Re: [RFC PATCH v2 3/3] fb_defio: do not use deprecated page->mapping, index fields

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

Andrew - Ugh sorry - please disregard the below, I sent it to the wrong
thread. It's Saturday and I'm tired and brain not working :>)

Let me resend this against the correct non-RFC thread!

On Sat, Feb 01, 2025 at 05:01:15PM +0000, Lorenzo Stoakes wrote:
> On Mon, Jan 13, 2025 at 11:15:48PM +0000, Lorenzo Stoakes wrote:
> > With the introduction of mapping_wrprotect_page() there is no need to use
> > folio_mkclean() in order to write-protect mappings of frame buffer pages,
> > and therefore no need to inappropriately set kernel-allocated page->index,
> > mapping fields to permit this operation.
> >
> > Instead, store the pointer to the page cache object for the mapped driver
> > in the fb_deferred_io object, and use the already stored page offset from
> > the pageref object to look up mappings in order to write-protect them.
> >
> > This is justified, as for the page objects to store a mapping pointer at
> > the point of assignment of pages, they must all reference the same
> > underlying address_space object. Since the life time of the pagerefs is
> > also the lifetime of the fb_deferred_io object, storing the pointer here
> > makes snese.
> >
> > This eliminates the need for all of the logic around setting and
> > maintaining page->index,mapping which we remove.
> >
> > This eliminates the use of folio_mkclean() entirely but otherwise should
> > have no functional change.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>
> Andrew -
>
> Sorry to be a pain but could you please apply the attached fix-patch to
> avoid build bot failures when randconfig generates invalid
> configurations. The defio mechanism entirely relies upon the page faulting
> mechanism, and thus an MMU to function.
>
> This was previously masked, because folio_mkclean() happens to have a
> !CONFIG_MMU stub. It really doesn't make sense to apply such a stub for
> mapping_wrprotect_page() for architectures without an MMU.
>
> Instead, correctly express the actual dependency in Kconfig, which should
> prevent randconfig from doing the wrong thing and also helps document this
> fact about defio.
>
> Thanks!
>
> ----8<----
> From 32abcfbb8dea92d9a8a99e6a86f45a1823a75c59 Mon Sep 17 00:00:00 2001
> From: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Date: Sat, 1 Feb 2025 16:56:02 +0000
> Subject: [PATCH] fbdev: have CONFIG_FB_DEFERRED_IO depend on CONFIG_MMU
>
> Frame buffer deferred I/O is entirely reliant on the page faulting
> mechanism (and thus, an MMU) to function.
>
> Express this dependency in the Kconfig, as otherwise randconfig could
> generate invalid configurations resulting in build errors.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202502020030.MnEJ847Z-lkp@intel.com/
> ---
>  drivers/video/fbdev/core/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/drivers/video/fbdev/core/Kconfig b/drivers/video/fbdev/core/Kconfig
> index d554d8c543d4..154804914680 100644
> --- a/drivers/video/fbdev/core/Kconfig
> +++ b/drivers/video/fbdev/core/Kconfig
> @@ -135,6 +135,7 @@ config FB_SYSMEM_FOPS
>  config FB_DEFERRED_IO
>  	bool
>  	depends on FB_CORE
> +	depends on MMU
>
>  config FB_DMAMEM_HELPERS
>  	bool
> --
> 2.48.1