Re: [PATCH v3 3/4] mm: prevent KSM from breaking VMA merging for new VMAs

[Lorenzo Stoakes]

On Mon, Jun 23, 2025 at 10:37:53AM +0200, Vlastimil Babka wrote:
> On 6/20/25 14:48, Lorenzo Stoakes wrote:
> > Hi Andrew,
> >
> > Sending a fix-patch for this commit due to a reported syzbot issue which
> > highlighted a bug in the implementation.
> >
> > I discuss the syzbot report at [0].
> >
> > [0]: https://lore.kernel.org/all/a55beb72-4288-4356-9642-76ab35a2a07c@lucifer.local/
> >
> > There's a very minor conflict around the map->vm_flags vs. map->flags change,
> > easily resolvable, but if you need a respin let me know.
> >
> > I ran through all mm self tests included the newly introduced one in 4/4 and all
> > good.
> >
> > Thanks, Lorenzo
> >
> > ----8<----
> > From 4d9dde3013837595d733b5059c2d6474261654d6 Mon Sep 17 00:00:00 2001
> > From: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > Date: Fri, 20 Jun 2025 13:21:03 +0100
> > Subject: [PATCH] mm/vma: correctly invoke late KSM check after mmap hook
> >
> > Previously we erroneously checked whether KSM was applicable prior to
> > invoking the f_op->mmap() hook in the case of not being able to perform
> > this check early.
> >
> > This is problematic, as filesystems such as hugetlb, which use anonymous
> > memory and might otherwise get KSM'd, set VM_HUGETLB in the f_op->mmap()
> > hook.
> >
> > Correct this by checking at the appropriate time.
> >
> > Reported-by: syzbot+a74a028d848147bc5931@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://lore.kernel.org/all/6853fc57.a00a0220.137b3.0009.GAE@xxxxxxxxxx/
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
>
> I've checked the version in mm tree, LGTM.
>
> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
>

Cheers, appreciated!