This patch follows the discussions on previous documentation patch threads [1][2]. It presents the exception case of shared memory management from the pagemap's point of view. It briefly describes what is missing, why it is missing and alternatives to the pagemap for page info retrieval in user space. In short, the kernel does not keep track of PTEs for swapped out shared pages within the processes that references them. Thus, the proc/pid/pagemap tool cannot print the swap destination of the shared memory pages, instead setting the pagemap entry to zero for both non-allocated and swapped out pages. This can create confusion for users who need information on swapped out pages. The reasons why maintaining the PTEs of all swapped out shared pages among all processes while maintaining similar performance is not a trivial task, or a desirable change, have been discussed extensively [1][3][4][5]. There are also arguments for why this arguably missing information should eventually be exposed to the user in either a future pagemap patch, or by an alternative tool. [1]: https://marc.info/?m=162878395426774 [2]: https://lore.kernel.org/lkml/20210920164931.175411-1-tiberiu.georgescu@xxxxxxxxxxx/ [3]: https://lore.kernel.org/lkml/20210730160826.63785-1-tiberiu.georgescu@xxxxxxxxxxx/ [4]: https://lore.kernel.org/lkml/20210807032521.7591-1-peterx@xxxxxxxxxx/ [5]: https://lore.kernel.org/lkml/20210715201651.212134-1-peterx@xxxxxxxxxx/ Tiberiu A Georgescu (1): Documentation: update pagemap with shmem exceptions Documentation/admin-guide/mm/pagemap.rst | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) -- 2.33.0.363.g4c719308ce
