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 removal of these fields is imminent, rendering this series more urgent than it might first appear. 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 mapping_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. non-RFC: * Kajtar kindly smoke-tested the defio side of this change and confirmed that it appears to work correctly. I am therefore stripping the RFC and putting forward as a non-RFC series. RFC v2: * Updated Jaya Kumar's email on cc - the MAINTAINERS section is apparently incorrect. * Corrected rmap_walk_file() comment to refer to folios as per Matthew. * Reference folio->mapping rather than folio_mapping(folio) in rmap_walk_file() as per Matthew. * Reference folio->index rather than folio_pgoff(folio) in rmap_walk_file() as per Matthew. * Renamed rmap_wrprotect_file_page() to mapping_wrprotect_page() as per Matthew. * Fixed kerneldoc and moved to implementation as per Matthew. * Updated mapping_wrprotect_page() to take a struct page pointer as per David. * Removed folio lock when invoking mapping_wrprotect_page() in fb_deferred_io_work() as per Matthew. * Removed compound_nr() in fb_deferred_io_work() as per Matthew. RFC v1: https://lore.kernel.org/all/1e452b5b65f15a9a5d0c2ed3f5f812fdd1367603.1736352361.git.lorenzo.stoakes@xxxxxxxxxx/ *** BLURB HERE *** Lorenzo Stoakes (3): mm: refactor rmap_walk_file() to separate out traversal logic mm: provide mapping_wrprotect_page() function fb_defio: do not use deprecated page->mapping, index fields drivers/video/fbdev/core/fb_defio.c | 38 ++----- include/linux/fb.h | 1 + include/linux/rmap.h | 3 + mm/rmap.c | 152 +++++++++++++++++++++++----- 4 files changed, 141 insertions(+), 53 deletions(-) -- 2.48.1
