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Message-ID: <afce1bdf-6b5f-3393-cafa-81148277773d@redhat.com>
Date: Wed, 30 Nov 2022 11:24:34 +0100
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Cc: James Houghton <jthoughton@...gle.com>,
Jann Horn <jannh@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Rik van Riel <riel@...riel.com>,
Nadav Amit <nadav.amit@...il.com>,
Miaohe Lin <linmiaohe@...wei.com>,
Muchun Song <songmuchun@...edance.com>,
Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH 03/10] mm/hugetlb: Document huge_pte_offset usage
On 29.11.22 20:35, Peter Xu wrote:
> huge_pte_offset() is potentially a pgtable walker, looking up pte_t* for a
> hugetlb address.
>
> Normally, it's always safe to walk a generic pgtable as long as we're with
> the mmap lock held for either read or write, because that guarantees the
> pgtable pages will always be valid during the process.
>
> But it's not true for hugetlbfs, especially shared: hugetlbfs can have its
> pgtable freed by pmd unsharing, it means that even with mmap lock held for
> current mm, the PMD pgtable page can still go away from under us if pmd
> unsharing is possible during the walk.
>
> So we have two ways to make it safe even for a shared mapping:
>
> (1) If we're with the hugetlb vma lock held for either read/write, it's
> okay because pmd unshare cannot happen at all.
>
> (2) If we're with the i_mmap_rwsem lock held for either read/write, it's
> okay because even if pmd unshare can happen, the pgtable page cannot
> be freed from under us.
>
> Document it.
>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
> include/linux/hugetlb.h | 32 ++++++++++++++++++++++++++++++++
> 1 file changed, 32 insertions(+)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 551834cd5299..81efd9b9baa2 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -192,6 +192,38 @@ extern struct list_head huge_boot_pages;
>
> pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
> unsigned long addr, unsigned long sz);
> +/*
> + * huge_pte_offset(): Walk the hugetlb pgtable until the last level PTE.
> + * Returns the pte_t* if found, or NULL if the address is not mapped.
> + *
> + * Since this function will walk all the pgtable pages (including not only
> + * high-level pgtable page, but also PUD entry that can be unshared
> + * concurrently for VM_SHARED), the caller of this function should be
> + * responsible of its thread safety. One can follow this rule:
> + *
> + * (1) For private mappings: pmd unsharing is not possible, so it'll
> + * always be safe if we're with the mmap sem for either read or write.
> + * This is normally always the case, IOW we don't need to do anything
> + * special.
Maybe worth mentioning that hugetlb_vma_lock_read() and friends already
optimize for private mappings, to not take the VMA lock if not required.
Was happy to spot that optimization in there already :)
--
Thanks,
David / dhildenb
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