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

[David Hildenbrand <david@redhat.com>]

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.

-- 
Thanks,

David / dhildenb