Re: [PATCH v11 1/5] mm: softdirty: Add pgtable_soft_dirty_supported()

[David Hildenbrand <david@redhat.com>]

On 12.09.25 11:21, Chunyan Zhang wrote:
> On Fri, 12 Sept 2025 at 16:41, David Hildenbrand <david@redhat.com> wrote:
>>
>> [...]
>>
>>>>> +/*
>>>>> + * We should remove the VM_SOFTDIRTY flag if the soft-dirty bit is
>>>>> + * unavailable on which the kernel is running, even if the architecture
>>>>> + * provides the resource and soft-dirty is compiled in.
>>>>> + */
>>>>> +#ifdef CONFIG_MEM_SOFT_DIRTY
>>>>> +     if (!pgtable_soft_dirty_supported())
>>>>> +             mnemonics[ilog2(VM_SOFTDIRTY)][0] = 0;
>>>>> +#endif
>>>>
>>>> You can now drop the ifdef.
>>>
>>> Ok, you mean define VM_SOFTDIRTY 0x08000000 no matter if
>>> MEM_SOFT_DIRTY is compiled in, right?
>>>
>>> Then I need memcpy() to set mnemonics[ilog2(VM_SOFTDIRTY)] here.
>>
>> The whole hunk will not be required when we make sure VM_SOFTDIRTY never
>> gets set, correct?
> 
> Oh no, this hunk code does not set vmflag.
> The mnemonics[ilog2(VM_SOFTDIRTY)] is for show_smap_vma_flags(),
> something like below:
> # cat /proc/1/smaps
> 5555605c7000-555560680000 r-xp 00000000 fe:00 19
>    /bin/busybox
> ...
> VmFlags: rd ex mr mw me sd
> 
> 'sd' is for soft-dirty
> 
> I think this is still needed, right?

If nobody sets VM_SOFTDIRTY in vma->vm_flags, then we will never print it.

So you can just leave the "#ifdef CONFIG_MEM_SOFT_DIRTY" as is to handle 
the VM_SOFTDIRTY=0 case.

So you should not have to change anything in show_smap_vma_flags().

[...]

>>>> That should be handled with the above never-set-VM_SOFTDIRTY.
>>>
>>> We don't need to check if (!pgtable_soft_dirty_supported()) if I
>>> understand correctly.
>> Hm, let me think about that. No, I think this has to stay as the comment
>> says, so this case here is special.
> 
> I will cook a new version and then we can discuss further based on the
> new patch.


Sounds good!


-- 
Cheers

David / dhildenb