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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Mar 08, 2019 at 01:36:33PM -0800, 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(), 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 a problem (also described in [1] and
> [2]) with interactions between get_user_pages ("gup") and filesystems.
> 
> Problem description: let's start with a bug report. Below, is what happens
> sometimes, under memory pressure, when a driver pins some pages via gup,
> and then marks those pages dirty, and releases them. Note that the gup
> documentation actually recommends that pattern. The problem is that the
> filesystem may do a writeback while the pages were gup-pinned, and then the
> filesystem believes that the pages are clean. So, when the driver later
> marks the pages as dirty, that conflicts with the filesystem's page
> tracking and results in a BUG(), like this one that I experienced:
> 
>     kernel BUG at /build/linux-fQ94TU/linux-4.4.0/fs/ext4/inode.c:1899!
>     backtrace:
>         ext4_writepage
>         __writepage
>         write_cache_pages
>         ext4_writepages
>         do_writepages
>         __writeback_single_inode
>         writeback_sb_inodes
>         __writeback_inodes_wb
>         wb_writeback
>         wb_workfn
>         process_one_work
>         worker_thread
>         kthread
>         ret_from_fork
> 
> ...which is due to the file system asserting that there are still buffer
> heads attached:
> 
>         ({                                                      \
>                 BUG_ON(!PagePrivate(page));                     \
>                 ((struct buffer_head *)page_private(page));     \
>         })
> 
> Dave Chinner's description of this is very clear:
> 
>     "The fundamental issue is that ->page_mkwrite must be called on every
>     write access to a clean file backed page, not just the first one.
>     How long the GUP reference lasts is irrelevant, if the page is clean
>     and you need to dirty it, you must call ->page_mkwrite before it is
>     marked writeable and dirtied. Every. Time."
> 
> This is just one symptom of the larger design problem: real filesystems
> that actually write to a backing device, do not actually support
> get_user_pages() being called on their pages, and letting hardware write
> directly to those pages--even though that pattern has been going on since
> about 2005 or so.
> 
> The steps are to fix it 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.
> 
> [1] https://lwn.net/Articles/774411/ : "DMA and get_user_pages()"
> [2] https://lwn.net/Articles/753027/ : "The Trouble with get_user_pages()"
> 
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Christopher Lameter <cl@linux.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Dave Chinner <david@fromorbit.com>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: Jerome Glisse <jglisse@redhat.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Mike Rapoport <rppt@linux.ibm.com>
> Cc: Ralph Campbell <rcampbell@nvidia.com>
> 
> Reviewed-by: Jan Kara <jack@suse.cz>
> Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>    # docs
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
>  include/linux/mm.h | 24 ++++++++++++++
>  mm/gup.c           | 82 ++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 106 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5801ee849f36..353035c8b115 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -993,6 +993,30 @@ static inline void put_page(struct page *page)
>  		__put_page(page);
>  }
>  
> +/**
> + * put_user_page() - release a gup-pinned page
> + * @page:            pointer to page to be released
> + *
> + * 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.
> + *
> + * put_user_page() and put_page() are not interchangeable, despite this early
> + * implementation that makes them look the same. put_user_page() calls must
> + * be perfectly matched up with get_user_page() calls.
> + */
> +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/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.

> +
> +		put_user_page(page);
> +	}
> +}
> +
> +/**
> + * put_user_pages_dirty() - release and dirty an array of gup-pinned pages
> + * @pages:  array of pages to be marked dirty and released.
> + * @npages: number of pages in the @pages array.
> + *
> + * "gup-pinned page" refers to a page that has had one of the get_user_pages()
> + * variants called on that page.
> + *
> + * 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.
> + *
> + */
> +void put_user_pages_dirty(struct page **pages, unsigned long npages)
> +{
> +	__put_user_pages_dirty(pages, npages, set_page_dirty);

Have you checked if compiler is clever enough eliminate indirect function
call here? Maybe it's better to go with an opencodded approach and get rid
of callbacks?


> +}
> +EXPORT_SYMBOL(put_user_pages_dirty);
> +
> +/**
> + * put_user_pages_dirty_lock() - release and dirty an array of gup-pinned pages
> + * @pages:  array of pages to be marked dirty and released.
> + * @npages: number of pages in the @pages array.
> + *
> + * 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().
> + *
> + */
> +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() - release an array of gup-pinned pages.
> + * @pages:  array of pages to be marked dirty and released.
> + * @npages: number of pages in the @pages array.
> + *
> + * For each page in the @pages array, release the page using put_user_page().
> + *
> + * Please see the put_user_page() documentation for details.
> + */
> +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.

> +}
> +EXPORT_SYMBOL(put_user_pages);
> +
>  static struct page *no_page_table(struct vm_area_struct *vma,
>  		unsigned int flags)
>  {
> -- 
> 2.21.0
> 

-- 
 Kirill A. Shutemov