Re: [RFC PATCH v2 0/4] mm/zsmalloc: reduce zs_free() latency on swap release path

[Nhat Pham]

On Tue, Apr 21, 2026 at 10:18 AM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> On Tue, Apr 21, 2026 at 11:55 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
> >
>
> Thanks for adding me to the Cc list :), Barry started this idea with
> ZRAM, which looks very interesting to me.
>
> > On Tue, Apr 21, 2026 at 5:16 AM Wenchao Hao <haowenchao22@xxxxxxxxx> wrote:
> > >
> > > Swap freeing can be expensive when unmapping a VMA containing
> > > many swap entries. This has been reported to significantly
> > > delay memory reclamation during Android's low-memory killing,
> > > especially when multiple processes are terminated to free
> > > memory, with slot_free() accounting for more than 80% of
> > > the total cost of freeing swap entries.
> > >
> > > Two earlier attempts by Lei and Zhiguo added a new thread in the mm core
> > > to asynchronously collect and free swap entries [1][2], but the
> > > design itself is fairly complex.
> > >
> > > When anon folios and swap entries are mixed within a
> > > process, reclaiming anon folios from killed processes
> > > helps return memory to the system as quickly as possible,
> > > so that newly launched applications can satisfy their
> > > memory demands. It is not ideal for swap freeing to block
> > > anon folio freeing. On the other hand, swap freeing can
> > > still return memory to the system, although at a slower
> > > rate due to memory compression.
> >
> > Is this correct? I don't think we do decompression in
> > zswap_invalidate() path. We do decompression in zswap_load(), but as a
> > separate step from zswap_invalidate().
>
> It's not about decompression. I think what Wenchao means here is that:
> freeing the swap entry also releases the backing compression data, but
> compared to freeing an actual folio (which bring back a free folio to
> reduce memory pressure), you may need to free a lot of swap entries to
> free one whole folio, because the compressed data could be much
> smaller than folio and with fragmentation. And swap entry freeing is
> still not fast enough to be ignored.

Ah I see yeah. That's the not "as much bang-for-your-buck" as folio
freeing category. I agree on this point.

>
> >
> > zswap/zsmalloc entry freeing is decoupled from decompression. For
> > example, on process teardown, we free the zsmalloc memory but never
> > decompress (if we do then it's a bug to be fixed lol, but I doubt it).
> >
> > Zsmalloc freeing might not be worth as much bang-for-your-buck wise
> > compared to anon folio freeing, but if it's "expensive", then I think
> > that points to a different root-cause: zsmalloc's poor scalability in
> > the free path.
>
> That's a very nice insight. I had an idea previously that can we have
> something like a zs free bulk? Freeing handles one by one does seem
> expensive.
> https://lore.kernel.org/linux-mm/adt3Q_SRToF6fb3W@KASONG-MC4/
>
> It might be tricky to do so though.
>
> It will be best if we can speed up everything, doing things async
> doesn't reduce the total amount of work, and might cause more trouble
> like worker overhead or delayed freeing causing more memory pressure,
> if the workqueue didn't run in time. Or maybe a process is almost
> completely swapped out, then this won't help at all.
>
> I'm not against the async idea, they might combine well.

Completely agree! I was thinking about batching the free operations
for zsmalloc. Right now seems like even if we have a contiguous range
of swap slots to be freed, we call one
zram_slot_free_notify/zswap_invalidate at a time, which then call
zs_free one at a time? I wonder if there's any batching opportunity
here. Might be complicated with the pool lock and class lock dance in
zs_free() though :)

And yeah the async stuff is orthogonal too.

>
> >
> > I've stared at this code path for a bit, because my other patch series
> > (vswap - see [1]) was reported to display regression on the free path
> > on the usemem benchmark. And one of the issues was the contention
> > between compaction (both systemwide compaction, i.e zs_page_migrate,
> > and zsmalloc's internal compaction, but mostly the former).:
> >
> > * zs_free read-acquires pool->lock, and compaction write-acquires the
> > same lock. So the compaction thread will make all zs free-ers wait for
> > it. I saw this read lock delay when I perfed the free step of usemem.
> >
> > * If this lock has fair queue-ing semantics (I have not checked), then
> > if there a compaction is behind a bunch of zs_free in the queue, then
> > all the subsequent zs_free's ers are blocked :)
> >
> > * I'm also curious about cache-friendliness of this rwlock, bouncing
> > across CPUs, if you have multiple processes being torn down
> > concurrently.
>
> That's interesting, when I mentioned zs free bulk I was thinking that,
> if we have a percpu queue, at least we may try read lock that on every
> enqueue, free the whole queue if successful, then release the lock.
> I'm sure there are more ways to optimize that, just a random idea :)

Yep! Would be nice to have some perf trace to pinpoint where the overhead is.

On my end, I perfed the free phase of usemem. It varies a bit based on
exact build config, kernel version, or even between runs, but the
cheapest I've seen for the pool lock contention overhead is about 3%
of the free phase (this is on baseline, not vswap kernel). That's
pretty big (bigger than vswap overhead even on the kernels with vswap,
which is kinda silly). Obviously the host was very overcommitted, so
compaction was running in the background at the same time, but
still...