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

[John Hubbard <jhubbard@nvidia.com>]

On 3/19/19 7:06 AM, Kirill A. Shutemov 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.
> 
> Fair enough. Should we get it into a comment here?

How's this read to you? I reworded and slightly expanded Jerome's 
description:

diff --git a/mm/gup.c b/mm/gup.c
index d1df7b8ba973..86397ae23922 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -61,6 +61,24 @@ static void __put_user_pages_dirty(struct page **pages,
        for (index = 0; index < npages; index++) {
                struct page *page = compound_head(pages[index]);
 
+               /*
+                * Checking PageDirty at this point may race with
+                * clear_page_dirty_for_io(), but that's OK. Two key cases:
+                *
+                * 1) This code sees the page as already dirty, so it skips
+                * the call to sdf(). That could happen because
+                * clear_page_dirty_for_io() called page_mkclean(),
+                * followed by set_page_dirty(). However, now the page is
+                * going to get written back, which meets the original
+                * intention of setting it dirty, so all is well:
+                * clear_page_dirty_for_io() goes on to call
+                * TestClearPageDirty(), and write the page back.
+                *
+                * 2) This code sees the page as clean, so it calls sdf().
+                * The page stays dirty, despite being written back, so it
+                * gets written back again in the next writeback cycle.
+                * This is harmless.
+                */
                if (!PageDirty(page))
                        sdf(page);

> 
>>>> +void put_user_pages(struct page **pages, unsigned long npages)
>>>> +{
>>>> +	unsigned long index;
>>>> +
>>>> +	for (index = 0; index < npages; index++)
>>>> +		put_user_page(pages[index]);
>>>
>>> I believe there's an room for improvement for compound pages.
>>>
>>> If there's multiple consequential pages in the array that belong to the
>>> same compound page we can get away with a single atomic operation to
>>> handle them all.
>>
>> Yes maybe just add a comment with that for now and leave this kind of
>> optimization to latter ?
> 
> Sounds good to me.
> 

Here's a comment for that:

@@ -127,6 +145,11 @@ void put_user_pages(struct page **pages, unsigned long npages)
 {
        unsigned long index;
 
+       /*
+        * TODO: this can be optimized for huge pages: if a series of pages is
+        * physically contiguous and part of the same compound page, then a
+        * single operation to the head page should suffice.
+        */
        for (index = 0; index < npages; index++)
                put_user_page(pages[index]);
 }


thanks,
-- 
John Hubbard
NVIDIA