Re: [PATCH v3 0/2] mm/swap: skip empty clusters in the swapoff scan

[Baoquan He]

On 08/07/26 at 03:41pm, Youngjun Park wrote:
> On Thu, Aug 06, 2026 at 01:06:55PM -0700, Andrew Morton wrote:
> > On Fri,  7 Aug 2026 04:32:26 +0900 Youngjun Park <youngjun.park@xxxxxxx> wrote:
> > 
> > > find_next_to_unuse() walks a swap device one offset at a time.  Slot
> > > state now lives in a per cluster swap table, so patch 2 dismisses an
> > > empty cluster with one counter read instead of SWAPFILE_CLUSTER table
> > > reads.
> > 
> > Thanks.
> > 
> > Can you help us understand how significant this change is for users? 
> > If "not very" then I'd prefer to defer consideraton of the series until
> > after 7.3-rc1.
> 
> Hello Andrew
> 
> "Not very" in the common case, though there is a case where the win is clear.
> No bug and no user report.
> 
> For now I would rather defer to after 7.3-rc1.
> 
> And for your reference, here is the details.
> 
> Every swapoff does a little less work now, because the scan steps over an
> unused area one cluster at a time.
> But IMHO most of the swapoff time goes to unuse_mm() and to reading the pages back in.
> 
> The gain shows on a large swap device that is almost empty, when the last
> pages still in use are near the end of it.  The scan has to walk up to
> them, and today it looks at every slot on the way.  Now the empty clusters
> in between are skipped in one step.
> 
> I have no measured times yet, since that case has to be set up on purpose.
> What I did is the arithmetic for the case that skips best,
> For example 1T of swap with 256M slots with SWAPFILE_CLUSTER = 512
> and everything free but the far end:
> 
>   - today:          256M table reads
>   - with the skip:  512K counter reads


Maybe just use time to measure swapoff time consuming, just like below
as I did on a kvm guest, I guess a bare metal machine with larger system
ram could be more obvious?

root@fedora:~# free -h
               total        used        free      shared  buff/cache   available
Mem:           3.8Gi       181Mi       3.6Gi       924Ki        72Mi       3.7Gi
Swap:          2.0Gi        18Mi       2.0Gi
root@fedora:~# time swapoff /dev/vdb

real	0m0.101s
user	0m0.001s
sys	0m0.017s
root@fedora:~# swapon /dev/vdb
root@fedora:~# time swapoff /dev/vdb

real	0m0.014s
user	0m0.003s
sys	0m0.001s

Not sure if Andrew is asking for this.


> 
> That should be around half a second of scan saved.

Yeah, a concrete number is shown.