netdev - Re: [PATCH v12 04/22] mm: devmap: refactor 1-based refcounting for ZONE

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Date:   Wed, 15 Jan 2020 13:19:41 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, <bpf@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>, <kvm@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <linux-media@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>, <netdev@...r.kernel.org>,
        <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v12 04/22] mm: devmap: refactor 1-based refcounting for
 ZONE_DEVICE pages

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