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 <tiberiu.georgescu@xxxxxxxxxxx> Reviewed-by: Ivan Teterevkov <ivan.teterevkov@xxxxxxxxxxx> Reviewed-by: Florian Schmidt <florian.schmidt@xxxxxxxxxxx> Reviewed-by: Carl Waldspurger <carl.waldspurger@xxxxxxxxxxx> Reviewed-by: Jonathan Davies <jonathan.davies@xxxxxxxxxxx> --- 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..ea3f88f3c18d 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 swapped or none can be deduced with the +lseek system call. For a single page, the algorithm is: + +0. If the pagemap entry of the page has bit 63 (page present) set, the page + is present. +1. Otherwise, get an fd to the file where the page is backed. For anonymous + shared pages, the file can be found in ``/proc/pid/map_files/``. +2. Call lseek with LSEEK_DATA flag and seek to the virtual address of the page + you wish to inspect. If it overshoots the PAGE_SIZE, the page is NONE. +3. Otherwise, the page is in swap. + Other notes =========== -- 2.33.0.363.g4c719308ce
