From 5b32e44e8b885e673b1a1a748f4dfb33208d7f08 Mon Sep 17 00:00:00 2001 From: Tiberiu A Georgescu Date: Thu, 23 Sep 2021 06:46:18 +0000 Subject: [PATCH] Documentation: update pagemap with shmem exceptions Mentioning the current missing information in the pagemap and alternatives on how to retrieve it, in case someone stumbles upon unexpected behaviour. Signed-off-by: Tiberiu A Georgescu Reviewed-by: Ivan Teterevkov Reviewed-by: Florian Schmidt Reviewed-by: Carl Waldspurger Reviewed-by: Jonathan Davies Reviewed-by: David Hildenbrand Reviewed-by: Peter Xu Link: https://lore.kernel.org/r/20210923064618.157046-2-tiberiu.georgescu@nutanix.com Signed-off-by: Jonathan Corbet --- Documentation/admin-guide/mm/pagemap.rst | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst index fb578fbbb76c..4581527c07ae 100644 --- a/Documentation/admin-guide/mm/pagemap.rst +++ b/Documentation/admin-guide/mm/pagemap.rst @@ -196,6 +196,28 @@ you can go through every map in the process, find the PFNs, look those up in kpagecount, and tally up the number of pages that are only referenced once. +Exceptions for Shared Memory +============================ + +Page table entries for shared pages are cleared when the pages are zapped or +swapped out. This makes swapped out pages indistinguishable from never-allocated +ones. + +In kernel space, the swap location can still be retrieved from the page cache. +However, values stored only on the normal PTE get lost irretrievably when the +page is swapped out (i.e. SOFT_DIRTY). + +In user space, whether the page is present, swapped or none can be deduced with +the help of lseek and/or mincore system calls. + +lseek() can differentiate between accessed pages (present or swapped out) and +holes (none/non-allocated) by specifying the SEEK_DATA flag on the file where +the pages are backed. For anonymous shared pages, the file can be found in +``/proc/pid/map_files/``. + +mincore() can differentiate between pages in memory (present, including swap +cache) and out of memory (swapped out or none/non-allocated). + Other notes ===========