Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[John Hubbard]

On 12/3/18 11:53 PM, Mike Rapoport wrote:
> Hi John,
> 
> Thanks for having documentation as a part of the patch. Some kernel-doc
> nits below.
> 
> On Mon, Dec 03, 2018 at 04:17:19PM -0800, john.hubbard@xxxxxxxxx wrote:
>> From: John Hubbard <jhubbard@xxxxxxxxxx>
>>
>> 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(), and also as a replacement
>> for open-coded loops that release multiple pages.
>> These may be used for subsequent performance improvements,
>> via batching of pages to be released.
>>
>> This is the first step of fixing the problem described in [1]. The steps
>> are:
>>
>> 1) (This patch): provide put_user_page*() routines, intended to be used
>>    for releasing pages that were pinned via get_user_pages*().
>>
>> 2) Convert all of the call sites for get_user_pages*(), to
>>    invoke put_user_page*(), instead of put_page(). This involves dozens of
>>    call sites, and will take some time.
>>
>> 3) After (2) is complete, use get_user_pages*() and put_user_page*() to
>>    implement tracking of these pages. This tracking will be separate from
>>    the existing struct page refcounting.
>>
>> 4) Use the tracking and identification of these pages, to implement
>>    special handling (especially in writeback paths) when the pages are
>>    backed by a filesystem. Again, [1] provides details as to why that is
>>    desirable.
>>
>> [1] https://lwn.net/Articles/753027/ : "The Trouble with get_user_pages()"
>>
>> Reviewed-by: Jan Kara <jack@xxxxxxx>
>>
>> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
>> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
>> Cc: Christopher Lameter <cl@xxxxxxxxx>
>> Cc: Jason Gunthorpe <jgg@xxxxxxxx>
>> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>> Cc: Jan Kara <jack@xxxxxxx>
>> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
>> Cc: Jerome Glisse <jglisse@xxxxxxxxxx>
>> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
>> Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
>> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
>> ---
>>  include/linux/mm.h | 20 ++++++++++++
>>  mm/swap.c          | 80 ++++++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 100 insertions(+)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 5411de93a363..09fbb2c81aba 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -963,6 +963,26 @@ static inline void put_page(struct page *page)
>>  		__put_page(page);
>>  }
>>
>> +/*
>> + * put_user_page() - release a page that had previously been acquired via
>> + * a call to one of the get_user_pages*() functions.
> 
> Please add @page parameter description, otherwise kernel-doc is unhappy

Hi Mike,

Sorry I missed these kerneldoc points from your earlier review! I'll fix it
up now and it will show up in the next posting.

> 
>> + *
>> + * Pages that were pinned via get_user_pages*() must be released via
>> + * either put_user_page(), or one of the put_user_pages*() routines
>> + * below. This is so that eventually, pages that are pinned via
>> + * get_user_pages*() can be separately tracked and uniquely handled. In
>> + * particular, interactions with RDMA and filesystems need special
>> + * handling.
>> + */
>> +static inline void put_user_page(struct page *page)
>> +{
>> +	put_page(page);
>> +}
>> +
>> +void put_user_pages_dirty(struct page **pages, unsigned long npages);
>> +void put_user_pages_dirty_lock(struct page **pages, unsigned long npages);
>> +void put_user_pages(struct page **pages, unsigned long npages);
>> +
>>  #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
>>  #define SECTION_IN_PAGE_FLAGS
>>  #endif
>> diff --git a/mm/swap.c b/mm/swap.c
>> index aa483719922e..bb8c32595e5f 100644
>> --- a/mm/swap.c
>> +++ b/mm/swap.c
>> @@ -133,6 +133,86 @@ void put_pages_list(struct list_head *pages)
>>  }
>>  EXPORT_SYMBOL(put_pages_list);
>>
>> +typedef int (*set_dirty_func)(struct page *page);
>> +
>> +static void __put_user_pages_dirty(struct page **pages,
>> +				   unsigned long npages,
>> +				   set_dirty_func sdf)
>> +{
>> +	unsigned long index;
>> +
>> +	for (index = 0; index < npages; index++) {
>> +		struct page *page = compound_head(pages[index]);
>> +
>> +		if (!PageDirty(page))
>> +			sdf(page);
>> +
>> +		put_user_page(page);
>> +	}
>> +}
>> +
>> +/*
>> + * put_user_pages_dirty() - for each page in the @pages array, make
>> + * that page (or its head page, if a compound page) dirty, if it was
>> + * previously listed as clean. Then, release the page using
>> + * put_user_page().
>> + *
>> + * Please see the put_user_page() documentation for details.
>> + *
>> + * set_page_dirty(), which does not lock the page, is used here.
>> + * Therefore, it is the caller's responsibility to ensure that this is
>> + * safe. If not, then put_user_pages_dirty_lock() should be called instead.
>> + *
>> + * @pages:  array of pages to be marked dirty and released.
>> + * @npages: number of pages in the @pages array.
> 
> Please put the parameters description next to the brief function
> description, as described in [1]
> 
> [1] https://www.kernel.org/doc/html/latest/doc-guide/kernel-doc.html#function-documentation
> 

OK. 

> 
>> + *
>> + */
>> +void put_user_pages_dirty(struct page **pages, unsigned long npages)
>> +{
>> +	__put_user_pages_dirty(pages, npages, set_page_dirty);
>> +}
>> +EXPORT_SYMBOL(put_user_pages_dirty);
>> +
>> +/*
>> + * put_user_pages_dirty_lock() - for each page in the @pages array, make
>> + * that page (or its head page, if a compound page) dirty, if it was
>> + * previously listed as clean. Then, release the page using
>> + * put_user_page().
>> + *
>> + * Please see the put_user_page() documentation for details.
>> + *
>> + * This is just like put_user_pages_dirty(), except that it invokes
>> + * set_page_dirty_lock(), instead of set_page_dirty().
>> + *
>> + * @pages:  array of pages to be marked dirty and released.
>> + * @npages: number of pages in the @pages array.
> 
> Ditto

OK.

> 
>> + *
>> + */
>> +void put_user_pages_dirty_lock(struct page **pages, unsigned long npages)
>> +{
>> +	__put_user_pages_dirty(pages, npages, set_page_dirty_lock);
>> +}
>> +EXPORT_SYMBOL(put_user_pages_dirty_lock);
>> +
>> +/*
>> + * put_user_pages() - for each page in the @pages array, release the page
>> + * using put_user_page().
>> + *
>> + * Please see the put_user_page() documentation for details.
>> + *
>> + * @pages:  array of pages to be marked dirty and released.
>> + * @npages: number of pages in the @pages array.
>> + *
> 
> And here as well :)

OK.


thanks,
-- 
John Hubbard
NVIDIA