Re: [PATCH v8 11/15] khugepaged: allow madvise_collapse to check all anonymous mTHP orders

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/7/8 12:37, Nico Pache wrote:
> On Fri, Jul 4, 2025 at 3:14 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>>
>> On Fri, 4 Jul 2025 14:11:13 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> wrote:
>>
>>> On 2025/7/2 13:57, Nico Pache wrote:
>>>> Allow madvise_collapse to scan/collapse all mTHP orders without the
>>>> strict requirement of needing the PMD-order enabled.
>>>>
>>>> Signed-off-by: Nico Pache <npache@redhat.com>
>>>
>>> I am afraid we should drop this patch from the series, since Hugh
>>> explicitly opposed my modification of the madvise_collapse control logic
>>> in this thread[1].
>>
>> Well it drops easily enough.  I don't know if it compiles yet.
>>
>> Nico, can you confirm that you're OK with the droppage and that the
>> series is still good without this patch?
> Yes that is fine, it shouldn't conflict at all. Although this isn't
> the only patch that will need modification... Ill reply to Baolin
> inline here.
>>
>>> Moreover, since we have not yet clarified how to handle the conflict
>>> between the semantics of madvise_collapse and the THP interfaces, we
>>> should keep the current logic[2] (means madvise_collapse still only
>>> allows PMD collapse).
> Hi Baolin,
> 
> Sorry I saw most of that discussion but missed that point by David.
> 
> We should come to an agreement on what the proper approach is, because
> if not we'll ALWAYS have this problem and NEVER solve it (pun
> intended).
> 
> Jokes aside, if we are leaving MADV_COLLAPSE untouched then I need to
> make some slight modifications to this series so that MADV_COLLAPSE
> never tries anything other than PMD collapse (should be a pretty small
> change). Dropping this commit alone does not achieve that, but rather
> just allows MADV_COLLAPSE to work if PMD is disabled, and one other
> mTHP size is enabled. On second thought this isn't great either as
> MADV_COLLAPSE ignores sysfs, so if you have any mTHP size enabled, it
> will still continue to collapse to other PMD+mTHP sizes... wow this
> really is a mess.

Yes, this is what I mentioned earlier about the conflict between 
MADV_COLLAPSE and mTHP collapse :(

>>> If madvise_collapse is to support mTHP collapse, there will be more
>>> semantic conflicts to discuss.
> I guess a V9 is inevitable, I will drop mTHP support for
> MADV_COLLAPSE, and drop this patch too. I'll let this series sit for a
> week or so to gather reviews (and make sure nothing else is missing).

Sure.