linux-kernel - Re: [RFC PATCH 2/2] mm/gup: introduce vaddr_pin_pages

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Message-ID: <38d2ff2f-4a69-e8bd-8f7c-41f1dbd80fae@nvidia.com>
Date:   Mon, 12 Aug 2019 17:07:32 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Ira Weiny <ira.weiny@...el.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jérôme Glisse <jglisse@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-rdma@...r.kernel.org>
Subject: Re: [RFC PATCH 2/2] mm/gup: introduce vaddr_pin_pages_remote()

On 8/12/19 4:49 PM, Ira Weiny wrote:
> On Sun, Aug 11, 2019 at 06:50:44PM -0700, john.hubbard@...il.com wrote:
>> From: John Hubbard <jhubbard@...dia.com>
...
>> diff --git a/drivers/infiniband/core/umem_odp.c b/drivers/infiniband/core/umem_odp.c
>> index 53085896d718..fdff034a8a30 100644
>> --- a/drivers/infiniband/core/umem_odp.c
>> +++ b/drivers/infiniband/core/umem_odp.c
>> @@ -534,7 +534,7 @@ static int ib_umem_odp_map_dma_single_page(
>>   	}
>>   
>>   out:
>> -	put_user_page(page);
>> +	vaddr_unpin_pages(&page, 1, &umem_odp->umem.vaddr_pin);
>>   
>>   	if (remove_existing_mapping) {
>>   		ib_umem_notifier_start_account(umem_odp);
>> @@ -635,9 +635,10 @@ int ib_umem_odp_map_dma_pages(struct ib_umem_odp *umem_odp, u64 user_virt,
>>   		 * complex (and doesn't gain us much performance in most use
>>   		 * cases).
>>   		 */
>> -		npages = get_user_pages_remote(owning_process, owning_mm,
>> +		npages = vaddr_pin_pages_remote(owning_process, owning_mm,
>>   				user_virt, gup_num_pages,
>> -				flags, local_page_list, NULL, NULL);
>> +				flags, local_page_list, NULL, NULL,
>> +				&umem_odp->umem.vaddr_pin);
> 
> Thinking about this part of the patch... is this pin really necessary?  This
> code is not doing a long term pin.  The page just needs a reference while we
> map it into the devices page tables.  Once that is done we should get notifiers
> if anything changes and we can adjust.  right?
> 

OK, now it's a little interesting: the FOLL_PIN is necessary, but maybe not
FOLL_LONGTERM. Illustrating once again that it's actually necessary to allow
these flags to vary independently.

And that leads to another API refinement idea: let's set FOLL_PIN within the
vaddr_pin_pages*() wrappers, and set FOLL_LONGTER in the *callers* of those
wrappers, yes?

thanks,
-- 
John Hubbard
NVIDIA