Re: [PATCH] mm: dump_page: add debugfs file for dumping page state by pfn
From: Konstantin Khlebnikov
Date: Mon May 25 2020 - 11:58:29 EST
On 25/05/2020 18.35, Vlastimil Babka wrote:
On 5/25/20 4:19 PM, Konstantin Khlebnikov wrote:
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.
Yeah that's indeed useful, however I'm less sure if kernel log is the proper way
to extract the data. For example IIRC with the page_owner file can "seek to pfn"
to dump it, although that makes it somewhat harder to use.
Or we could write pfn to one file and read the dump from another one? But that's
not atomic.
Perhaps if we could do something like "cat /sys/kernel/debug/dump_page/<pfn>"
without all the pfns being actually listed in the dump_page directory with "ls"?
Is that possible?
Too much code for me. =)
This could be kind of ftrace tracer which iterates over pages and dumps them,
but anyway looks ridiculously overengineered.
This one hack connects existing 'pagemap' with existing 'dump_page', so almost free.
For complicated cases there is gdb and special tool drgn https://github.com/osandov/drgn
Writing script which parses all that stuff from kernel log isn't big deal either.
I have one with 100+ lines regexp for all kinds of kernel splats.
Will publish when find time for polishing.
# 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)