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

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Date:   Fri, 5 Oct 2018 13:51:07 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>, <john.hubbard@...il.com>
CC:     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 12:49 PM, 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.
> 
> After the call sites are changed, then these routines will be updated to do more.
> [2], above has slightly more detail about that.
> 
> 

Also, I plan to respin again pretty soon, because someone politely pointed out offline
that even in this small patchset, I've botched the handling of the --npages loop, sigh. 
(Thanks, Ralph!)

The original form:

    while(--npages)

was correct, but now it's not so much.

thanks,
-- 
John Hubbard
NVIDIA