linux-kernel - Re: [PATCH v3] mm/page_alloc: clear page->private in free_pages

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Message-ID: <c0b93b3f-bc4f-42f6-8287-72d015a0a79b@kernel.org>
Date: Mon, 9 Feb 2026 16:46:54 +0100
From: "David Hildenbrand (Arm)" <david@...nel.org>
To: Vlastimil Babka <vbabka@...e.cz>,
 Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>, linux-mm@...ck.org
Cc: akpm@...ux-foundation.org, surenb@...gle.com, mhocko@...e.com,
 jackmanb@...gle.com, hannes@...xchg.org, ziy@...dia.com, npiggin@...il.com,
 linux-kernel@...r.kernel.org, kasong@...cent.com, hughd@...gle.com,
 chrisl@...nel.org, ryncsn@...il.com, stable@...r.kernel.org,
 willy@...radead.org
Subject: Re: [PATCH v3] mm/page_alloc: clear page->private in
 free_pages_prepare()

On 2/9/26 12:17, Vlastimil Babka wrote:
> On 2/7/26 23:08, David Hildenbrand (Arm) wrote:
>>>
>>> -               /*
>>> -                * page->private should not be set in tail pages. Fix up
>>> and warn once
>>> -                * if private is unexpectedly set.
>>> -                */
>>> -               if (unlikely(new_folio->private)) {
>>> -                       VM_WARN_ON_ONCE_PAGE(true, new_head);
>>> -                       new_folio->private = NULL;
>>> -               }
>>
>> BTW, I wonder whether we should bring that check back for non-device folios.
> 
> If the rule is now that when upon freeing in free_pages_prepare() we clear
> private in the head page and not tail pages (where we expect the owner of
> the page to do it), maybe that check for tail pages should be done in the
> is_check_pages_enabled() part of free_pages_prepare().
> 
> Or should the check be also in the split path because somebody can set a
> tail private between allocation and split? (and not just inherit it from a
> previous allocation that didn't clear it?).

We ran into that check in the past, when folio->X overlayed 
page->private in a tail, and would actually have to be zeroed out.

So it should be part of this splitting code I think.

-- 
Cheers,

David