err = pin_user_pages(data & PAGE_MASK, dma->nr_pages, gup_flags,
> +			     dma->pages, NULL);
>  
>  	if (err != dma->nr_pages) {
>  		dma->nr_pages = (err >= 0) ? err : 0;
> -- 
> 2.38.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch