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 12:46:56 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jan Kara <jack@...e.cz>
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>,
        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 11/25/19 12:59 AM, Jan Kara wrote:
> 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...
> 

OK. 

>> @@ -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
> 

Yes. Fixed up locally. The function now looks like this (for this patch, not for
the entire series, which renames "put" to "unpin"):


static void mm_iommu_unpin(struct mm_iommu_table_group_mem_t *mem)
{
	long i;
	struct page *page = NULL;

	if (!mem->hpas)
		return;

	for (i = 0; i < mem->entries; ++i) {
		if (!mem->hpas[i])
			continue;

		page = pfn_to_page(mem->hpas[i] >> PAGE_SHIFT);
		if (!page)
			continue;

		put_user_pages_dirty_lock(&page, 1,
				mem->hpas[i] & MM_IOMMU_TABLE_GROUP_PAGE_DIRTY);

		mem->hpas[i] = 0;
	}
}

thanks,
-- 
John Hubbard
NVIDIA