linux-kernel - Re: [PATCH] khugepaged: Optimize __collapse_huge_page_copy_succeeded() for large folios by PTE batching

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Message-ID: <42aa7d12-3f84-4ee6-a067-5aee30ec677d@lucifer.local>
Date: Wed, 18 Jun 2025 18:10:28 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org, ziy@...dia.com,
        baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
        npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] khugepaged: Optimize
 __collapse_huge_page_copy_succeeded() for large folios by PTE batching

On Wed, Jun 18, 2025 at 06:14:22PM +0200, David Hildenbrand wrote:
> On 18.06.25 12:26, Dev Jain wrote:
> > +
> >   			/*
> >   			 * ptl mostly unnecessary, but preempt has to
> >   			 * be disabled to update the per-cpu stats
> >   			 * inside folio_remove_rmap_pte().
> >   			 */
> >   			spin_lock(ptl);
>
> Existing code: The PTL locking should just be moved outside of the loop.

Do we really want to hold the PTL for the duration of the loop? Are we sure
it's safe to do so? Are there any locks taken in other functions that might
sleep that'd mean holding a spinlock would be a problem?