Re: [PATCH v2 03/10] mm/hugetlb: Document huge_pte_offset usage

[David Hildenbrand <david@redhat.com>]

On 07.12.22 21:49, John Hubbard wrote:
> On 12/7/22 12:30, 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(+)
> 
> Looks good, with a couple of minor wording tweaks below that you might
> consider folding in, but either way,
> 
> Reviewed-by: John Hubbard <jhubbard@nvidia.com>
> 
>>
>> 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:
> 
>        "responsible for"
> 
>> + *
>> + *  (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.
> 
> mmap sem is sooo two years ago! :)
> 
>> + *      This is normally always the case, IOW we don't need to do anything
> 
> "normally always" hurts my sense of logic. And "IOW" is for typing very quickly
> in chats or email, not for long term documentation that is written rarely
> and read many times.
> 
>> + *      special.
> 
> So putting all that together, maybe:
> 
>    *  (1) For private mappings: pmd unsharing is not possible, so holding the
>    *      mmap_lock for either read or write is sufficient. Most callers already
>    *      hold the mmap_lock, so normally, no special action is required.

With that,

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb