Re: RFC: Block reservation for hugetlbfs

[Nick Piggin]

David Gibson wrote:
> These days, hugepages are demand-allocated at first fault time.
> There's a somewhat dubious (and racy) heuristic when making a new
> mmap() to check if there are enough available hugepages to fully
> satisfy that mapping.
>
> A particularly obvious case where the heuristic breaks down is where a
> process maps its hugepages not as a single chunk, but as a bunch of
> individually mmap()ed (or shmat()ed) blocks without touching and
> instantiating the blocks in between allocations.  In this case the
> size of each block is compared against the total number of available
> hugepages.  It's thus easy for the process to become overcommitted,
> because each block mapping will succeed, although the total number of
> hugepages required by all blocks exceeds the number available.  In
> particular, this defeats such a program which will detect a mapping
> failure and adjust its hugepage usage downward accordingly.
>
> The patch below is a draft attempt to address this problem, by
> strictly reserving a number of physical hugepages for hugepages inodes
> which have been mapped, but not instatiated.  MAP_SHARED mappings are
> thus "safe" - they will fail on mmap(), not later with a SIGBUS.
> MAP_PRIVATE mappings can still SIGBUS.
>
> This patch appears to address the problem at hand - it allows DB2 to
> start correctly, for instance, which previously suffered the failure
> described above.  I'm almost certain I'm missing some locking or other
> synchronization - I am entirely bewildered as to what I need to hold
> to safely update i_blocks as below.  Corrections for my ignorance
> solicited...
>
> Signed-off-by: David Gibson <dwg@xxxxxxxxxxx>

This introduces
tree_lock(r) -> hugetlb_lock

And we already have
hugetlb_lock -> lru_lock

So we now have tree_lock(r) -> lru_lock, which would deadlock
against lru_lock -> tree_lock(w), right?

From a quick glance it looks safe, but I'd _really_ rather not
introduce something like this.

--
SUSE Labs, Novell Inc.
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