Right now the only means by which we can write-protect a range using the reverse mapping is via folio_mkclean(). However this is not always the appropriate means of doing so, specifically in the case of the framebuffer deferred I/O logic (fb_defio enabled by CONFIG_FB_DEFERRED_IO). There, kernel pages are mapped read-only and write-protect faults used to batch up I/O operations. Each time the deferred work is done, folio_mkclean() is used to mark the framebuffer page as having had I/O performed on it. However doing so requires the kernel page (perhaps allocated via vmalloc()) to have its page->mapping, index fields set so the rmap can find everything that maps it in order to write-protect. This is problematic as firstly, these fields should not be set for kernel-allocated memory, and secondly these are not folios (it's not user memory) and page->index, mapping fields are now deprecated and soon to be removed. The implementers cannot be blamed for having used this however, as there is simply no other way of performing this operation correctly. This series fixes this - we provide the rmap_wrprotect_page() function to allow the reverse mapping to be used to look up mappings from the page cache object (i.e. its address_space pointer) at a specific offset. The fb_defio logic already stores this offset, and can simply be expanded to keep track of the page cache object, so the change then becomes straight-forward. This series should have no functional change. *** REVIEWERS NOTES: *** I do not have any hardware that uses fb_defio, so I'm asking for help with testing this series from those who do :) I have tested the mm side of this, and done a quick compile smoke test of the fb_defio side but this _very much_ requires testing on actual hardware to ensure everything behaves as expected. This is based on Andrew's tree [0] in the mm-unstable branch - I was thinking it'd be best to go through the mm tree (with fb_defio maintainer approval, of course!) as it relies upon the mm changes to work correctly. [0]: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/ Lorenzo Stoakes (3): mm: refactor rmap_walk_file() to separate out traversal logic mm: provide rmap_wrprotect_file_page() function fb_defio: do not use deprecated page->mapping, index fields drivers/video/fbdev/core/fb_defio.c | 34 +++---- include/linux/fb.h | 1 + include/linux/rmap.h | 20 +++++ mm/rmap.c | 135 ++++++++++++++++++++++------ 4 files changed, 141 insertions(+), 49 deletions(-) -- 2.47.1
