linux-kernel - Re: [PATCH v1 2/4] mm: Expose abnormal new_pte during move

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Message-ID: <20250530164411.GB25160@redhat.com>
Date: Fri, 30 May 2025 18:44:11 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Pu Lehui <pulehui@...weicloud.com>, mhiramat@...nel.org,
	peterz@...radead.org, akpm@...ux-foundation.org,
	Liam.Howlett@...cle.com, vbabka@...e.cz, jannh@...gle.com,
	pfalcato@...e.de, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org, pulehui@...wei.com
Subject: Re: [PATCH v1 2/4] mm: Expose abnormal new_pte during move_ptes

On 05/30, Lorenzo Stoakes wrote:
>
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -237,6 +237,8 @@ static int move_ptes(struct pagetable_move_control *pmc,
> >
> >  	for (; old_addr < old_end; old_pte++, old_addr += PAGE_SIZE,
> >  				   new_pte++, new_addr += PAGE_SIZE) {
> > +		WARN_ON_ONCE(!pte_none(*new_pte));
> > +
>
> I mean, we really really should not ever be seeing a mapped PTE here, so I think
> a WARN_ON_ONCE() is fine.
>
> We unmap anything ahead of time, and only I think this uprobe breakpoint
> installation would ever cause this to be the case.
>
> We can make this a VM_WARN_ON_ONCE() too I suppose, just in case there's
> something we're not thinking of, but I'd say at some point we'd want to change
> it to a WARN_ON_ONCE().

Note also that move_normal_pmd/move_normal_pud use WARN_ON_ONCE(!xxx_none(...)),
not VM_WARN_ON_ONCE().

Oleg.