Messages in this thread |  | | From | Dev Jain <> | | Subject | Re: [PATCH v2] arm64: Restrict pagetable teardown to avoid false warning | | Date | Tue, 20 May 2025 14:35:01 +0530 |
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On 19/05/2025 13:16, David Hildenbrand wrote: > 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@vger.kernel.org> >>> Reported-by: Ryan Roberts <ryan.roberts@arm.com> >>> Signed-off-by: Dev Jain <dev.jain@arm.com> >>> --- >>> 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).
Indeed; there is no locking, but this portion of the vmalloc VA space has been allocated to us exclusively, so we know there can be no one else racing.
> >> 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))
Yes, I saw that. I know that we don't technically need READ_ONCE(). I'm just proposng that for arm64 code we should be consistent with what it already does. See Commit 20a004e7b017 ("arm64: mm: Use READ_ONCE/WRITE_ONCE when accessing page tables")
So I'll just use pmdp_get()? (Hopefully my reply comes fine, I am replying from the terminal)
Thanks, Ryan
> > > :) > > Acked-by: David Hildenbrand <david@redhat.com> >
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