[v3,1/1] Documentation: update pagemap with shmem exceptions

Commit Message

Tiberiu A Georgescu Sept. 23, 2021, 6:46 a.m. UTC

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 <[e-mail]>
Reviewed-by: Ivan Teterevkov <[e-mail]>
Reviewed-by: Florian Schmidt <[e-mail]>
Reviewed-by: Carl Waldspurger <[e-mail]>
Reviewed-by: Jonathan Davies <[e-mail]>
---
 Documentation/admin-guide/mm/pagemap.rst | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

Comments

Peter Xu Sept. 23, 2021, 3 p.m. UTC | #1

On Thu, Sep 23, 2021 at 06:46:18AM +0000, Tiberiu A Georgescu wrote:
> 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 <[e-mail]>
> Reviewed-by: Ivan Teterevkov <[e-mail]>
> Reviewed-by: Florian Schmidt <[e-mail]>
> Reviewed-by: Carl Waldspurger <[e-mail]>
> Reviewed-by: Jonathan Davies <[e-mail]>

Reviewed-by: Peter Xu <[e-mail]>

Thanks,

David Hildenbrand Sept. 23, 2021, 3:02 p.m. UTC | #2

On 23.09.21 17:00, Peter Xu wrote:
> On Thu, Sep 23, 2021 at 06:46:18AM +0000, Tiberiu A Georgescu wrote:
>> 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 <[e-mail]>
>> Reviewed-by: Ivan Teterevkov <[e-mail]>
>> Reviewed-by: Florian Schmidt <[e-mail]>
>> Reviewed-by: Carl Waldspurger <[e-mail]>
>> Reviewed-by: Jonathan Davies <[e-mail]>
> 
> Reviewed-by: Peter Xu <[e-mail]>

LGTM as well; I like that we only give hints instead of listing steps to 
perform as in the previous version.

Reviewed-by: David Hildenbrand <[e-mail]>

Patch

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
 ===========