Hi guys, It'd be good if somebody from the drm side could look at this as you are using fields that are deprecated and will be made unavailable (in a non-optional fashion), and relatively soon :) It'd be good to confirm this works on real hardware (none of mine supports this sadly, I tried 3 separate machines...). In effect I provide a whole new mechanism just so defio can do the right thing and retain the exact same behaviour, so just need to confirm it does what you need from our side. Thanks, Lorenzo On Mon, Jan 13, 2025 at 11:15:45PM +0000, Lorenzo Stoakes wrote: > 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 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. > > *** 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/ > > 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/ > > 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(-) > > -- > 2.48.0