On Wed, Mar 30, 2016 at 5:51 PM, Olga Kornievskaia <aglo@xxxxxxxxx> wrote: > 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. I'm saying why not just turn off the WCC functionality on the server then. -- 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
