linux-kernel - Re: [PATCH v2 2/3] mm: introduce put_user

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Date:   Fri, 5 Oct 2018 17:03:06 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
CC:     <john.hubbard@...il.com>, Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jan Kara <jack@...e.cz>, <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, Al Viro <viro@...iv.linux.org.uk>,
        Jerome Glisse <jglisse@...hat.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v2 2/3] mm: introduce put_user_page[s](), placeholder
 versions

On 10/5/18 2:48 PM, Jason Gunthorpe wrote:
> On Fri, Oct 05, 2018 at 12:49:06PM -0700, John Hubbard wrote:
>> On 10/5/18 8:17 AM, Jason Gunthorpe wrote:
>>> On Thu, Oct 04, 2018 at 09:02:24PM -0700, john.hubbard@...il.com wrote:
>>>> From: John Hubbard <jhubbard@...dia.com>
>>>>
>>>> Introduces put_user_page(), which simply calls put_page().
>>>> This provides a way to update all get_user_pages*() callers,
>>>> so that they call put_user_page(), instead of put_page().
>>>>
>>>> Also introduces put_user_pages(), and a few dirty/locked variations,
>>>> as a replacement for release_pages(), for the same reasons.
>>>> These may be used for subsequent performance improvements,
>>>> via batching of pages to be released.
>>>>
>>>> This prepares for eventually fixing the problem described
>>>> in [1], and is following a plan listed in [2], [3], [4].
>>>>
>>>> [1] https://lwn.net/Articles/753027/ : "The Trouble with get_user_pages()"
>>>>
>>>> [2] https://lkml.kernel.org/r/20180709080554.21931-1-jhubbard@nvidia.com
>>>>     Proposed steps for fixing get_user_pages() + DMA problems.
>>>>
>>>> [3]https://lkml.kernel.org/r/20180710082100.mkdwngdv5kkrcz6n@quack2.suse.cz
>>>>     Bounce buffers (otherwise [2] is not really viable).
>>>>
>>>> [4] https://lkml.kernel.org/r/20181003162115.GG24030@quack2.suse.cz
>>>>     Follow-up discussions.
>>>>
>> [...]
>>>>  
>>>> +/* Placeholder version, until all get_user_pages*() callers are updated. */
>>>> +static inline void put_user_page(struct page *page)
>>>> +{
>>>> +	put_page(page);
>>>> +}
>>>> +
>>>> +/* For get_user_pages*()-pinned pages, use these variants instead of
>>>> + * release_pages():
>>>> + */
>>>> +static inline void put_user_pages_dirty(struct page **pages,
>>>> +					unsigned long npages)
>>>> +{
>>>> +	while (npages) {
>>>> +		set_page_dirty(pages[npages]);
>>>> +		put_user_page(pages[npages]);
>>>> +		--npages;
>>>> +	}
>>>> +}
>>>
>>> Shouldn't these do the !PageDirty(page) thing?
>>>
>>
>> Well, not yet. This is the "placeholder" patch, in which I planned to keep
>> the behavior the same, while I go to all the get_user_pages call sites and change 
>> put_page() and release_pages() over to use these new routines.
> 
> Hmm.. Well, if it is the right thing to do here, why not include it and
> take it out of callers when doing the conversion?
> 
> If it is the wrong thing, then let us still take it out of callers
> when doing the conversion :)
> 
> Just seems like things will be in a better place to make future
> changes if all the call sights are de-duplicated and correct.
> 

OK, yes. Let me send out a v3 with that included, then.

thanks,
-- 
John Hubbard
NVIDIA