[PATCH v3 1/1] Documentation: update pagemap with shmem exceptions
From: Tiberiu A Georgescu <hidden>
Date: 2021-09-23 06:46:41
Also in:
linux-mm, lkml
Subsystem:
documentation, memory management - misc, the rest · Maintainers:
Jonathan Corbet, Andrew Morton, David Hildenbrand, Linus Torvalds
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 <redacted> Reviewed-by: Ivan Teterevkov <redacted> Reviewed-by: Florian Schmidt <redacted> Reviewed-by: Carl Waldspurger <redacted> Reviewed-by: Jonathan Davies <redacted> --- 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 ===========
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2.33.0.363.g4c719308ce