Re: [PATCH v2] arm64: Restrict pagetable teardown to avoid false warning

[David Hildenbrand]

On 19.05.25 11:08, Ryan Roberts wrote:
> On 18/05/2025 10:54, Dev Jain wrote:
> > Commit 9c006972c3fe removes the pxd_present() checks because the caller
>
> nit: please use the standard format for describing commits: Commit 9c006972c3fe
> ("arm64: mmu: drop pXd_present() checks from pXd_free_pYd_table()")
>
> > checks pxd_present(). But, in case of vmap_try_huge_pud(), the caller only
> > checks pud_present(); pud_free_pmd_page() recurses on each pmd through
> > pmd_free_pte_page(), wherein the pmd may be none. Thus it is possible to
> > hit a warning in the latter, since pmd_none => !pmd_table(). Thus, add
> > a pmd_present() check in pud_free_pmd_page().
> >
> > This problem was found by code inspection.
> >
> > This patch is based on 6.15-rc6.
>
> nit: please remove this to below the "---", its not part of the commit log.
>
> > Fixes: 9c006972c3fe (arm64: mmu: drop pXd_present() checks from pXd_free_pYd_table())
>
> nit: remove empty line; the tags should all be in a single block with no empty
> lines.
>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Reported-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> > Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> > ---
> > v1->v2:
> >   - Enforce check in caller
> >
> >   arch/arm64/mm/mmu.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index ea6695d53fb9..5b1f4cd238ca 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -1286,7 +1286,8 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
> >   	next = addr;
> >   	end = addr + PUD_SIZE;
> >   	do {
> > -		pmd_free_pte_page(pmdp, next);
> > +		if (pmd_present(*pmdp))
>
> pmd_free_pte_page() is using READ_ONCE() to access the *pmdp to ensure it can't
> be torn. I suspect we don't technically need that in these functions because
> there can be no race with a writer.

Yeah, if there is no proper locking in place the function would already 
seriously mess up (double freeing etc).

> But the arm64 arch code always uses
> READ_ONCE() for dereferencing pgtable entries for safely. Perhaps we should be
> consistent here?

mm/vmalloc.c:   if (pmd_present(*pmd) && !pmd_free_pte_page(pmd, addr))


:)

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb