Writing back compressed pages [LWN.net]

Writing back compressed pages

Posted Feb 25, 2026 5:25 UTC (Wed) by intelfx (subscriber, #130118)
In reply to: Writing back compressed pages by PeeWee
Parent article: Modernizing swapping: virtual swap spaces

> Are you absolutely positive?

Entirely.

> Maybe it's a more recent change?

Depends on how recent we are talking. Linux 5.10 old enough?

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Writing back compressed pages

Posted Feb 25, 2026 6:05 UTC (Wed) by PeeWee (subscriber, #175777) [Link] (4 responses)

> Are you absolutely positive?
Entirely.

Found it:

		error = swap_writeout(folio, plug);
		if (error != AOP_WRITEPAGE_ACTIVATE) {
			/* folio has been unlocked */
			return error;
		}


		/*
		 * The intention here is to avoid holding on to the swap when
		 * zswap was unable to compress and unable to writeback; but
		 * it will be appropriate if other reactivate cases are added.
		 */

> Maybe it's a more recent change?
Depends on how recent we are talking. Linux 5.10 old enough?

That may very well be the case. I am running Ubuntu LTS which is not exactly bleeding edge.

Thanks for finally giving me a definitive answer to that question!

Writing back compressed pages

Posted Feb 25, 2026 9:31 UTC (Wed) by intelfx (subscriber, #130118) [Link] (3 responses)

It wasn't really "definitive", more like an upper bound that I could give relatively quickly... Tmpfs is zswappable at least up to Linux 4.9 (stretch); this was a good excuse to play around with a clustered Incus and imagebuilder but I won't bother checking further.

> I am running Ubuntu LTS which is not exactly bleeding edge.

So I'd say unless your Ubuntu LTS is like 14.04, you've just got it misconfigured somehow.

Writing back compressed pages

Posted Feb 25, 2026 9:35 UTC (Wed) by intelfx (subscriber, #130118) [Link] (1 responses)

> So I'd say unless your Ubuntu LTS is like 14.04, you've just got it misconfigured somehow.

(That said, given that you say your workloads are incompressible, it's all purely academic anyway. Oh well, still an excuse to build some images.)

Writing back compressed pages

Posted Feb 25, 2026 11:44 UTC (Wed) by PeeWee (subscriber, #175777) [Link]

I've just realized that my memory might also be clouded by my tinkering with memory.zswap.writeback=0. But I am pretty certain I saw that behavior before that cgroup knob even existed. And there seem to have been problems with accounting & cgroup control. I don't know how relevant they would have been for this case, but there is a set of commits under that umbrella linked to from the kernelnewbies page for the v5.19 release. I found those by accident while researching when writeback disabling was introduced, to narrow the time frame.

Anyway, I am starting to feel like I am abusing this thread/forum, so I'll leave it at that. I am very grateful for all your input and effort! Now I can explore some more use cases I had ruled out before. And if, against expectations, I do see the erroneous behavior, I'll know for sure that it must be an error of some kind.

Writing back compressed pages

Posted Feb 25, 2026 11:50 UTC (Wed) by PeeWee (subscriber, #175777) [Link]

It wasn't really "definitive"

Oh, now I get it. The "definitive" was meant in reference to the question if tmpfs somehow bypasses zswap. It does not matter too much, when it was made zswappable.