[PATCH] pagemap: update documentation

Konstantin Khlebnikov
2015-07-16T18:47:48Z

Notes about recent changes.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
Documentation/vm/pagemap.txt | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/Documentation/vm/pagemap.txt b/Documentation/vm/pagemap.txt
index 3cfbbb333ea1..aab39aa7dd8f 100644
--- a/Documentation/vm/pagemap.txt
+++ b/Documentation/vm/pagemap.txt
@@ -16,12 +16,17 @@ There are three components to pagemap:
* Bits 0-4 swap type if swapped
* Bits 5-54 swap offset if swapped
* Bit 55 pte is soft-dirty (see Documentation/vm/soft-dirty.txt)
- * Bit 56 page exlusively mapped
+ * Bit 56 page exclusively mapped (since 4.2)
* Bits 57-60 zero
- * Bit 61 page is file-page or shared-anon
+ * Bit 61 page is file-page or shared-anon (since 3.5)
* Bit 62 page swapped
* Bit 63 page present

+ Since Linux 4.0 only users with the CAP_SYS_ADMIN capability can get PFNs:
+ for unprivileged users from 4.0 till 4.2 open fails with -EPERM, starting
+ from from 4.2 PFN field is zeroed if user has no CAP_SYS_ADMIN capability.
+ Reason: information about PFNs helps in exploiting Rowhammer vulnerability.
+
If the page is not present but in swap, then the PFN contains an
encoding of the swap file number and the page's offset into the
swap. Unmapped pages return a null PFN. This allows determining
@@ -160,3 +165,8 @@ Other notes:
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.
+
+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.