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SubjectRe: [patch][resend] MAP_HUGETLB munmap fails with size not 2MB aligned
On 03/26/2015 08:39 PM, Davide Libenzi wrote:
> On Thu, 26 Mar 2015, David Rientjes wrote:
>
>> Yes, this munmap() behavior of lengths <= hugepage_size - PAGE_SIZE for a
>> hugetlb vma is long standing and there may be applications that break as a
>> result of changing the behavior: a database that reserves all allocated
>> hugetlb memory with mmap() so that it always has exclusive access to those
>> hugepages, whether they are faulted or not, and maintains its own hugepage
>> pool (which is common), may test the return value of munmap() and depend
>> on it returning -EINVAL to determine if it is freeing memory that was
>> either dynamically allocated or mapped from the hugetlb reserved pool.
>
> You went a long way to create such a case.
> But, in your case, that application will erroneously considering hugepage
> mmaped memory, as dynamically allocated, since it will always get EINVAL,
> unless it passes an aligned size. Aligned size, which a fix like the one
> posted in the patch will still leave as success.
> OTOH, an application, which might be more common than the one you posted,
> which calls munmap() to release a pointer which it validly got from a
> previous mmap(), will leak huge pages as all the issued munmaps will fail.
>
>
>> If we were to go back in time and decide this when the munmap() behavior
>> for hugetlb vmas was originally introduced, that would be valid. The
>> problem is that it could lead to userspace breakage and that's a
>> non-starter.
>>
>> What we can do is improve the documentation and man-page to clearly
>> specify the long-standing behavior so that nobody encounters unexpected
>> results in the future.
>
> This way you will leave the mmap API with broken semantics.
> In any case, I am done arguing.
> I will leave to Andrew to sort it out, and to Michael Kerrisk to update
> the mmap man pages with the new funny behaviour.

+ CC's

You know that people don't always magically CC themselves, or read all of
lkml/linux-mm? :)

>
>
> - Davide
>
>
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