linux-kernel - Re: [PATCH v11 1/5] mm: softdirty: Add pgtable_soft_dirty

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Message-ID: <b2334e92-9d04-4d27-aac1-fec91d5ee464@redhat.com>
Date: Fri, 12 Sep 2025 15:32:42 +0200
From: David Hildenbrand <david@...hat.com>
To: Chunyan Zhang <zhang.lyra@...il.com>
Cc: Chunyan Zhang <zhangchunyan@...as.ac.cn>,
 linux-riscv@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Alexandre Ghiti <alex@...ti.fr>, Deepak Gupta <debug@...osinc.com>,
 Ved Shanbhogue <ved@...osinc.com>, Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 Andrew Morton <akpm@...ux-foundation.org>, Peter Xu <peterx@...hat.com>,
 Arnd Bergmann <arnd@...db.de>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Axel Rasmussen <axelrasmussen@...gle.com>, Yuanchu Xie <yuanchu@...gle.com>
Subject: Re: [PATCH v11 1/5] mm: softdirty: Add pgtable_soft_dirty_supported()

On 12.09.25 11:21, Chunyan Zhang wrote:
> On Fri, 12 Sept 2025 at 16:41, David Hildenbrand <david@...hat.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