Re: [PATCH] mm/hmm: Simplify hmm_vma_walk_pud slightly

[Steven Price <steven.price@arm.com>]

On 12/03/2020 15:11, Jason Gunthorpe wrote:
> On Thu, Mar 12, 2020 at 02:40:08PM +0000, Steven Price wrote:
>> On 12/03/2020 14:27, Jason Gunthorpe wrote:
>>> On Thu, Mar 12, 2020 at 10:28:13AM +0000, Steven Price wrote:
>>>> By refactoring to deal with the !pud_huge(pud) || !pud_devmap(pud)
>>>> condition early it's possible to remove the 'ret' variable and remove a
>>>> level of indentation from half the function making the code easier to
>>>> read.
>>>>
>>>> No functional change.
>>>>
>>>> Signed-off-by: Steven Price <steven.price@arm.com>
>>>> Thanks to Jason's changes there were only two code paths left using
>>>> the out_unlock label so it seemed like a good opportunity to
>>>> refactor.
>>>
>>> Yes, I made something very similar, what do you think of this:
>>>
>>> https://github.com/jgunthorpe/linux/commit/93f0ed42ab3f9ceb27b58fb7c7c3ecaf60f16b36
>>
>> Even better! Sorry I didn't realise you'd already done this. I just saw that
>> the function was needlessly complicated after your fix, so I thought I'd do
>> a drive-by cleanup since part of the mess was my fault! :)
> 
> No worries, I've got a lot of patches for hmm_range_fault right now,
> just trying to organize them, test them and post them. Haven't posted
> that one yet.
> 
> Actually, while you are looking at this, do you think we should be
> adding at least READ_ONCE in the pagewalk.c walk_* functions? The
> multiple references of pmd, pud, etc without locking seems sketchy to
> me.

I agree it seems worrying. I'm not entirely sure whether the holding of 
mmap_sem is sufficient, this isn't something that I changed so I've just 
been hoping that it's sufficient since it seems to have been working 
(whether that's by chance because the compiler didn't generate multiple 
reads I've no idea). For walking the kernel's page tables the lack of 
READ_ONCE is also not great, but at least for PTDUMP we don't care too 
much about accuracy and it should be crash proof because there's no RCU 
grace period. And again the code I was replacing didn't have any special 
protection.

I can't see any harm in updating the code to include READ_ONCE and I'm 
happy to review a patch.

Thanks,

Steve