On Wed, 20 Aug 2014 21:42:55 -0400 Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx> wrote: > 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? > Would that work? PG_private seems to be set when the page is being written, not when the COMMIT is happening. That is marked with the NFS_INO_COMMIT flag. So what my code does is wait a little while for NFS_INO_COMMIT to be released. I tried to leave the flow largely unchanged for a non-local server and only introduced a timeout for a local server. That probably creates the complexity that is bothering you(?). NeilBrown
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