On Wed, Aug 20, 2014 at 9:11 PM, NeilBrown <neilb@xxxxxxx> wrote: > On Wed, 20 Aug 2014 20:45:16 -0400 Trond Myklebust > <trond.myklebust@xxxxxxxxxxxxxxx> wrote: > >> On Mon, 2014-08-18 at 16:22 +1000, NeilBrown wrote: >> > Support for loop-back mounted NFS filesystems is useful when NFS is >> > used to access shared storage in a high-availability cluster. >> > >> > If the node running the NFS server fails, some other node can mount the >> > filesystem and start providing NFS service. If that node already had >> > the filesystem NFS mounted, it will now have it loop-back mounted. >> > >> > nfsd can suffer a deadlock when allocating memory and entering direct >> > reclaim. >> > While direct reclaim does not write to the NFS filesystem it can send >> > and wait for a COMMIT through nfs_release_page(). >> > >> > This patch modifies nfs_release_page() and the functions it calls so >> > that if the COMMIT is sent to the local host (i.e. is routed over the >> > loop-back interface) then nfs_release_page() will not wait forever for >> > that COMMIT to complete. This is achieved using a new flag >> > FLUSH_COND_CONNECTED. When this is set the flush is conditional and >> > will only wait if the client is connected to a non-local server. >> > >> > Aborting early should be safe as kswapd uses the same code but never >> > waits for the COMMIT. >> > >> > Always aborting early could upset the balance of memory management so >> > when the commit was sent to the local host we still wait but with a >> > 100ms timeout. This is enough to significantly slow down any process >> > that is allocating lots of memory and often long enough to let the >> > commit complete. >> > >> > In those rare cases where it is nfsd, or something that nfsd is >> > waiting for, that is calling nfs_release_page(), 100ms is not so long >> > that throughput will be seriously affected. >> > >> > When fail-over happens a remote (foreign) client will first become >> > disconnected and then turn into a local client. >> > To prevent deadlocks from happening at this point, we still have a >> > timeout when the COMMIT has been sent to a remote client. In this case >> > the timeout is longer (1 second). >> > >> > So when a node that has mounted a remote filesystem loses the >> > connection, nfs_release_page() will stop blocking and start failing. >> > Memory allocators will then be able to make progress without blocking >> > in NFS. Any process which writes to a file will still be blocked in >> > balance_dirty_pages(). >> > >> > This patch makes use of the new 'private' field in >> > "struct wait_bit_key" to store the start time of a commit, so the >> > 'action' function called by __wait_on_bit() knows how much longer >> > it is appropriate to wait. >> > >> >> This puts way too much RPC connection logic in the NFS layer: we really >> should not have to care. Is there any reason why we could not just use >> RPC_TASK_SOFTCONN to have the commit fail if the connection is broken? > > I tried to keep as much in the RPC layer as I could.... > > There really is a big difference between talking to an 'nfsd' on the same > machine and talking to one on a different machine. In the first case you > need to be cautious of deadlocks, in the second you don't. That is a > difference that matters to NFS, not to RPC. > > I guess we could always have a timeout, even when connected to a remote > server. We would just end up blocking somewhere else when the server was not > responding. > > I don't think SOFTCONN is really appropriate. We don't want the COMMIT to > stop being retried. We just don't want the memory reclaim code to block > waiting for the COMMIT. > >> >> Note that all this has to come with a huge WARNING: all your data may be >> lost even if you think you just fsync()ed it. Is there any difference >> between doing this and using the 'async' export option on the knfsd >> side? >> > > I don't think this patch risks losing data at all - that certainly isn't the > intent. > This patch only affects the behaviour of ->releasepage, and only allows it to > fail instead of block. It is perfectly acceptable for releasepage to fail > and it doesn't result in data loss. It just means that the page is busy. > So why not just change the behaviour of ->releasepage() to always initiate a non-blocking commit and then wait up to 1 second for the PG_private bit to be released? -- Trond Myklebust Linux NFS client maintainer, PrimaryData trond.myklebust@xxxxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
