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

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Message-ID: <4c7dc187-bd04-47c5-b619-b91b83cca913@redhat.com>
Date: Wed, 18 Jun 2025 19:23:29 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 18.06.25 19:10, Lorenzo Stoakes wrote:
> 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?

It's a very weird thing to not hold the PTL while walking page tables, 
and then only grabbing it for clearing entries just to make selected 
functions happy ...

I mostly spotted the release_pte_folio(), which I think should be fine 
with a spinlock held. I missed the free_folio_and_swap_cache(), not sure 
if that is problematic.

Interestingly, release_pte_folio() does a

a) node_stat_mod_folio
b) folio_unlock
c) folio_putback_lru

... and folio_putback_lru() is documented to "lru_lock must not be held, 
interrupts must be enabled". Hmmmm. I suspect that doc is wrong.

So yeah, maybe better keep that weird looking locking like it is :)

-- 
Cheers,

David / dhildenb