Might be too late in this thread, but in case you are going to continue and/or
repost:
[CC += linux-api@xxxxxxxxxxxxxxx]
(also linux-man and Michael to match my other reply)
Since this is a kernel-user-space API change, please CC linux-api@. The
kernel source file Documentation/SubmitChecklist notes that all Linux kernel
patches that change userspace interfaces should be CCed to
linux-api@xxxxxxxxxxxxxxx, so that the various parties who are interested in API
changes are informed. For further information, see
https://www.kernel.org/doc/man-pages/linux-api-ml.html
On 03/26/2015 09:03 PM, David Rientjes wrote:
> On Thu, 26 Mar 2015, Davide Libenzi 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.
>
> There was a patch proposed last week to add reserved pools to the
> hugetlbfs mount option specifically for the case where a large database
> wants sole reserved access to the hugepage pool. This is why hugetlbfs
> pages become reserved on mmap(). In that case, the database never wants
> to do munmap() and instead maintains its own hugepage pool.
>
> That makes the usual database case, mmap() all necessary hugetlb pages to
> reserve them, even easier since they have historically had to maintain
> this pool amongst various processes.
>
> Is there a process out there that tests for munmap(ptr) == EINVAL and, if
> true, returns ptr to its hugepage pool? I can't say for certain that none
> exist, that's why the potential for breakage exists.
>
>> 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.
>>
>
> That application would have to be ignoring an EINVAL return value.
>
>> > 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.
>>
>
> The behavior is certainly not new, it has always been the case for
> munmap() on hugetlb vmas.
>
> In a strict POSIX interpretation, it refers only to pages in the sense of
> what is returned by sysconf(_SC_PAGESIZE). Such vmas are not backed by
> any pages of size sysconf(_SC_PAGESIZE), so this behavior is undefined.
> It would be best to modify the man page to explicitly state this for
> MAP_HUGETLB.
>
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