RFE: zfs doesn't mark its native kernel slabs as being reclaimable

[siebenmann]

The native kernel slab caches can be marked as containing data that is likely to be reclaimable under memory pressure. Slabs marked this way have their space show up in /proc/meminfo's SReclaimable (as opposed to SUnreclaim, which is slab memory not marked this way) and show up in the NR_SLAB_RECLAIMABLE page state. It appears that reclaimable slabs affect memory accounting in a few places beyond /proc/meminfo.

Normal Linux filesystems mark much of their slab usage this way by setting the SLAB_RECLAIM_ACCOUNT flag on slab creation. ZFS doesn't, perhaps partly because there is no equivalent Solaris/SPL flag for this marking. Especially if more of ZFS's memory allocation is moving to native kernel slabs, it might be good to somehow mark appropriate ZFS slabs this way.

(I suspect that not all ZFS slabs are reclaimable so it'd be incorrect to just have the SPL mark all of them that way by default.)

[behlendorf]

Thanks for pointing this out. As we shift more fully on to the Linux interfaces we should definitely find a way to take advantage of this.