Re: [PATCH v3 5/6] drm/gem-shmem: Track folio accessed/dirty status in mmap

[Boris Brezillon <boris.brezillon@collabora.com>]

On Wed, 20 May 2026 10:11:03 -0300
Igor Torrente <igor.torrente@collabora.com> wrote:

> On 2/9/26 10:27, Thomas Zimmermann wrote:
> > Invoke folio_mark_accessed() in mmap page faults to add the folio to
> > the memory manager's LRU list. Userspace invokes mmap to get the memory
> > for software rendering. Compositors do the same when creating the final
> > on-screen image, so keeping the pages in LRU makes sense. Avoids paging
> > out graphics buffers when under memory pressure.
> >
> > In pfn_mkwrite, further invoke the folio_mark_dirty() to add the folio
> > for writeback should the underlying file be paged out from system memory.
> > This rarely happens in practice, yet it would corrupt the buffer content.
> >
> > This has little effect on a system's hardware-accelerated rendering, which
> > only mmaps for an initial setup of textures, meshes, shaders, etc.
> >
> > v3:
> > - rewrite for VM_PFNMAP
> > v2:
> > - adapt to changes in drm_gem_shmem_try_mmap_pmd()
> >
> > Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> > Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>  
> 
> Hi Thomas,
> 
> I have and Radxa ROCK 5B+ with Mali-G610 GPU and I'm developing the VDRM 
> (native context) driver
> for panthor.ko.
> 
> Recently I update my kernel from 7.0 to the 7.1-rc3, and qemu started 
> freezing with KVM error below
> when running any graphical application with Venus or Panthor VDRM. For 
> some reason Virgl is not affected.
> 
> ```
> error: kvm run failed Bad address
> PC=0000ffffb4a86d70 X00=0000ffffb4be5000 X01=0000ffffb2279bb0
> X02=0000000000000600 X03=0000ffffb4be5000 X04=0000ffffb227a1b0
> X05=0000ffffb4be5600 X06=0000000000000001 X07=0000aaaaf384fd80
> X08=dcc58d0daf897753 X09=0000000000000002 X10=0000ffffb4b96758
> X11=000000002533f84b X12=0000000000001060 X13=0000000000000000
> X14=0000000000000000 X15=0000000000000010 X16=0000ffffb3880128
> X17=0000ffffb4a86c80 X18=0000000000000000 X19=0000aaaaf384f8a0
> X20=0000aaaaf37cec80 X21=0000aaaaf37c7210 X22=0000aaaaf37c9550
> X23=0000aaaaf37c74e0 X24=0000ffffb3b47fc8 X25=0000ffffb3b47fc8
> X26=0000ffffb3b43c78 X27=0000aaaaf37e1840 X28=0000aaaaf37cee50
> X29=0000ffffecbf1a60 X30=0000ffffb364b530  SP=0000ffffecbf1a60
> PSTATE=0000000020001000 --C- EL0t
> ```
> 
> After bisecting I reach to your commit.
> 
> ```
> 28e3918179aa75cfd4490b3b0281496ed91d829a  drm/gem-shmem:
> Track folio accessed/dirty status in mmap
> ```
> 
> It still reproducible in the 7.1-rc4 (ab5fce87a778).
> 
> Appling the following patch (Boris suggestion) fixes the issue for me.
> 
> ```
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c 
> b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 545933c7f712..1e036f69c1ce 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -696,7 +696,7 @@ const struct vm_operations_struct 
> drm_gem_shmem_vm_ops = {
>   #endif
>          .open = drm_gem_shmem_vm_open,
>          .close = drm_gem_shmem_vm_close,
> -       .pfn_mkwrite = drm_gem_shmem_pfn_mkwrite,
> +//     .pfn_mkwrite = drm_gem_shmem_pfn_mkwrite,
>   };
>   EXPORT_SYMBOL_GPL(drm_gem_shmem_vm_ops);
> ```
> 
> I'm not familiar with the drm_gem, code so I don't any more insightful 
> information
> to share.  But hopefully we can find a better fix for this.

Hm, given how close we are from the end of the release cycle, it might
be safer to revert those .pfn_mkwrite changes and consider them again
once all the details have been sorted out (it's the second time around
we hit issues on this patch, and the lack of reviews from MM devs is
scaring me bit to be honest).

Thomas, any opinion?