linux-kernel - Re: [PATCH mm-unstable v1 14/20] drm/etnaviv: remove FOLL

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Date:   Wed, 16 Nov 2022 11:49:43 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-alpha@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-ia64@...r.kernel.org, linux-mips@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org,
        linux-um@...ts.infradead.org, etnaviv@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-samsung-soc@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-media@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-perf-users@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Peter Xu <peterx@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, Nadav Amit <namit@...are.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Shuah Khan <shuah@...nel.org>,
        Lucas Stach <l.stach@...gutronix.de>,
        David Airlie <airlied@...il.com>,
        Oded Gabbay <ogabbay@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christoph Hellwig <hch@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Russell King <linux+etnaviv@...linux.org.uk>,
        Christian Gmeiner <christian.gmeiner@...il.com>
Subject: Re: [PATCH mm-unstable v1 14/20] drm/etnaviv: remove FOLL_FORCE usage

On Wed, Nov 16, 2022 at 11:26:53AM +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()).
> 
> commit cd5297b0855f ("drm/etnaviv: Use FOLL_FORCE for userptr")
> documents that FOLL_FORCE | FOLL_WRITE was really only used for reliable
> R/O pinning.
> 
> 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: Daniel Vetter <daniel.vetter@...ll.ch>
> Cc: Lucas Stach <l.stach@...gutronix.de>
> Cc: Russell King <linux+etnaviv@...linux.org.uk>
> Cc: Christian Gmeiner <christian.gmeiner@...il.com>
> Cc: David Airlie <airlied@...il.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>

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

Also ack for merging through whatever tree suits best, since I guess this
should all land together.
-Daniel

> ---
>  drivers/gpu/drm/etnaviv/etnaviv_gem.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.c b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> index cc386f8a7116..efe2240945d0 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> @@ -638,6 +638,7 @@ static int etnaviv_gem_userptr_get_pages(struct etnaviv_gem_object *etnaviv_obj)
>  	struct page **pvec = NULL;
>  	struct etnaviv_gem_userptr *userptr = &etnaviv_obj->userptr;
>  	int ret, pinned = 0, npages = etnaviv_obj->base.size >> PAGE_SHIFT;
> +	unsigned int gup_flags = FOLL_LONGTERM;
>  
>  	might_lock_read(&current->mm->mmap_lock);
>  
> @@ -648,14 +649,15 @@ static int etnaviv_gem_userptr_get_pages(struct etnaviv_gem_object *etnaviv_obj)
>  	if (!pvec)
>  		return -ENOMEM;
>  
> +	if (!userptr->ro)
> +		gup_flags |= FOLL_WRITE;
> +
>  	do {
>  		unsigned num_pages = npages - pinned;
>  		uint64_t ptr = userptr->ptr + pinned * PAGE_SIZE;
>  		struct page **pages = pvec + pinned;
>  
> -		ret = pin_user_pages_fast(ptr, num_pages,
> -					  FOLL_WRITE | FOLL_FORCE | FOLL_LONGTERM,
> -					  pages);
> +		ret = pin_user_pages_fast(ptr, num_pages, gup_flags, pages);
>  		if (ret < 0) {
>  			unpin_user_pages(pvec, pinned);
>  			kvfree(pvec);
> -- 
> 2.38.1
> 

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