On 29.01.21 09:51, Michal Hocko wrote:

On Fri 29-01-21 09:21:28, Mike Rapoport wrote:

quoted

On Thu, Jan 28, 2021 at 02:01:06PM +0100, Michal Hocko wrote:

quoted

On Thu 28-01-21 11:22:59, Mike Rapoport wrote:

quoted

And hugetlb pools may be also depleted by anybody by calling
mmap(MAP_HUGETLB) and there is no any limiting knob for this, while
secretmem has RLIMIT_MEMLOCK.

Yes it can fail. But it would fail at the mmap time when the reservation
fails. Not during the #PF time which can be at any time.

It may fail at $PF time as well:

hugetlb_fault()
         hugeltb_no_page()
                 ...
                 alloc_huge_page()
                         alloc_gigantic_page()
                                 cma_alloc()
                                         -ENOMEM;

I would have to double check. From what I remember cma allocator is an
optimization to increase chances to allocate hugetlb pages when
overcommiting because pages should be normally pre-allocated in the pool
and reserved during mmap time. But even if a hugetlb page is not pre
allocated then this will get propagated as SIGBUS unless that has
changed.

It's an optimization to allocate gigantic pages dynamically later (so 
not using memblock during boot). Not just for overcommit, but for any 
kind of allocation.

The actual allocation from cma should happen when setting nr_pages:

nr_hugepages_store_common()->set_max_huge_pages()->alloc_pool_huge_page()...->alloc_gigantic_page()

The path described above seems to be trying to overcommit gigantic 
pages, something that can be expected to SIGBUS. Reservations are 
handled via the pre-allocated pool.

-- 
Thanks,

David / dhildenb


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