Re: [PATCH v4 1/1] mm: introduce put_user_page*(), placeholder versions

[Tom Talpey <tom@talpey.com>]

On 3/19/2019 3:45 PM, Jerome Glisse wrote:
> On Tue, Mar 19, 2019 at 03:43:44PM -0500, Tom Talpey wrote:
>> On 3/19/2019 4:03 AM, Ira Weiny wrote:
>>> On Tue, Mar 19, 2019 at 04:36:44PM +0100, Jan Kara wrote:
>>>> On Tue 19-03-19 17:29:18, Kirill A. Shutemov wrote:
>>>>> On Tue, Mar 19, 2019 at 10:14:16AM -0400, Jerome Glisse wrote:
>>>>>> On Tue, Mar 19, 2019 at 09:47:24AM -0400, Jerome Glisse wrote:
>>>>>>> On Tue, Mar 19, 2019 at 03:04:17PM +0300, Kirill A. Shutemov wrote:
>>>>>>>> On Fri, Mar 08, 2019 at 01:36:33PM -0800, john.hubbard@gmail.com wrote:
>>>>>>>>> From: John Hubbard <jhubbard@nvidia.com>
>>>>>>>
>>>>>>> [...]
>>>>>>>
>>>>>>>>> diff --git a/mm/gup.c b/mm/gup.c
>>>>>>>>> index f84e22685aaa..37085b8163b1 100644
>>>>>>>>> --- a/mm/gup.c
>>>>>>>>> +++ b/mm/gup.c
>>>>>>>>> @@ -28,6 +28,88 @@ struct follow_page_context {
>>>>>>>>>    	unsigned int page_mask;
>>>>>>>>>    };
>>>>>>>>> +typedef int (*set_dirty_func_t)(struct page *page);
>>>>>>>>> +
>>>>>>>>> +static void __put_user_pages_dirty(struct page **pages,
>>>>>>>>> +				   unsigned long npages,
>>>>>>>>> +				   set_dirty_func_t sdf)
>>>>>>>>> +{
>>>>>>>>> +	unsigned long index;
>>>>>>>>> +
>>>>>>>>> +	for (index = 0; index < npages; index++) {
>>>>>>>>> +		struct page *page = compound_head(pages[index]);
>>>>>>>>> +
>>>>>>>>> +		if (!PageDirty(page))
>>>>>>>>> +			sdf(page);
>>>>>>>>
>>>>>>>> How is this safe? What prevents the page to be cleared under you?
>>>>>>>>
>>>>>>>> If it's safe to race clear_page_dirty*() it has to be stated explicitly
>>>>>>>> with a reason why. It's not very clear to me as it is.
>>>>>>>
>>>>>>> The PageDirty() optimization above is fine to race with clear the
>>>>>>> page flag as it means it is racing after a page_mkclean() and the
>>>>>>> GUP user is done with the page so page is about to be write back
>>>>>>> ie if (!PageDirty(page)) see the page as dirty and skip the sdf()
>>>>>>> call while a split second after TestClearPageDirty() happens then
>>>>>>> it means the racing clear is about to write back the page so all
>>>>>>> is fine (the page was dirty and it is being clear for write back).
>>>>>>>
>>>>>>> If it does call the sdf() while racing with write back then we
>>>>>>> just redirtied the page just like clear_page_dirty_for_io() would
>>>>>>> do if page_mkclean() failed so nothing harmful will come of that
>>>>>>> neither. Page stays dirty despite write back it just means that
>>>>>>> the page might be write back twice in a row.
>>>>>>
>>>>>> Forgot to mention one thing, we had a discussion with Andrea and Jan
>>>>>> about set_page_dirty() and Andrea had the good idea of maybe doing
>>>>>> the set_page_dirty() at GUP time (when GUP with write) not when the
>>>>>> GUP user calls put_page(). We can do that by setting the dirty bit
>>>>>> in the pte for instance. They are few bonus of doing things that way:
>>>>>>       - amortize the cost of calling set_page_dirty() (ie one call for
>>>>>>         GUP and page_mkclean()
>>>>>>       - it is always safe to do so at GUP time (ie the pte has write
>>>>>>         permission and thus the page is in correct state)
>>>>>>       - safe from truncate race
>>>>>>       - no need to ever lock the page
>>>>>>
>>>>>> Extra bonus from my point of view, it simplify thing for my generic
>>>>>> page protection patchset (KSM for file back page).
>>>>>>
>>>>>> So maybe we should explore that ? It would also be a lot less code.
>>>>>
>>>>> Yes, please. It sounds more sensible to me to dirty the page on get, not
>>>>> on put.
>>>>
>>>> I fully agree this is a desirable final state of affairs.
>>>
>>> I'm glad to see this presented because it has crossed my mind more than once
>>> that effectively a GUP pinned page should be considered "dirty" at all times
>>> until the pin is removed.  This is especially true in the RDMA case.
>>
>> But, what if the RDMA registration is readonly? That's not uncommon, and
>> marking dirty unconditonally would add needless overhead to such pages.
> 
> Yes and this is only when FOLL_WRITE is set ie when you are doing GUP and
> asking for write. Doing GUP and asking for read is always safe.

Aha, ok great.

I guess it does introduce something for callers to be aware of, if
they GUP very large regions. I suppose if they're sufficiently aware
of the situation, e.g. pnfs LAYOUT_COMMIT notifications, they could
walk lists and reset page_dirty for untouched pages before releasing.
That's their issue though, and agreed it's safest for the GUP layer
to mark.

Tom.