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SubjectRe: [PATCH v2 2/3] mm: introduce put_user_page[s](), placeholder versions
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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@gmail.com wrote:
>>> From: John Hubbard <jhubbard@nvidia.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

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