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.
0f8975e mm: soft-dirty bits for user memory changes tracking
Documentation/filesystems/proc.txt | 7 ++++-
Documentation/vm/soft-dirty.txt | 36 ++++++++++++++++++++++++++
arch/Kconfig | 3 +++
arch/x86/Kconfig | 1 +
arch/x86/include/asm/pgtable.h | 24 +++++++++++++++--
arch/x86/include/asm/pgtable_types.h | 12 +++++++++
fs/proc/task_mmu.c | 47 ++++++++++++++++++++++++++++++----
include/asm-generic/pgtable.h | 22 ++++++++++++++++
mm/Kconfig | 12 +++++++++
mm/huge_memory.c | 2 +-
mm/mremap.c | 2 +-
11 files changed, 158 insertions(+), 10 deletions(-)