On Fri, 6 Sep 2013 11:48:45 -0500 Quentin Barnes <qbarnes@xxxxxxxxx> wrote: > Jeff, can your try out my test program in the base note on your > RHEL5.9 or later RHEL5.x kernels? > > I reverified that running the test on a 2.6.18-348.16.1.el5 x86_64 > kernel (latest released RHEL5.9) does not show the problem for me. > Based on what you and Trond have said in this thread though, I'm > really curious why it doesn't have the problem. > I can confirm what you see on RHEL5. One difference is that RHEL5's page_mkwrite handler does not do wait_on_page_writeback. That was added as part of the stable pages work that went in a while back, so that may be the main difference. Adding that in doesn't seem to materially change things though. In any case, what I see is that the initial program just ends up with a two calls to nfs_vm_page_mkwrite(). They both push out a WRITE and then things settle down (likely because the page is still marked dirty). Eventually, another write occurs and the dirty page gets pushed out to the server in a small flurry of WRITEs to the same range.Then, things settle down again until there's another small flurry of activity. My suspicion is that there is a race condition involved here, but I'm unclear on where it is. I'm not 100% convinced this is a bug, but page fault semantics aren't my strong suit. You may want to consider opening a "formal" RH support case if you have interest in getting Trond's patch backported, and/or following up on why RHEL5 behaves the way it does. > On Fri, Sep 6, 2013 at 8:36 AM, Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > On Thu, 5 Sep 2013 17:34:20 -0500 > > Quentin Barnes <qbarnes@xxxxxxxxx> wrote: > > > >> On Thu, Sep 05, 2013 at 09:57:24PM +0000, Myklebust, Trond wrote: > >> > On Thu, 2013-09-05 at 16:36 -0500, Quentin Barnes wrote: > >> > > On Thu, Sep 05, 2013 at 08:02:01PM +0000, Myklebust, Trond wrote: > >> > > > On Thu, 2013-09-05 at 14:11 -0500, Quentin Barnes wrote: > >> > > > > On Thu, Sep 05, 2013 at 12:03:03PM -0500, Malahal Naineni wrote: > >> > > > > > Neil Brown posted a patch couple days ago for this! > >> > > > > > > >> > > > > > http://thread.gmane.org/gmane.linux.nfs/58473 > >> > > > > > >> > > > > I tried Neil's patch on a v3.11 kernel. The rebuilt kernel still > >> > > > > exhibited the same 1000s of WRITEs/sec problem. > >> > > > > > >> > > > > Any other ideas? > >> > > > > >> > > > Yes. Please try the attached patch. > >> > > > >> > > Great! That did the trick! > >> > > > >> > > Do you feel this patch could be worthy of pushing it upstream in its > >> > > current state or was it just to verify a theory? > >> > > > >> > > > >> > > In comparing the nfs_flush_incompatible() implementations between > >> > > RHEL5 and v3.11 (without your patch), the guts of the algorithm seem > >> > > more or less logically equivalent to me on whether or not to flush > >> > > the page. Also, when and where nfs_flush_incompatible() is invoked > >> > > seems the same. Would you provide a very brief pointer to clue me > >> > > in as to why this problem didn't also manifest circa 2.6.18 days? > >> > > >> > There was no nfs_vm_page_mkwrite() to handle page faults in the 2.6.18 > >> > days, and so the risk was that your mmapped writes could end up being > >> > sent with the wrong credentials. > >> > >> Ah! You're right that nfs_vm_page_mkwrite() was missing from > >> the original 2.6.18, so that makes sense, however, Red Hat had > >> backported that function starting with their RHEL5.9(*) kernels, > >> yet the problem doesn't manifest on RHEL5.9. Maybe the answer lies > >> somewhere in RHEL5.9's do_wp_page(), or up that call path, but > >> glancing through it, it all looks pretty close though. > >> > >> > >> (*) That was the source I using when comparing with the 3.11 source > >> when studying your patch since it was the last kernel known to me > >> without the problem. > >> > > > > I'm pretty sure RHEL5 has a similar problem, but it's unclear to me why > > you're not seeing it there. I have a RHBZ open vs. RHEL5 but it's marked > > private at the moment (I'll see about opening it up). I brought this up > > upstream about a year ago with this strawman patch: > > > > http://article.gmane.org/gmane.linux.nfs/51240 > > > > ...at the time Trond said he was working on a set of patches to track > > the open/lock stateid on a per-req basis. Did that approach not pan > > out? > > > > Also, do you need to do a similar fix to nfs_can_coalesce_requests? > > > > Thanks, > > -- > > Jeff Layton <jlayton@xxxxxxxxxx> > -- > 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 -- Jeff Layton <jlayton@xxxxxxxxxx> -- 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
