Re: [PATCH v3 2/4] mm/zswap: Implement proactive writeback

[Yosry Ahmed <yosry@kernel.org>]

On Wed, Jun 03, 2026 at 07:22:36PM +0800, Hao Jia wrote:
> 
> 
> On 2026/5/30 09:40, Yosry Ahmed wrote:
> > On Fri, May 29, 2026 at 12:58:09PM -0700, Nhat Pham wrote:
> > > On Tue, May 26, 2026 at 4:46 AM Hao Jia <jiahao.kernel@gmail.com> wrote:
> > > > 
> > > > From: Hao Jia <jiahao1@lixiang.com>
> > > > 
> > > > Zswap currently writes back pages to backing swap reactively, triggered
> > > > either by the shrinker or when the pool reaches its size limit. There is
> > > > no mechanism to control the amount of writeback for a specific memory
> > > > cgroup. However, users may want to proactively write back zswap pages,
> > > > e.g., to free up memory for other applications or to prepare for
> > > > memory-intensive workloads.
> > > > 
> > > > Introduce a "zswap_writeback_only" key to the memory.reclaim cgroup
> > > > interface. When specified, this key bypasses standard memory reclaim
> > > > and exclusively performs proactive zswap writeback up to the requested
> > > > budget. If omitted, the default reclaim behavior remains unchanged.
> > > > 
> > > > Example usage:
> > > >    # Write back 100MB of pages from zswap to the backing swap
> > > >    echo "100M zswap_writeback_only" > memory.reclaim
> > > 
> > > Hmmm, so this 100MB is the pre-compression size? i.e if this 100 MB
> > > compresses to 25 MB, then you're only freeing 25 MB?
> > > 
> > > I'm ok-ish with this, but can you document it?
> > 
> > That's a good point. I think pre-compressed size doesn't make sense to
> > be honest. We should care about how much memory we are actually trying
> > to save by doing writeback here.
> > 
> > The pre-compressed size is only useful in determining the blast radius,
> > how many actual pages are going to have slower page faults now. But
> > then, I don't think there's a reasonable way for userspace to decide
> > that.
> > 
> > I understand passing in the compressed size is tricky because we need to
> > keep track of the size of the compressed pages we end up writing back,
> > but it should be doable.
> 
> Agreed. Using pre-compressed size is probably easier to implement. IIRC,
> interfaces like ZRAM writeback_limit are also calculated using the
> pre-compressed size.
> 
> I'll clarify this in the documentation in the next version.
> 
> > 
> > If we really want pre-compressed size here, then yes we need to make it
> > very clear, and I vote that we use a separate interface in this case
> > because memory.reclaim having different meanings for the amount of
> > memory written to it is extremely counter-intuitive.
> > 
> Agree. This would indeed break the semantics of memory.reclaim. I will use a
> separate interface for proactive writeback in the next version.

But doesn't it make more sense to specify the compressed size, which is
ultimately the amount of memory you actually want to reclaim.

Johannes, Nhat, WDYT?