[PATCH v3 3/4] fbdev/deferred-io: Support contiguous kernel memory framebuffers

[Michael Kelley]

From: Michael Kelley <mhklinux at outlook.com> Sent: Tuesday, June 3, 2025 10:25 AM
> 
> From: David Hildenbrand <david at redhat.com> Sent: Tuesday, June 3, 2025 12:55 AM
> >
> > On 03.06.25 03:49, Michael Kelley wrote:
> > > From: David Hildenbrand <david at redhat.com> Sent: Monday, June 2, 2025 2:48 AM
> > >>

[snip]

> > >>> @@ -182,20 +221,34 @@ static vm_fault_t fb_deferred_io_track_page(struct fb_info *info, unsigned long
> > >>>    	}
> > >>>
> > >>>    	/*
> > >>> -	 * We want the page to remain locked from ->page_mkwrite until
> > >>> -	 * the PTE is marked dirty to avoid mapping_wrprotect_range()
> > >>> -	 * being called before the PTE is updated, which would leave
> > >>> -	 * the page ignored by defio.
> > >>> -	 * Do this by locking the page here and informing the caller
> > >>> -	 * about it with VM_FAULT_LOCKED.
> > >>> +	 * The PTE must be marked writable before the defio deferred work runs
> > >>> +	 * again and potentially marks the PTE write-protected. If the order
> > >>> +	 * should be switched, the PTE would become writable without defio
> > >>> +	 * tracking the page, leaving the page forever ignored by defio.
> > >>> +	 *
> > >>> +	 * For vmalloc() framebuffers, the associated struct page is locked
> > >>> +	 * before releasing the defio lock. mm will later mark the PTE writaable
> > >>> +	 * and release the struct page lock. The struct page lock prevents
> > >>> +	 * the page from being prematurely being marked write-protected.
> > >>> +	 *
> > >>> +	 * For FBINFO_KMEMFB framebuffers, mm assumes there is no struct page,
> > >>> +	 * so the PTE must be marked writable while the defio lock is held.
> > >>>    	 */
> > >>> -	lock_page(pageref->page);
> > >>> +	if (info->flags & FBINFO_KMEMFB) {
> > >>> +		unsigned long pfn = page_to_pfn(pageref->page);
> > >>> +
> > >>> +		ret = vmf_insert_mixed_mkwrite(vmf->vma, vmf->address,
> > >>> +					       __pfn_to_pfn_t(pfn, PFN_SPECIAL));
> > >>
> > >> Will the VMA have VM_PFNMAP or VM_MIXEDMAP set? PFN_SPECIAL is a
> > >> horrible hack.
> > >>
> > >> In another thread, you mention that you use PFN_SPECIAL to bypass the
> > >> check in vm_mixed_ok(), so VM_MIXEDMAP is likely not set?
> > >
> > > The VMA has VM_PFNMAP set, not VM_MIXEDMAP.  It seemed like
> > > VM_MIXEDMAP is somewhat of a superset of VM_PFNMAP, but maybe that's
> > > a wrong impression.
> >
> > VM_PFNMAP: nothing is refcounted except anon pages
> >
> > VM_MIXEDMAP: anything with a "struct page" (pfn_valid()) is refcounted
> >
> > pte_special() is a way for GUP-fast to distinguish these refcounted (can
> > GUP) from non-refcounted (camnnot GUP) pages mapped by PTEs without any
> > locks or the VMA being available.
> >
> > Setting pte_special() in VM_MIXEDMAP on ptes that have a "struct page"
> > (pfn_valid()) is likely very bogus.
> 
> OK, good to know.
> 
> >
> > > vm_mixed_ok() does a thorough job of validating the
> > > use of __vm_insert_mixed(), and since what I did was allowed, I thought
> > > perhaps it was OK. Your feedback has set me straight, and that's what I
> > > needed. :-)
> >
> > What exactly are you trying to achieve? :)
> >
> > If it's mapping a page with a "struct page" and *not* refcounting it,
> > then vmf_insert_pfn() is the current way to achieve that in a VM_PFNMAP
> > mapping. It will set pte_special() automatically for you.
> >
> 
> Yes, that's what I'm using to initially create the special PTE in the
> .fault callback.
> 
> > >
> > > But the whole approach is moot with Alistair Popple's patch set that
> > > eliminates pfn_t. Is there an existing mm API that will do mkwrite on a
> > > special PTE in a VM_PFNMAP VMA? I didn't see one, but maybe I missed
> > > it. If there's not one, I'll take a crack at adding it in the next version of my
> > > patch set.
> >
> > I assume you'd want vmf_insert_pfn_mkwrite(), correct? Probably
> > vmf_insert_pfn_prot() can be used by adding PAGE_WRITE to pgprot. (maybe
> > :) )
> 
> Ok, I'll look at that more closely. The sequence is that the special
> PTE gets created with vmf_insert_pfn(). Then when the page is first
> written to, the .pfn_mkwrite callback is invoked by mm. The question
> is the best way for that callback to mark the existing PTE as writable.
> 

FWIW, vmf_insert_pfn_prot() won't work. It calls insert_pfn() with
the "mkwrite" parameter set to 'false', in which case insert_pfn()
does nothing if the PTE already exists.

So I would need to create a new API that does appropriate validation
for a VM_PFNMAP VMA, and then calls insert_pfn() with the "mkwrite"
parameter set to 'true'.

Michael