linux-kernel - [PATCH v6 12/12] mm: dump_page(): additional diagnostics for huge pinned pages

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Date:   Mon, 10 Feb 2020 16:15:36 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        <linux-doc@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <linux-kselftest@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
        <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCH v6 12/12] mm: dump_page(): additional diagnostics for huge pinned pages

As part of pin_user_pages() and related API calls, pages are
"dma-pinned". For the case of compound pages of order > 1, the per-page
accounting of dma pins is accomplished via the 3rd struct page in the
compound page. In order to support debugging of any pin_user_pages()-
related problems, enhance dump_page() so as to report the pin count
in that case.

Documentation/core-api/pin_user_pages.rst is also updated accordingly.

Cc: Jan Kara <jack@...e.cz>
Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Cc: Matthew Wilcox <willy@...radead.org>
Signed-off-by: John Hubbard <jhubbard@...dia.com>
---
 Documentation/core-api/pin_user_pages.rst |  7 +++++++
 mm/debug.c                                | 21 ++++++++++++++++-----
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
index 5c8a5f89756b..2e939ff10b86 100644
--- a/Documentation/core-api/pin_user_pages.rst
+++ b/Documentation/core-api/pin_user_pages.rst
@@ -238,6 +238,13 @@ long-term [R]DMA pins in place, or during pin/unpin transitions.
 (...unless it was already out of balance due to a long-term RDMA pin being in
 place.)
 
+Other diagnostics
+=================
+
+dump_page() has been enhanced slightly, to handle these new counting fields, and
+to better report on compound pages in general. Specifically, for compound pages
+with order > 1, the exact (hpage_pinned_refcount) pincount is reported.
+
 References
 ==========
 
diff --git a/mm/debug.c b/mm/debug.c
index f5ffb0784559..2189357f0987 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -85,11 +85,22 @@ void __dump_page(struct page *page, const char *reason)
 	mapcount = PageSlab(head) ? 0 : page_mapcount(page);
 
 	if (compound)
-		pr_warn("page:%px refcount:%d mapcount:%d mapping:%p "
-			"index:%#lx head:%px order:%u compound_mapcount:%d\n",
-			page, page_ref_count(head), mapcount,
-			mapping, page_to_pgoff(page), head,
-			compound_order(head), compound_mapcount(page));
+		if (hpage_pincount_available(page)) {
+			pr_warn("page:%px refcount:%d mapcount:%d mapping:%p "
+				"index:%#lx head:%px order:%u "
+				"compound_mapcount:%d compound_pincount:%d\n",
+				page, page_ref_count(head), mapcount,
+				mapping, page_to_pgoff(page), head,
+				compound_order(head), compound_mapcount(page),
+				compound_pincount(page));
+		} else {
+			pr_warn("page:%px refcount:%d mapcount:%d mapping:%p "
+				"index:%#lx head:%px order:%u "
+				"compound_mapcount:%d\n",
+				page, page_ref_count(head), mapcount,
+				mapping, page_to_pgoff(page), head,
+				compound_order(head), compound_mapcount(page));
+		}
 	else
 		pr_warn("page:%px refcount:%d mapcount:%d mapping:%p index:%#lx\n",
 			page, page_ref_count(page), mapcount,
-- 
2.25.0