# \[PATCH\] Documentation: update pagemap with SOFT\_DIRTY \& UFFD\_WP shmem issue - From: Tiberiu A Georgescu \ - Date: 2021-07-30 15:51:57 - List: linux-doc - Page: https://ratatoskr.run/linux-doc/2021/07/8526493 - Thread: https://ratatoskr.run/linux-doc/2021/07/8526493/t.md ```text Mentioning the current missing functionality of the pagemap, 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 --- Documentation/admin-guide/mm/pagemap.rst | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst index fb578fbbb76c..627f3832b3a2 100644 --- a/Documentation/admin-guide/mm/pagemap.rst +++ b/Documentation/admin-guide/mm/pagemap.rst @@ -207,3 +207,9 @@ Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is always 12 at most architectures). Since Linux 3.11 their meaning changes after first clear of soft-dirty bits. Since Linux 4.2 they are used for flags unconditionally. + +Note that the page table entries for swappable and non-syncable pages are +cleared when those pages are zapped or swapped out. This makes information +about the page disappear from the pagemap. The location of the swapped +page can still be retrieved from the page cache, but flags like SOFT_DIRTY +and UFFD_WP are lost irretrievably. -- 2.32.0.380.geb27b338a3 ```