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

David Hildenbrand
2022-11-30T16:33:03Z

On 30.11.22 17:25, Peter Xu wrote:
> On Wed, Nov 30, 2022 at 05:11:36PM +0100, David Hildenbrand wrote:
>> On 30.11.22 17:09, Peter Xu wrote:
>>> On Wed, Nov 30, 2022 at 11:24:34AM +0100, David Hildenbrand wrote:
>>>> 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@redhat.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.
>>>
>>> Yes we can. I assume this is not super urgent so I'll hold a while to see
>>> whether there's anything else that needs amending for the documents.
>>>
>>> Btw, even with hugetlb_vma_lock_read() checking SHARED for a private only
>>> code path it's still better to not take the lock at all, because that still
>>> contains a function jump which will be unnecesary.
>>
>> IMHO it makes coding a lot more consistent and less error-prone when not
>> care about whether to the the lock or not (as an optimization) and just
>> having this handled "automatically".
>>
>> Optimizing a jump out would rather smell like a micro-optimization.
>
> Or we can move the lock helpers into the headers, too.

Ah, yes.

--
Thanks,

David / dhildenb