Re: nullfs and ZFS issues

On Tue, Apr 19, 2022 at 11:47:22AM +0200, Mateusz Guzik wrote:
| Try this: https://people.freebsd.org/~mjg/vnlru_free_pick.diff
| 
| this is not committable but should validate whether it works fine

As a POC it's working.  I see the vnode count for the nullfs and
ZFS go up.  The ARC cache also goes up until it exceeds the ARC max.
size tten the vnodes for nullfs and ZFS goes down.  The ARC cache goes
down as well.  This all repeats over and over.  The systems seems
healthy.  No excessive running of arc_prune or arc_evict.

My only comment is that the vnode freeing seems a bit agressive.
Going from ~15,000 to ~200 vnode for nullfs and the same for ZFS.
The ARC drops from 70M to 7M (max is set at 64M) for this unit
test.

Thanks,

Doug A.
 
| On 4/19/22, Mateusz Guzik <mjguzik@gmail.com> wrote:
| > On 4/19/22, Mateusz Guzik <mjguzik@gmail.com> wrote:
| >> On 4/19/22, Doug Ambrisko <ambrisko@ambrisko.com> wrote:
| >>> I've switched my laptop to use nullfs and ZFS.  Previously, I used
| >>> localhost NFS mounts instead of nullfs when nullfs would complain
| >>> that it couldn't mount.  Since that check has been removed, I've
| >>> switched to nullfs only.  However, every so often my laptop would
| >>> get slow and the the ARC evict and prune thread would consume two
| >>> cores 100% until I rebooted.  I had a 1G max. ARC and have increased
| >>> it to 2G now.  Looking into this has uncovered some issues:
| >>>      -	nullfs would prevent vnlru_free_vfsops from doing anything
| >>> 	when called from ZFS arc_prune_task
| >>>      -	nullfs would hang onto a bunch of vnodes unless mounted with
| >>> 	nocache
| >>>      -	nullfs and nocache would break untar.  This has been fixed now.
| >>>
| >>> With nullfs, nocache and settings max vnodes to a low number I can
| >>> keep the ARC around the max. without evict and prune consuming
| >>> 100% of 2 cores.  This doesn't seem like the best solution but it
| >>> better then when the ARC starts spinning.
| >>>
| >>> Looking into this issue with bhyve and a md drive for testing I create
| >>> a brand new zpool mounted as /test and then nullfs mount /test to /mnt.
| >>> I loop through untaring the Linux kernel into the nullfs mount, rm -rf
| >>> it
| >>> and repeat.  I set the ARC to the smallest value I can.  Untarring the
| >>> Linux kernel was enough to get the ARC evict and prune to spin since
| >>> they couldn't evict/prune anything.
| >>>
| >>> Looking at vnlru_free_vfsops called from ZFS arc_prune_task I see it
| >>>   static int
| >>>   vnlru_free_impl(int count, struct vfsops *mnt_op, struct vnode *mvp)
| >>>   {
| >>> 	...
| >>>
| >>>         for (;;) {
| >>> 	...
| >>>                 vp = TAILQ_NEXT(vp, v_vnodelist);
| >>> 	...
| >>>
| >>>                 /*
| >>>                  * Don't recycle if our vnode is from different type
| >>>                  * of mount point.  Note that mp is type-safe, the
| >>>                  * check does not reach unmapped address even if
| >>>                  * vnode is reclaimed.
| >>>                  */
| >>>                 if (mnt_op != NULL && (mp = vp->v_mount) != NULL &&
| >>>                     mp->mnt_op != mnt_op) {
| >>>                         continue;
| >>>                 }
| >>> 	...
| >>>
| >>> The vp ends up being the nulfs mount and then hits the continue
| >>> even though the passed in mvp is on ZFS.  If I do a hack to
| >>> comment out the continue then I see the ARC, nullfs vnodes and
| >>> ZFS vnodes grow.  When the ARC calls arc_prune_task that calls
| >>> vnlru_free_vfsops and now the vnodes go down for nullfs and ZFS.
| >>> The ARC cache usage also goes down.  Then they increase again until
| >>> the ARC gets full and then they go down again.  So with this hack
| >>> I don't need nocache passed to nullfs and I don't need to limit
| >>> the max vnodes.  Doing multiple untars in parallel over and over
| >>> doesn't seem to cause any issues for this test.  I'm not saying
| >>> commenting out continue is the fix but a simple POC test.
| >>>
| >>
| >> I don't see an easy way to say "this is a nullfs vnode holding onto a
| >> zfs vnode". Perhaps the routine can be extrended with issuing a nullfs
| >> callback, if the module is loaded.
| >>
| >> In the meantime I think a good enough(tm) fix would be to check that
| >> nothing was freed and fallback to good old regular clean up without
| >> filtering by vfsops. This would be very similar to what you are doing
| >> with your hack.
| >>
| >
| > Now that I wrote this perhaps an acceptable hack would be to extend
| > struct mount with a pointer to "lower layer" mount (if any) and patch
| > the vfsops check to also look there.
| >
| >>
| >>> It appears that when ZFS is asking for cached vnodes to be
| >>> free'd nullfs also needs to free some up as well so that
| >>> they are free'd on the VFS level.  It seems that vnlru_free_impl
| >>> should allow some of the related nullfs vnodes to be free'd so
| >>> the ZFS ones can be free'd and reduce the size of the ARC.
| >>>
| >>> BTW, I also hacked the kernel and mount to show the vnodes used
| >>> per mount ie. mount -v:
| >>>   test on /test (zfs, NFS exported, local, nfsv4acls, fsid
| >>> 2b23b2a1de21ed66,
| >>> vnodes: count 13846 lazy 0)
| >>>   /test on /mnt (nullfs, NFS exported, local, nfsv4acls, fsid
| >>> 11ff002929000000, vnodes: count 13846 lazy 0)
| >>>
| >>> Now I can easily see how the vnodes are used without going into ddb.
| >>> On my laptop I have various vnet jails and nullfs mount my homedir into
| >>> them so pretty much everything goes through nullfs to ZFS.  I'm limping
| >>> along with the nullfs nocache and small number of vnodes but it would be
| >>> nice to not need that.
| >>>
| >>> Thanks,
| >>>
| >>> Doug A.
| >>>
| >>>
| >>
| >>
| >> --
| >> Mateusz Guzik <mjguzik gmail.com>
| >>
| >
| >
| > --
| > Mateusz Guzik <mjguzik gmail.com>
| >
| 
| 
| -- 
| Mateusz Guzik <mjguzik gmail.com>