Messages in this thread | | | Date | Sat, 15 Jun 2002 11:00:15 +0200 | From | Jens Axboe <> | Subject | Re: bio_chain: proposed solution for bio_alloc failure and large IO simplification |
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On Sat, Jun 15 2002, Adam J. Richter wrote: > Jens Axboe wrote: > >The I/O path allocations all use GFP_NOIO (or GFP_NOFS), which all have > >__GFP_WAIT set. So the bio allocations will try normal allocation first, > >then fall back to the bio pool. If the bio pool is also empty, we will > >block waiting for entries to be freed there. So there never will be a > >failure. > > I did not realize that allocation with __GFP_WAIT was guaranteed > to _never_ fail.
See the mempool implementation. Even before bio used mempool, it used the exact same logic to make sure it never fails. Basically the heuristic in both cases is:
repeat: bio = normal_alloc if (bio) return bio
bio = get_from_pool if (bio) return bio
if (gfp_mask & __GFP_WAIT) start disk i/o goto repeat;
> Even so, if __GFP_WAIT never fails, then it can deadlock (for > example, some other device driver has a memory leak). Under a > scheme like bio_chain (provided that it is changed not to call a > memory allocator that can deadlock), the only way you deadlock is > if there really is deadlock bug in the lower layers that process > the underlying request.
This whole dead lock debate has been done to death before, I suggest you find the mempool discussions in the lkml archives from the earlier 2.5 series. Basically we maintain deadlock free allocation although we never fail allocs by saying that a single bio allocation is short lived (or at least not held indefinetely). That plus a reserve of XX bios makes sure that someone will always return a bio to the pool sooner or later and at least the get_from_pool alloc above will succeed sooner or later even if vm pressure is ridicilous.
-- Jens Axboe
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