Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork()

John Hubbard
2020-09-23T20:19:11Z

On 9/23/20 8:44 AM, Peter Xu wrote:
> On Wed, Sep 23, 2020 at 04:01:14PM +0200, Jan Kara wrote:
>> On Wed 23-09-20 09:50:04, Peter Xu wrote:
...
>>>> But the problem is that if you apply mm->has_pinned check on file pages,
>>>> you can get false negatives now. And that's not acceptable...
>>>
>>> Do you mean the case where proc A pinned page P from a file, then proc B
>>> mapped the same page P on the file, then fork() on proc B?
>>
>> Yes.

aha, thanks for spelling out the false negative problem.

>>
>>> If proc B didn't explicitly pinned page P in B's address space too,
>>> shouldn't we return "false" for page_likely_dma_pinned(P)? Because if
>>> proc B didn't pin the page in its own address space, I'd think it's ok to
>>> get the page replaced at any time as long as the content keeps the same.
>>> Or couldn't we?
>>
>> So it depends on the reason why you call page_likely_dma_pinned(). For your
>> COW purposes the check is correct but e.g. for "can filesystem safely
>> writeback this page" the page_likely_dma_pinned() would be wrong. So I'm
>> not objecting to the mechanism as such. I'm mainly objecting to the generic
>> function name which suggests something else than what it really checks and
>> thus it could be used in wrong places in the future... That's why I'd
>> prefer to restrict the function to PageAnon pages where there's no risk of
>> confusion what the check actually does.
>
> How about I introduce the helper as John suggested, but rename it to
>
> page_maybe_dma_pinned_by_mm()
>
> ?
>
> Then we also don't need to judge on which is more likely to happen (between
> "maybe" and "likely", since that will confuse me if I only read these words..).
>

You're right, it is too subtle of a distinction after all. I agree that sticking
with "_maybe_" avoids that confusion.

> I didn't use any extra suffix like "cow" because I think it might be useful for
> things besides cow. Fundamentally the new helper will be mm-based, so "by_mm"
> seems to suite better to me.
>
> Does that sound ok?
>

Actually, Jan nailed it. I just wasn't understanding his scenario, but now that
I do, and considering your other point about wording, I think we end up with:

anon_page_maybe_pinned()

as a pretty good name for a helper function. (We don't want "_mm" because that
refers more to the mechanism used internally, rather than the behavior of the
function. "anon_" adds more meaning.)

...now I better go and try to grok what Jason is recommending for the new
meaning of FOLL_PIN, in another tributary of this thread. I don't *think* it affects
this naming point, though. :)

thanks,
--
John Hubbard
NVIDIA