We seem to have managed to keep XFS going without the memory reservation
scheme - and the way we do I/O on metadata right now means there is always
a memory allocation in that path. At the moment the only thing I can kill
the system with is make -j bzImage it eventually grinds to a halt with
the swapper waiting for a request slot in the block layer but the system
is in such a mess that I have not been able to diagnose it further than
that.
A lot of careful use of GFP flags on memory allocation was necessary to
get to this point, the GFP_NOIO and GFP_NOFS finally made this deadlock
clean.
Steve
>
> It's not just ext3 --- highmem bounce buffering and soft raid buffers
> have the same problem, and work around it by doing their own internal
> preallocation of emergency buffers. Loop devices and nbd will have a
> similar problem if you use those for swap or writable mmaps, as will
> NFS.
>
> One proposed suggestion is to do per-zone memory reservations for the
> VM's use: Ben LaHaise has prototype code for that and we'll be testing
> to see if it makes for an improvement when used with ext3.
>
> Cheers,
> Stephen
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