Messages in this thread | | | Date | Tue, 21 Feb 2006 15:18:59 +1100 | From | Nick Piggin <> | Subject | Re: RFC: Block reservation for hugetlbfs |
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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@au1.ibm.com> >
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.
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