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authorPavel Emelyanov <xemul@parallels.com>2013-07-03 15:01:20 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2013-07-03 16:07:26 -0700
commit0f8975ec4db2c8b5bd111b211292ca9be0feb6b8 (patch)
tree47bb0acc9fc3e783ad9cf33097a6636190f5e42b /Documentation/filesystems
parent2b0a9f017548f05e42fbf7e67c4a626c1ebd5e12 (diff)
mm: soft-dirty bits for user memory changes tracking
The soft-dirty is a bit on a PTE which helps to track which pages a task writes to. In order to do this tracking one should 1. Clear soft-dirty bits from PTEs ("echo 4 > /proc/PID/clear_refs) 2. Wait some time. 3. Read soft-dirty bits (55'th in /proc/PID/pagemap2 entries) To do this tracking, the writable bit is cleared from PTEs when the soft-dirty bit is. Thus, after this, when the task tries to modify a page at some virtual address the #PF occurs and the kernel sets the soft-dirty bit on the respective PTE. Note, that although all the task's address space is marked as r/o after the soft-dirty bits clear, the #PF-s that occur after that are processed fast. This is so, since the pages are still mapped to physical memory, and thus all the kernel does is finds this fact out and puts back writable, dirty and soft-dirty bits on the PTE. Another thing to note, is that when mremap moves PTEs they are marked with soft-dirty as well, since from the user perspective mremap modifies the virtual memory at mremap's new address. Signed-off-by: Pavel Emelyanov <xemul@parallels.com> Cc: Matt Mackall <mpm@selenic.com> Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com> Cc: Glauber Costa <glommer@parallels.com> Cc: Marcelo Tosatti <mtosatti@redhat.com> Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com> Cc: Stephen Rothwell <sfr@canb.auug.org.au> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/filesystems')
-rw-r--r--Documentation/filesystems/proc.txt7
1 files changed, 6 insertions, 1 deletions
diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index fd8d0d594fc7..fcc22c982a25 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -473,7 +473,8 @@ This file is only present if the CONFIG_MMU kernel configuration option is
enabled.
The /proc/PID/clear_refs is used to reset the PG_Referenced and ACCESSED/YOUNG
-bits on both physical and virtual pages associated with a process.
+bits on both physical and virtual pages associated with a process, and the
+soft-dirty bit on pte (see Documentation/vm/soft-dirty.txt for details).
To clear the bits for all the pages associated with the process
> echo 1 > /proc/PID/clear_refs
@@ -482,6 +483,10 @@ To clear the bits for the anonymous pages associated with the process
To clear the bits for the file mapped pages associated with the process
> echo 3 > /proc/PID/clear_refs
+
+To clear the soft-dirty bit
+ > echo 4 > /proc/PID/clear_refs
+
Any other value written to /proc/PID/clear_refs will have no effect.
The /proc/pid/pagemap gives the PFN, which can be used to find the pageflags