[PATCH] mm: dump_page: add debugfs file for dumping page state by pfn

[Konstantin Khlebnikov]

Tool 'page-types' could list pages mapped by process or file cache pages,
but it shows only limited amount of state exported via procfs.

Let's employ existing helper dump_page() to reach remaining information:
writing pfn into /sys/kernel/debug/dump_page dumps state into kernel log.

# echo 0x37c43c > /sys/kernel/debug/dump_page
# dmesg | tail -6
 page:ffffcb0b0df10f00 refcount:1 mapcount:0 mapping:000000007755d3d9 index:0x30
 0xffffffffae4239e0 name:"libGeoIP.so.1.6.9"
 flags: 0x200000000020014(uptodate|lru|mappedtodisk)
 raw: 0200000000020014 ffffcb0b187fd288 ffffcb0b189e6248 ffff9528a04afe10
 raw: 0000000000000030 0000000000000000 00000001ffffffff 0000000000000000
 page dumped because: debugfs request

With CONFIG_PAGE_OWNER=y shows also stacks for last page alloc and free:

 page:ffffea0018fff480 refcount:1 mapcount:1 mapping:0000000000000000 index:0x7f9f28f62
 anon flags: 0x100000000080034(uptodate|lru|active|swapbacked)
 raw: 0100000000080034 ffffea00184140c8 ffffea0018517d88 ffff8886076ba161
 raw: 00000007f9f28f62 0000000000000000 0000000100000000 ffff888bfc79f000
 page dumped because: debugfs request
 page->mem_cgroup:ffff888bfc79f000
 page_owner tracks the page as allocated
 page last allocated via order 0, migratetype Movable, gfp_mask 0x100dca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO)
  prep_new_page+0x139/0x1a0
  get_page_from_freelist+0xde9/0x14e0
  __alloc_pages_nodemask+0x18b/0x360
  alloc_pages_vma+0x7c/0x270
  __handle_mm_fault+0xd40/0x12b0
  handle_mm_fault+0xe7/0x1e0
  do_page_fault+0x2d5/0x610
  page_fault+0x2f/0x40
 page last free stack trace:
  free_pcp_prepare+0x11e/0x1c0
  free_unref_page_list+0x71/0x180
  release_pages+0x31e/0x480
  tlb_flush_mmu+0x44/0x150
  tlb_finish_mmu+0x3d/0x70
  exit_mmap+0xdd/0x1a0
  mmput+0x70/0x140
  do_exit+0x33f/0xc40
  do_group_exit+0x3a/0xa0
  __x64_sys_exit_group+0x14/0x20
  do_syscall_64+0x48/0x130
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
---
 Documentation/admin-guide/mm/pagemap.rst |    3 +++
 Documentation/vm/page_owner.rst          |   10 ++++++++++
 mm/debug.c                               |   27 +++++++++++++++++++++++++++
 3 files changed, 40 insertions(+)

diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
index 340a5aee9b80..663ad5490d72 100644
--- a/Documentation/admin-guide/mm/pagemap.rst
+++ b/Documentation/admin-guide/mm/pagemap.rst
@@ -205,3 +205,6 @@ Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
 always 12 at most architectures). Since Linux 3.11 their meaning changes
 after first clear of soft-dirty bits. Since Linux 4.2 they are used for
 flags unconditionally.
+
+Page state could be dumped into kernel log by writing pfn in text form
+into /sys/kernel/debug/dump_page.
diff --git a/Documentation/vm/page_owner.rst b/Documentation/vm/page_owner.rst
index 0ed5ab8c7ab4..d4d4dc64c19d 100644
--- a/Documentation/vm/page_owner.rst
+++ b/Documentation/vm/page_owner.rst
@@ -88,3 +88,13 @@ Usage
 
    See the result about who allocated each page
    in the ``sorted_page_owner.txt``.
+
+Notes
+=====
+
+To lookup pages in file cache or mapped in process you could use interface
+pagemap documented in Documentation/admin-guide/mm/pagemap.rst or tool
+page-types in the tools/vm directory.
+
+Page state could be dumped into kernel log by writing pfn in text form
+into /sys/kernel/debug/dump_page.
diff --git a/mm/debug.c b/mm/debug.c
index 2189357f0987..5803f2b63d95 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -14,6 +14,7 @@
 #include <linux/migrate.h>
 #include <linux/page_owner.h>
 #include <linux/ctype.h>
+#include <linux/debugfs.h>
 
 #include "internal.h"
 
@@ -147,6 +148,32 @@ void dump_page(struct page *page, const char *reason)
 }
 EXPORT_SYMBOL(dump_page);
 
+#ifdef CONFIG_DEBUG_FS
+static int dump_page_set(void *data, u64 pfn)
+{
+	struct page *page;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	page = pfn_to_online_page(pfn);
+	if (!page)
+		return -ENXIO;
+
+	dump_page(page, "debugfs request");
+	return 0;
+}
+DEFINE_DEBUGFS_ATTRIBUTE(dump_page_fops, NULL, dump_page_set, "%llx\n");
+
+static int __init dump_page_debugfs(void)
+{
+	debugfs_create_file_unsafe("dump_page", 0200, NULL, NULL,
+				   &dump_page_fops);
+	return 0;
+}
+late_initcall(dump_page_debugfs);
+#endif /* CONFIG_DEBUG_FS */
+
 #ifdef CONFIG_DEBUG_VM
 
 void dump_vma(const struct vm_area_struct *vma)