Re: [PATCH/RFC 00/19] Support loop-back NFS mounts

[NeilBrown]

On Thu, 17 Apr 2014 11:27:39 +1000 Dave Chinner <david@xxxxxxxxxxxxx> wrote:

> On Thu, Apr 17, 2014 at 10:20:48AM +1000, NeilBrown wrote:
> > A good example is the deadlock with the flush-* threads.
> > flush-* will lock a page, and  then call ->writepage.  If ->writepage
> > allocates memory it can enter reclaim, call ->releasepage on NFS, and block
> > waiting for a COMMIT to complete.
> > The COMMIT might already be running, performing fsync on that same file that
> > flush-* is flushing.  It locks each page in turn.  When it  gets to the page
> > that flush-* has locked, it will deadlock.
> 
> It's nfs_release_page() again....
> 
> > In general, if nfsd is allowed to block on local filesystem, and local
> > filesystem is allowed to block on NFS, then a deadlock can happen.
> > We would need a clear hierarchy
> > 
> >    __GFP_NETFS > __GFP_FS > __GFP_IO
> > 
> > for it to work.  I'm not sure the extra level really helps a lot and it would
> > be a lot of churn.
> 
> I think you are looking at this the wrong way - it's not the other
> filesystems that have to avoid memory reclaim recursion, it's the
> NFS client mount that is on loopback that needs to avoid recursion.
> 
> IMO, the fix should be that the NFS client cannot block on messages sent to the NFSD
> on the same host during memory reclaim. That is, nfs_release_page()
> cannot send commit messages to the server if the server is on
> localhost. Instead, it just tells memory reclaim that it can't
> reclaim that page.
> 
> If nfs_release_page() no longer blocks in memory reclaim, and all
> these nfsd-gets-blocked-in-GFP_KERNEL-memory-allocation recursion
> problems go away. Do the same for all the other memory reclaim
> operations in the NFS client, and you've got a solution that should
> work without needing to walk all over the rest of the kernel....

Maybe.
It is nfs_release_page() today. I wonder if it could be other things another
day.  I want to be sure I have a solution that really makes sense.

However ... the thing that nfs_release_page is doing it sending a COMMIT to
tell the server to flush to stable storage.  It does that so that if the
server crashes, then the client can re-send.
Of course when it is a loop-back mount the client is the server so the COMMIT
is completely pointless.  If the client notices that it is sending a COMMIT
to itself, it can simply assume a positive reply.

You are right, that would make the patch set a lot less intrusive.  I'll give
it some serious thought - thanks.

NeilBrown

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