linux-kernel - Re: [PATCH] mm/mremap: correctly account old mapping after MREMAP

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Message-ID: <9b2c6986-da81-4a02-830b-17554b6f2cbe@lucifer.local>
Date: Tue, 14 Oct 2025 10:57:04 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Pedro Falcato <pfalcato@...e.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mremap: correctly account old mapping after
 MREMAP_DONTUNMAP remap

On Tue, Oct 14, 2025 at 10:41:18AM +0100, Pedro Falcato wrote:
> On Mon, Oct 13, 2025 at 05:58:36PM +0100, Lorenzo Stoakes wrote:
> > Commit b714ccb02a76 ("mm/mremap: complete refactor of move_vma()")
> > mistakenly introduced a new behaviour - clearing the VM_ACCOUNT flag of the
> > old mapping when a mapping is mremap()'d with the MREMAP_DONTUNMAP flag
> > set.
> >
> > While we always clear the VM_LOCKED and VM_LOCKONFAULT flags for the old
> > mapping (the page tables have been moved, so there is no data that could
> > possibly be locked in memor), there is no reason to touch any other VMA
>                         memory
> > flags.
> >
> > This is because after the move the old mapping is in a state as if it were
> > freshly mapped. This implies that the attributes of the mapping ought to
> > remain the same, including whether or not the mapping is accounted.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Reviewed-by: Pedro Falcato <pfalcato@...e.de>

Thanks!

>
> Looks good, thanks! Should we add a test for it?

I considered that, but this is fundamental behaviour and so is very unlikely to
regress so doesn't seem really worth it.

>
> --
> Pedro

Cheers, Lorenzo