Re: [PATCH v3 2/4] mm/zswap: Implement proactive writeback
From: Hao Jia
Date: Wed Jun 03 2026 - 22:18:00 EST
On 2026/6/4 02:14, Nhat Pham wrote:
On Wed, Jun 3, 2026 at 10:58 AM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
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@xxxxxxxxx> wrote:
From: Hao Jia <jiahao1@xxxxxxxxxxx>
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.
Agree. This would indeed break the semantics of memory.reclaim. I will use a
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.
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.
I personally prefer compressed size to pre-compressed size. That's
kinda what user cares about, no?
One thing we can do is let users prescribe a compressed size, but
internally, we can multiply that by the average compression ratio.
That gives us a guesstimate of how many pages we need to reclaim, and
you can follow the rest of your implementation as is (perhaps with
short-circuit when we reach the goal with fewer pages reclaimed).
Got it. I will change it to use the compressed size in the next version.
Yosry, Nhat, should we continue using the zswap_writeback_only key to trigger proactive writeback?
Thanks,
Hao