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

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 20.05.26 um 16:44 schrieb Boris Brezillon:
> 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?

Sure, please go ahead. The mmap change in this series seems rather fragile.

Best regards
Thomas

>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)