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

Kirill A. Shutemov
2019-03-19T12:04:27Z

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