linux-kernel - Re: [PATCH v12 mm-new 07/15] khugepaged: generalize collapse_huge

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Message-ID: <1c05c28a-a4f7-4562-8799-6f08a03bd3eb@arm.com>
Date: Fri, 7 Nov 2025 08:39:03 +0530
From: Dev Jain <dev.jain@....com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Nico Pache <npache@...hat.com>
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Subject: Re: [PATCH v12 mm-new 07/15] khugepaged: generalize
 collapse_huge_page for mTHP collapse

> ----------[snip]------------
>
>> +
>> +		spin_lock(pmd_ptl);
> We're duplicating this in both branches, why not do outside if/else?
>
>> +		WARN_ON_ONCE(!pmd_none(*pmd));
> Hmm so the PMD entry will still always be empty on mTHP collapse? Surely we
> could be collapsing more than one mTHP into an existing PTE table no? I may
> be missing something here/confused :)

After this code path isolates the PTE table, we don't want any other code path
doing "Hey, I see an empty PMD, let's install a PTE table here". One of the
reasons why all the heavy locking is required here.

Also, I want to ask a question about WARN vs BUG_ON: suppose that the
race I described above occurs. After khugepaged isolates the PTE table, someone
faults in a PTE table there, and eventually writes data in the underlying folios.
Then the buggy khugepaged nukes out that table and installs a new one, installing
an mTHP folio which had old data. How do we decide whether such a condition is
worthy of a BUG_ON (leading to system crash) vs letting this pass with WARN?


>
> ------------[snip]----------
>