On Wed, Oct 29, 2025 at 03:23:27PM -0600, Nico Pache wrote:

On Wed, Oct 29, 2025 at 12:59 PM Lorenzo Stoakes
[off-list ref] wrote:

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On Tue, Oct 28, 2025 at 08:47:12PM -0600, Nico Pache wrote:

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On Tue, Oct 28, 2025 at 1:00 PM Lorenzo Stoakes

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Right, well I agree if we can make this 0/511 thing work, let's do that.

Ok, great, some consensus! I will go ahead with that solution.

:) awesome.

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Just to make sure we are all on the same page,

I am still stabilising my understanding of the creep issue, see the thread
where David kindly + patiently goes in detail, I think I am at a
(pre-examining algorithm itself) broad understanding of this.

I added some details of the creep issue in my other replies, hopefully
that also helps!

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the max_ptes_none value will be treated as 0 for anything other than
PMD collapse, or in the case of 511. Or will the max_ptes_none only
work for mTHP collapse when it is 0.

511 implies always collapse zero/none, 0 implies never, as I understand it.

0 implies only collapse if a given mTHP size is fully occupied by
present PTES. Since we start at PMD and work our way down we will
always end up with a PMD range of fully occupied mTHPs, potentially of
all different sizes.

Yeah this was my understanding, I mean terminology is tricky here (+ I am
probably not being entirely clear tbh), so I mean less so '0 means no
collapse' but rather '0 means no collapse of zero/none' but of course can
allow for collapse of present PTEs (within the same VMA).

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static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan)
{
unsigned int max_ptes_none;

/* ignore max_ptes_none limits */
if (full_scan)
return HPAGE_PMD_NR - 1;

if (order == HPAGE_PMD_ORDER)
return khugepaged_max_ptes_none;

if (khugepaged_max_ptes_none != HPAGE_PMD_NR - 1)
return 0;

return max_ptes_none >> (HPAGE_PMD_ORDER - order);
}

Here's the implementation for the first approach, looks like Baolin
was able to catch up and beat me to the other solution while I was
mulling over the thread lol

Broadly looks similar to Baolin's, I made some suggestions over there
though!

Thanks! They are both based on my current collapse_max_ptes_none! Just
a slight difference in behavior surrounding the two suggested
solutions by David.

Yes which is convenient as it's less delta for you!

I will still have to implement the logic for not attempting mTHP
collapses if it is any intermediate value (i.e. the function returns
-EINVAL).

Ack

-- Nico

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Cheers,
-- Nico

Thanks, Lorenzo

Cheers, Lorenzo