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- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Sat Jun 15 2002 - 22:00:33 EST