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

[David Hildenbrand <david@redhat.com>]

On 03.06.25 03:49, Michael Kelley wrote:
> From: David Hildenbrand <david@redhat.com> Sent: Monday, June 2, 2025 2:48 AM
>>
>> On 23.05.25 18:15, mhkelley58@gmail.com wrote:
>>> From: Michael Kelley <mhklinux@outlook.com>
>>>
>>> Current defio code works only for framebuffer memory that is allocated
>>> with vmalloc(). The code assumes that the underlying page refcount can
>>> be used by the mm subsystem to manage each framebuffer page's lifecycle,
>>> including freeing the page if the refcount goes to 0. This approach is
>>> consistent with vmalloc'ed memory, but not with contiguous kernel memory
>>> allocated via alloc_pages() or similar. The latter such memory pages
>>> usually have a refcount of 0 when allocated, and would be incorrectly
>>> freed page-by-page if used with defio. That free'ing corrupts the memory
>>> free lists and Linux eventually panics. Simply bumping the refcount after
>>> allocation doesn’t work because when the framebuffer memory is freed,
>>> __free_pages() complains about non-zero refcounts.
>>>
>>> Commit 37b4837959cb ("video: deferred io with physically contiguous
>>> memory") from the year 2008 purported to add support for contiguous
>>> kernel memory framebuffers. The motivating device, sh_mobile_lcdcfb, uses
>>> dma_alloc_coherent() to allocate framebuffer memory, which is likely to
>>> use alloc_pages(). It's unclear to me how this commit actually worked at
>>> the time, unless dma_alloc_coherent() was pulling from a CMA pool instead
>>> of alloc_pages(). Or perhaps alloc_pages() worked differently or on the
>>> arm32 architecture on which sh_mobile_lcdcfb is used.
>>>
>>> In any case, for x86 and arm64 today, commit 37b4837959cb9 is not
>>> sufficient to support contiguous kernel memory framebuffers. The problem
>>> can be seen with the hyperv_fb driver, which may allocate the framebuffer
>>> memory using vmalloc() or alloc_pages(), depending on the configuration
>>> of the Hyper-V guest VM (Gen 1 vs. Gen 2) and the size of the framebuffer.
>>>
>>> Fix this limitation by adding defio support for contiguous kernel memory
>>> framebuffers. A driver with a framebuffer allocated from contiguous
>>> kernel memory must set the FBINFO_KMEMFB flag to indicate such.
>>>
>>> Tested with the hyperv_fb driver in both configurations -- with a vmalloc()
>>> framebuffer and with an alloc_pages() framebuffer on x86. Also verified a
>>> vmalloc() framebuffer on arm64. Hardware is not available to me to verify
>>> that the older arm32 devices still work correctly, but the path for
>>> vmalloc() framebuffers is essentially unchanged.
>>>
>>> Even with these changes, defio does not support framebuffers in MMIO
>>> space, as defio code depends on framebuffer memory pages having
>>> corresponding 'struct page's.
>>>
>>> Fixes: 3a6fb6c4255c ("video: hyperv: hyperv_fb: Use physical memory for fb on HyperV Gen 1 VMs.")
>>> Signed-off-by: Michael Kelley <mhklinux@outlook.com>
>>> ---
>>> Changes in v3:
>>> * Moved definition of FBINFO_KMEMFB flag to a separate patch
>>>     preceeding this one in the patch set [Helge Deller]
>>> Changes in v2:
>>> * Tweaked code comments regarding framebuffers allocated with
>>>     dma_alloc_coherent() [Christoph Hellwig]
>>>
>>>    drivers/video/fbdev/core/fb_defio.c | 128 +++++++++++++++++++++++-----
>>>    1 file changed, 108 insertions(+), 20 deletions(-)
>>>
>>> diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
>>> index 4fc93f253e06..f8ae91a1c4df 100644
>>> --- a/drivers/video/fbdev/core/fb_defio.c
>>> +++ b/drivers/video/fbdev/core/fb_defio.c
>>> @@ -8,11 +8,40 @@
>>>     * for more details.
>>>     */
>>>
>>> +/*
>>> + * Deferred I/O ("defio") allows framebuffers that are mmap()'ed to user space
>>> + * to batch user space writes into periodic updates to the underlying
>>> + * framebuffer hardware or other implementation (such as with a virtualized
>>> + * framebuffer in a VM). At each batch interval, a callback is invoked in the
>>> + * framebuffer's kernel driver, and the callback is supplied with a list of
>>> + * pages that have been modified in the preceding interval. The callback can
>>> + * use this information to update the framebuffer hardware as necessary. The
>>> + * batching can improve performance and reduce the overhead of updating the
>>> + * hardware.
>>> + *
>>> + * Defio is supported on framebuffers allocated using vmalloc() and allocated
>>> + * as contiguous kernel memory using alloc_pages() or kmalloc(). These
>>> + * memory allocations all have corresponding "struct page"s. Framebuffers
>>> + * allocated using dma_alloc_coherent() should not be used with defio.
>>> + * Such allocations should be treated as a black box owned by the DMA
>>> + * layer, and should not be deconstructed into individual pages as defio
>>> + * does. Framebuffers in MMIO space are *not* supported because MMIO space
>>> + * does not have corrresponding "struct page"s.
>>> + *
>>> + * For framebuffers allocated using vmalloc(), struct fb_info must have
>>> + * "screen_buffer" set to the vmalloc address of the framebuffer. For
>>> + * framebuffers allocated from contiguous kernel memory, FBINFO_KMEMFB must
>>> + * be set, and "fix.smem_start" must be set to the physical address of the
>>> + * frame buffer. In both cases, "fix.smem_len" must be set to the framebuffer
>>> + * size in bytes.
>>> + */
>>> +
>>>    #include <linux/module.h>
>>>    #include <linux/kernel.h>
>>>    #include <linux/errno.h>
>>>    #include <linux/string.h>
>>>    #include <linux/mm.h>
>>> +#include <linux/pfn_t.h>
>>>    #include <linux/vmalloc.h>
>>>    #include <linux/delay.h>
>>>    #include <linux/interrupt.h>
>>> @@ -37,7 +66,7 @@ static struct page *fb_deferred_io_get_page(struct fb_info *info, unsigned long
>>>    	else if (info->fix.smem_start)
>>>    		page = pfn_to_page((info->fix.smem_start + offs) >> PAGE_SHIFT);
>>>
>>> -	if (page)
>>> +	if (page && !(info->flags & FBINFO_KMEMFB))
>>>    		get_page(page);
>>>
>>>    	return page;
>>> @@ -137,6 +166,15 @@ static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf)
>>>
>>>    	BUG_ON(!info->fbdefio->mapping);
>>>
>>> +	if (info->flags & FBINFO_KMEMFB)
>>> +		/*
>>> +		 * In this path, the VMA is marked VM_PFNMAP, so mm assumes
>>> +		 * there is no struct page associated with the page. The
>>> +		 * PFN must be directly inserted and the created PTE will be
>>> +		 * marked "special".
>>> +		 */
>>> +		return vmf_insert_pfn(vmf->vma, vmf->address, page_to_pfn(page));
>>> +
>>>    	vmf->page = page;
>>>    	return 0;
>>>    }
>>> @@ -163,13 +201,14 @@ EXPORT_SYMBOL_GPL(fb_deferred_io_fsync);
>>>
>>>    /*
>>>     * Adds a page to the dirty list. Call this from struct
>>> - * vm_operations_struct.page_mkwrite.
>>> + * vm_operations_struct.page_mkwrite or .pfn_mkwrite.
>>>     */
>>> -static vm_fault_t fb_deferred_io_track_page(struct fb_info *info, unsigned long offset,
>>> +static vm_fault_t fb_deferred_io_track_page(struct fb_info *info, struct vm_fault *vmf,
>>>    					    struct page *page)
>>>    {
>>>    	struct fb_deferred_io *fbdefio = info->fbdefio;
>>>    	struct fb_deferred_io_pageref *pageref;
>>> +	unsigned long offset = vmf->pgoff << PAGE_SHIFT;
>>>    	vm_fault_t ret;
>>>
>>>    	/* protect against the workqueue changing the page list */
>>> @@ -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.

> 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.

> 
> 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 
:) )

-- 
Cheers,

David / dhildenb