Re: [PATCH v1] mm/huge_memory: check pmd_special() only after pmd_present()

[David Hildenbrand]

On 26.09.24 17:42, David Hildenbrand wrote:
> We should only check for pmd_special() after we made sure that we
> have a present PMD. For example, if we have a migration PMD,
> pmd_special() might indicate that we have a special PMD although we
> really don't.
>
> This fixes confusing migration entries as PFN mappings, and not
> doing what we are supposed to do in the "is_swap_pmd()" case further
> down in the function -- including messing up COW, page table handling
> and accounting.
>
> Reported-by: syzbot+bf2c35fa302ebe3c7471@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/lkml/66f15c8d.050a0220.c23dd.000f.GAE@xxxxxxxxxx/
> Fixes: bc02afbd4d73 ("mm/fork: accept huge pfnmap entries")
> Cc: Peter Xu <peterx@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>
> I yet have to do more testing, but sending this out already.

Testing looks good. Andrew please queue this, it's a rather unpleasent 
behavior if we fork() with a PMD migration entry that should be fixed 
upstream soonish :)

(no idea how I could have missed CCing Andrew once more)

--
Cheers,

David / dhildenb