On Wed, Mar 30, 2016 at 5:45 PM, Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx> wrote: > On Wed, Mar 30, 2016 at 5:02 PM, Olga Kornievskaia <aglo@xxxxxxxxx> wrote: >> On Tue, Mar 29, 2016 at 3:47 PM, Olga Kornievskaia <aglo@xxxxxxxxx> wrote: >>> I think that patch introduces a problem. Since the checking for the >>> change in ctime was removed by the commit it leads to (improper) cache >>> invalidation in NFSv3. >>> >>> Test is write 10240bytes to the server then read it. Expectation is >>> not to see read on the wire. In the test the write is spread over >>> 3rpcs. >>> >>> On the 1nd reply >>> fattr->gencount=33 nfsi->gencount=32 generation_counter=35 >>> On the 2nd reply >>> fattr->gencount=34 nfsi->gencount=36 generation_counter=36 >>> >>> In the code when processing 2nd reply, >>> nfs_post_op_update_inode_force_wcc_locked() calls into >>> nfs_inode_attrs_need_update() it determines that it doesn't need to >>> update them (even though the size and the time have changed). so it >>> doesn't call nfs_wcc_update_inode() so the inode->i_version doesn't >>> get set to the ctime that was received in the 2nd reply. >>> >>> On the 3rd reply >>> fattr->gencount=37 nfsi->gencount=36 generation_counter=37 >>> >>> It leads to nfs_inode_attrs_need_update() returns 1 and in the >>> nfs_update_inode() the difference in the ctimes leads to invalidation. >>> fattr->gencount was update from nfs_writeback_update_node() -> >>> nfs_post_op_update_inode_force_wcc() calling nfs_fattr_set_barrier(). >>> >>> I'm not sure what appropriate values for "gencount" should have been. >>> But if the check for nfs_ctime_need_update() was still there in >>> nfs_inode_attrs_need_update() then the 2nd reply would have >>> appropriately updated the i_version and not lead to invalidation. >> >> Would like to add that this problem is not seen against the Linux >> server because it doesn't send "before" attributes. So code doesn't >> set the "pre_change_attr" which later doesn't make what's stored in >> inode->i_version. >> >> The problem also not seen for v4 because pre_change_attr is not gotten >> from the "before" attributes but instead from the previous value in >> inode->i_version which is then compared to the itself. >> >> If reverting the problematic commit is not the solution, then how >> about ignoring the "before" ctime attributes sent by the server. This >> also helps with the out-of-order RPCs. > > Why bother doing that on the client? These attributes aren't mandatory > to send... > Leads to poor client performances. Every large enough read invalidates the cache so all the reads go to the server always. -- 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
