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From: John Hubbard <jhubbard@nvidia.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Björn Töpel" <bjorn.topel@intel.com>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Dave Chinner" <david@fromorbit.com>,
	"David Airlie" <airlied@linux.ie>,
	"David S . Miller" <davem@davemloft.net>,
	"Ira Weiny" <ira.weiny@intel.com>, "Jan Kara" <jack@suse.cz>,
	"Jason Gunthorpe" <jgg@ziepe.ca>, "Jens Axboe" <axboe@kernel.dk>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	"Magnus Karlsson" <magnus.karlsson@intel.com>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
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	"Mike Kravetz" <mike.kravetz@oracle.com>,
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	"Vlastimil Babka" <vbabka@suse.cz>,
	bpf@vger.kernel.org, dri-devel@lists.freedesktop.org,
	kvm@vger.kernel.org, linux-block@vger.kernel.org,
	linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kselftest@vger.kernel.org, linux-media@vger.kernel.org,
	linux-rdma@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	netdev@vger.kernel.org, linux-mm@kvack.org,
	LKML <linux-kernel@vger.kernel.org>,
	"Christoph Hellwig" <hch@lst.de>
Subject: Re: [PATCH v12 04/22] mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages
Date: Wed, 15 Jan 2020 13:19:41 -0800
Message-ID: <4707f191-86f8-db4a-c3de-0a84b415b658@nvidia.com> (raw)
In-Reply-To: <20200115152306.GA19546@infradead.org>

On 1/15/20 7:23 AM, Christoph Hellwig wrote:
...
> 
> I'm really not sold on this scheme.  Note that I think it is
> particularly bad, but it also doesn't seem any better than what
> we had before, and it introduced quite a bit more code.
> 

Hi Christoph,

All by itself, yes. But the very next patch (which needs a little 
rework for other reasons, so not included here) needs to reuse some of 
these functions within __unpin_devmap_managed_user_page():

    page_is_devmap_managed()
    free_devmap_managed_page()

That patch was posted as part of the v11 series [1], and it did this:

+#ifdef CONFIG_DEV_PAGEMAP_OPS
+static bool __unpin_devmap_managed_user_page(struct page *page)
+{
+	int count;
+
+	if (!page_is_devmap_managed(page))
+		return false;
+
+	count = page_ref_sub_return(page, GUP_PIN_COUNTING_BIAS);
+
+	__update_proc_vmstat(page, NR_FOLL_PIN_RETURNED, 1);
+	/*
+	 * devmap page refcounts are 1-based, rather than 0-based: if
+	 * refcount is 1, then the page is free and the refcount is
+	 * stable because nobody holds a reference on the page.
+	 */
+	if (count == 1)
+		free_devmap_managed_page(page);
+	else if (!count)
+		__put_page(page);
+
+	return true;
+}
+#else
+static bool __unpin_devmap_managed_user_page(struct page *page)
+{
+	return false;
+}
+#endif /* CONFIG_DEV_PAGEMAP_OPS */
+
+/**
+ * unpin_user_page() - release a dma-pinned page
+ * @page:            pointer to page to be released
+ *
+ * Pages that were pinned via pin_user_pages*() must be released via either
+ * unpin_user_page(), or one of the unpin_user_pages*() routines. This is so
+ * that such pages can be separately tracked and uniquely handled. In
+ * particular, interactions with RDMA and filesystems need special handling.
+ */
+void unpin_user_page(struct page *page)
+{
+	page = compound_head(page);
+
+	/*
+	 * For devmap managed pages we need to catch refcount transition from
+	 * GUP_PIN_COUNTING_BIAS to 1, when refcount reach one it means the
+	 * page is free and we need to inform the device driver through
+	 * callback. See include/linux/memremap.h and HMM for details.
+	 */
+	if (__unpin_devmap_managed_user_page(page))
+		return;
+
+	if (page_ref_sub_and_test(page, GUP_PIN_COUNTING_BIAS))
+		__put_page(page);
+
+	__update_proc_vmstat(page, NR_FOLL_PIN_RETURNED, 1);
+}
+EXPORT_SYMBOL(unpin_user_page);


[1] https://lore.kernel.org/r/20191216222537.491123-24-jhubbard@nvidia.com  
    [PATCH v11 23/25] mm/gup: track FOLL_PIN pages

thanks,
-- 
John Hubbard
NVIDIA

  reply index

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-07 22:45 [PATCH v12 00/22] mm/gup: prereqs to track dma-pinned pages: FOLL_PIN John Hubbard
2020-01-07 22:45 ` [PATCH v12 01/22] mm/gup: factor out duplicate code from four routines John Hubbard
2020-01-07 22:45 ` [PATCH v12 02/22] mm/gup: move try_get_compound_head() to top, fix minor issues John Hubbard
2020-01-07 22:45 ` [PATCH v12 03/22] mm: Cleanup __put_devmap_managed_page() vs ->page_free() John Hubbard
2020-01-07 22:45 ` [PATCH v12 04/22] mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages John Hubbard
2020-01-15 15:23   ` Christoph Hellwig
2020-01-15 21:19     ` John Hubbard [this message]
2020-01-16  9:37       ` Christoph Hellwig
2020-01-16 20:30         ` John Hubbard
2020-01-07 22:45 ` [PATCH v12 05/22] goldish_pipe: rename local pin_user_pages() routine John Hubbard
2020-01-15 15:23   ` Christoph Hellwig
2020-01-07 22:45 ` [PATCH v12 06/22] mm: fix get_user_pages_remote()'s handling of FOLL_LONGTERM John Hubbard
2020-01-15 15:24   ` Christoph Hellwig
2020-01-07 22:45 ` [PATCH v12 07/22] vfio: fix FOLL_LONGTERM use, simplify get_user_pages_remote() call John Hubbard
2020-01-07 22:45 ` [PATCH v12 08/22] mm/gup: allow FOLL_FORCE for get_user_pages_fast() John Hubbard
2020-01-15 15:25   ` Christoph Hellwig
2020-01-07 22:45 ` [PATCH v12 09/22] IB/umem: use get_user_pages_fast() to pin DMA pages John Hubbard
2020-01-07 22:45 ` [PATCH v12 10/22] media/v4l2-core: set pages dirty upon releasing DMA buffers John Hubbard
2020-01-07 22:45 ` [PATCH v12 11/22] mm/gup: introduce pin_user_pages*() and FOLL_PIN John Hubbard
2020-01-15 15:30   ` Christoph Hellwig
2020-01-15 21:34     ` John Hubbard
2020-01-07 22:45 ` [PATCH v12 12/22] goldish_pipe: convert to pin_user_pages() and put_user_page() John Hubbard
2020-01-07 22:45 ` [PATCH v12 13/22] IB/{core,hw,umem}: set FOLL_PIN via pin_user_pages*(), fix up ODP John Hubbard
2020-01-07 22:45 ` [PATCH v12 14/22] mm/process_vm_access: set FOLL_PIN via pin_user_pages_remote() John Hubbard
2020-01-07 22:45 ` [PATCH v12 15/22] drm/via: set FOLL_PIN via pin_user_pages_fast() John Hubbard
2020-01-07 22:45 ` [PATCH v12 16/22] fs/io_uring: set FOLL_PIN via pin_user_pages() John Hubbard
2020-01-07 22:45 ` [PATCH v12 17/22] net/xdp: " John Hubbard
2020-01-07 22:45 ` [PATCH v12 18/22] media/v4l2-core: pin_user_pages (FOLL_PIN) and put_user_page() conversion John Hubbard
2020-01-07 22:45 ` [PATCH v12 19/22] vfio, mm: " John Hubbard
2020-01-07 22:45 ` [PATCH v12 20/22] powerpc: book3s64: convert to pin_user_pages() and put_user_page() John Hubbard
2020-01-07 22:45 ` [PATCH v12 21/22] mm/gup_benchmark: use proper FOLL_WRITE flags instead of hard-coding "1" John Hubbard
2020-01-07 22:45 ` [PATCH v12 22/22] mm, tree-wide: rename put_user_page*() to unpin_user_page*() John Hubbard
2020-01-15 15:26   ` Christoph Hellwig
2020-01-09 22:07 ` [PATCH v12 00/22] mm/gup: prereqs to track dma-pinned pages: FOLL_PIN John Hubbard
2020-01-14 20:15   ` John Hubbard
2020-01-14 23:29     ` Andrew Morton

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