Re: [PATCH v2] mm: page_alloc: move mlocked flag clearance into free_pages_prepare()

[Matthew Wilcox <willy@infradead.org>]

On Tue, Oct 22, 2024 at 02:14:39AM +0000, Roman Gushchin wrote:
> On Mon, Oct 21, 2024 at 09:34:24PM +0100, Matthew Wilcox wrote:
> > On Mon, Oct 21, 2024 at 05:34:55PM +0000, Roman Gushchin wrote:
> > > Fix it by moving the mlocked flag clearance down to
> > > free_page_prepare().
> > 
> > Urgh, I don't like this new reference to folio in free_pages_prepare().
> > It feels like a layering violation.  I'll think about where else we
> > could put this.
> 
> I agree, but it feels like it needs quite some work to do it in a nicer way,
> no way it can be backported to older kernels. As for this fix, I don't
> have better ideas...

Well, what is KVM doing that causes this page to get mapped to userspace?
Don't tell me to look at the reproducer as it is 403 Forbidden.  All I
can tell is that it's freed with vfree().

Is it from kvm_dirty_ring_get_page()?  That looks like the obvious thing,
but I'd hate to spend a lot of time on it and then discover I was looking
at the wrong thing.

The reason I'm interested in looking in this direction is that we're
separating pages from folios.  Pages allocated through vmalloc() won't
have refcounts, mapcounts, mlock bits, etc.  So it's quite important to
look at currently existing code and figure out how they can be modified
to work in this new environment.