Re: [PATCH 03/10] mm/hugetlb: Document huge_pte_offset usage

[David Hildenbrand]

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@xxxxxxxxxx>
> ---
>   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