linux-kernel - Re: [PATCH 3/5] mm/migrate.c: fix potential indeterminate pte entry in migrate_vma_insert

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Date:   Tue, 23 Mar 2021 22:07:23 +1100
From:   Alistair Popple <apopple@...dia.com>
To:     <linux-kernel@...r.kernel.org>
CC:     David Hildenbrand <david@...hat.com>,
        Miaohe Lin <linmiaohe@...wei.com>, <akpm@...ux-foundation.org>,
        <jglisse@...hat.com>, <shy828301@...il.com>, <linux-mm@...ck.org>
Subject: Re: [PATCH 3/5] mm/migrate.c: fix potential indeterminate pte entry in migrate_vma_insert_page()

On Tuesday, 23 March 2021 9:26:43 PM AEDT David Hildenbrand wrote:
> On 20.03.21 10:36, Miaohe Lin wrote:
> > If the zone device page does not belong to un-addressable device memory,
> > the variable entry will be uninitialized and lead to indeterminate pte
> > entry ultimately. Fix this unexpectant case and warn about it.
> 
> s/unexpectant/unexpected/
> 
> > 
> > Fixes: df6ad69838fc ("mm/device-public-memory: device memory cache 
coherent with CPU")
> > Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> > ---
> >   mm/migrate.c | 7 +++++++
> >   1 file changed, 7 insertions(+)
> > 
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 20a3bf75270a..271081b014cb 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -2972,6 +2972,13 @@ static void migrate_vma_insert_page(struct 
migrate_vma *migrate,
> >   
> >   			swp_entry = make_device_private_entry(page, vma->vm_flags & 
VM_WRITE);
> >   			entry = swp_entry_to_pte(swp_entry);
> > +		} else {
> > +			/*
> > +			 * For now we only support migrating to un-addressable
> > +			 * device memory.
> > +			 */
> > +			WARN_ON(1);
> > +			goto abort;
> 
> Fix it by crashing the kernel with panic_on_warn? :)
> 
> If this case can actual happen, than no WARN_ON() - rather a 
> pr_warn_once(). If this case cannot happen, why do we even care (it's 
> not a fix then)?

There is also already a check for this case in migrate_vma_pages(). The 
problem is it happens after the call to migrate_vma_insert_page(). I wonder if 
instead it would be better just to move the existing check to before that 
call?