netdev - Re: [PATCH v2 17/19] powerpc: book3s64: convert to pin_user_pages() and put_user

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Date:   Fri, 29 Nov 2019 12:23:15 +0100
From:   Jan Kara <jack@...e.cz>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, bpf@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, kvm@...r.kernel.org,
        linux-block@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-media@...r.kernel.org, linux-rdma@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 17/19] powerpc: book3s64: convert to pin_user_pages()
 and put_user_page()

On Mon 25-11-19 15:10:33, John Hubbard wrote:
> 1. Convert from get_user_pages() to pin_user_pages().
> 
> 2. As required by pin_user_pages(), release these pages via
> put_user_page(). In this case, do so via put_user_pages_dirty_lock().
> 
> That has the side effect of calling set_page_dirty_lock(), instead
> of set_page_dirty(). This is probably more accurate.

Maybe more accurate but it doesn't work for mm_iommu_unpin(). As I'm
checking mm_iommu_unpin() gets called from RCU callback which is executed
interrupt context and you cannot lock pages from such context. So you need
to queue work from the RCU callback and then do the real work from the
workqueue...

								Honza

> 
> As Christoph Hellwig put it, "set_page_dirty() is only safe if we are
> dealing with a file backed page where we have reference on the inode it
> hangs off." [1]
> 
> [1] https://lore.kernel.org/r/20190723153640.GB720@lst.de
> 
> Cc: Jan Kara <jack@...e.cz>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>  arch/powerpc/mm/book3s64/iommu_api.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/mm/book3s64/iommu_api.c b/arch/powerpc/mm/book3s64/iommu_api.c
> index 56cc84520577..fc1670a6fc3c 100644
> --- a/arch/powerpc/mm/book3s64/iommu_api.c
> +++ b/arch/powerpc/mm/book3s64/iommu_api.c
> @@ -103,7 +103,7 @@ static long mm_iommu_do_alloc(struct mm_struct *mm, unsigned long ua,
>  	for (entry = 0; entry < entries; entry += chunk) {
>  		unsigned long n = min(entries - entry, chunk);
>  
> -		ret = get_user_pages(ua + (entry << PAGE_SHIFT), n,
> +		ret = pin_user_pages(ua + (entry << PAGE_SHIFT), n,
>  				FOLL_WRITE | FOLL_LONGTERM,
>  				mem->hpages + entry, NULL);
>  		if (ret == n) {
> @@ -167,9 +167,8 @@ static long mm_iommu_do_alloc(struct mm_struct *mm, unsigned long ua,
>  	return 0;
>  
>  free_exit:
> -	/* free the reference taken */
> -	for (i = 0; i < pinned; i++)
> -		put_page(mem->hpages[i]);
> +	/* free the references taken */
> +	put_user_pages(mem->hpages, pinned);
>  
>  	vfree(mem->hpas);
>  	kfree(mem);
> @@ -212,10 +211,9 @@ static void mm_iommu_unpin(struct mm_iommu_table_group_mem_t *mem)
>  		if (!page)
>  			continue;
>  
> -		if (mem->hpas[i] & MM_IOMMU_TABLE_GROUP_PAGE_DIRTY)
> -			SetPageDirty(page);
> +		put_user_pages_dirty_lock(&page, 1,
> +				mem->hpas[i] & MM_IOMMU_TABLE_GROUP_PAGE_DIRTY);
>  
> -		put_page(page);
>  		mem->hpas[i] = 0;
>  	}
>  }
> -- 
> 2.24.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR