[PATCH 4/4] Documentation: update Documentation/vm/pagemap.txt

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

This patch adds a chapter about kpagecache interface.

ChangeLog:
- add len column in example output

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
 Documentation/vm/pagemap.txt | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git v3.15-rc5.orig/Documentation/vm/pagemap.txt v3.15-rc5/Documentation/vm/pagemap.txt
index 5948e455c4d2..12a871efd372 100644
--- v3.15-rc5.orig/Documentation/vm/pagemap.txt
+++ v3.15-rc5/Documentation/vm/pagemap.txt
@@ -150,3 +150,32 @@ once.
 Reading from any of the files will return -EINVAL if you are not starting
 the read on an 8-byte boundary (e.g., if you sought an odd number of bytes
 into the file), or if the size of the read is not a multiple of 8 bytes.
+
+
+kpagecache, from file perspective
+---------------------------------
+
+Similarly to pagemap, we have a interface /proc/kpagecache to let userspace
+know about pagecache profile for a given file. Unlike pagemap interface,
+we don't have to mmap() and fault in the target file, so the impact on other
+workloads (maybe the target of your analysis) is minimum.
+
+To use this interface, firstly we open it and write the name of the target
+file to it for setup. And then we can read the pagecache info of the file.
+The file contains the array of 64-bit entries for each page offset. Data
+format is like below:
+
+    * Bits  0-49  page frame number (PFN) if present
+    * Bits 50-59  zero (reserved)
+    * Bits 60-63  pagecache tags
+
+Good example is tools/vm/page-types.c, where we can get the list of pages
+belonging to the file like below:
+
+  $ dd if=/dev/urandom of=file bs=4096 count=2
+  $ date >> file
+  $ tools/vm/page-types -f file -Nl
+  voffset offset  len     tag     flags
+  0       640c7   1       0       __RU_l______________________________
+  1       640d7   1       0       __RU_l______________________________
+  2       640f4   1       1       ___UDlA_____________________________
-- 
1.9.0

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