On Wed, Mar 30, 2016 at 7:32 PM, Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx> wrote: > 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. One reasoning could be that providing cache consistency is client's responsibility. Server only provides needed information. While I can see that handling out-of-order RPCs might not be worth it because hopefully it doesn't happen often but handling in order RPCs and always invalidating the cache is a bug. -- 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
