On Sep 8, 2009, at 9:37 PM, Trond Myklebust wrote:
On Tue, 2009-09-08 at 21:01 -0400, Chuck Lever wrote:
On Sep 8, 2009, at 7:05 PM, Trond Myklebust wrote:
On Tue, 2009-09-08 at 18:43 -0400, Chuck Lever wrote:
On Sep 8, 2009, at 6:32 PM, Trond Myklebust wrote:
On Tue, 2009-09-08 at 18:05 -0400, Chuck Lever wrote:
Don't dive into memory reclaim in the NFS direct I/O paths,
otherwise
we can deadlock.
Reported by: Wengang Wang <wen.gang.wang@xxxxxxxxxx>
Fix-suggested-by: Zach Brown <zach.brown@xxxxxxxxxx>
Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
Wait... What??? How does an O_DIRECT read or write allocation
deadlock
with memory reclaim? Both the read and the write path call
nfs_direct_req_alloc() before they pin any user pages in memory.
This may be an issue only for loopback mounts where the backing
device
is an NFS O_DIRECT file. This type of deadlock may not be able to
happen in upstream kernels at this point.
I don't see how that makes any difference whatsoever. If the backing
device is a non-O_DIRECT file, then you have GFP_KERNEL allocation
of
the pages.
Anything that calls down into a filesystem on a read() or write()
path
had better not assume that it won't block.
Basically we're treating an O_DIRECT file just like a block device.
If the block I/O path blocks when a kernel file system calls in to do
a memory reclaim, we're in dutch.
Without a lot more changelog context that explains what you are
wanting
to do, why it is relevant to NFS (and O_DIRECT in particular), and why
you can't work around it in other ways (PF_MEMALLOC comes to mind),
I'm
not at all interested in applying this patch.
I didn't ask you to apply it, I just asked for your thoughts.
We have a dm target that uses an NFS file as a backing device. It
converts bios to NFS read and write requests using direct and async I/
O. It's a loopback block device with a local file system residing in
it.
(OK, so "loopback mount" was probably not a clear way to explain what
is going on).
Even so, it makes sense for this allocation to be consistent with
similar allocations in the other NFS I/O paths.
I don't buy the 'symmetry' argument. The reason for the GFP_NOFS in
the
nfs_writedata_alloc() is that you have a deadlock when the VM calls
->writepages() in order to reclaim memory.
That is not the case here, and so this is not a symmetrical case.
That is precisely the case here, in fact. The upper file system is
attempting to reclaim memory in the same kernel where the NFS client
is trying to allocate with GFP_KERNEL.
That's the "upper file system"'s problem, not ours... Stacking
filesystems causes issues. Screwing over the existing users of the
underlying filesystem is not a fix for those issues...
How does this change "screw over" the existing users of NFS O_DIRECT?
--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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