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

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Date:   Mon, 25 Nov 2019 09:59:15 +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 17/19] powerpc: book3s64: convert to pin_user_pages() and
 put_user_page()

On Sun 24-11-19 20:20:09, 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.
> 
> 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]
> 
> 3. Release each page in mem->hpages[] (instead of mem->hpas[]), because
> that is the array that pin_longterm_pages() filled in. This is more
> accurate and should be a little safer from a maintenance point of
> view.

Except that this breaks the code. hpages is unioned with hpas...

> [1] https://lore.kernel.org/r/20190723153640.GB720@lst.de
> 
> 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..196383e8e5a9 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(&mem->hpages[i], 1,
> +					  MM_IOMMU_TABLE_GROUP_PAGE_DIRTY);

And the dirtying condition is wrong here as well. Currently it is always
true.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR