Re: [PATCH 0/2] minimize swapping on zswap store failure

[Nhat Pham]

Wed, Apr 2, 2025 at 1:06 PM Joshua Hahn <joshua.hahnjy@xxxxxxxxx> wrote:
>
> On Mon, 16 Oct 2023 17:57:31 -0700 Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> > On Mon, Oct 16, 2023 at 5:35 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
>
> > I thought before about having a special list_head that allows us to
> > use the lower bits of the pointers as markers, similar to the xarray.
> > The markers can be used to place different objects on the same list.
> > We can have a list that is a mixture of struct page and struct
> > zswap_entry. I never pursued this idea, and I am sure someone will
> > scream at me for suggesting it. Maybe there is a less convoluted way
> > to keep the LRU ordering intact without allocating memory on the
> > reclaim path.
>
> Hi Yosry,
>
> Apologies for reviving an old thread, but I wasn't sure whether opening an
> entirely new thread was a better choice : -)
>
> So I've implemented your idea, using the lower 2 bits of the list_head's prev
> pointer (last bit indicates whether the list_head belongs to a page or a
> zswap_entry, and the second to last bit was repurposed for the second chance
> algorithm).
>
> For a very high level overview what I did in the patch:
> - When a page fails to compress, I remove the page mapping and tag both the
>   xarray entry (tag == set lowest bit to 1) and the page's list_head prev ptr,
>   then store the page directly into the zswap LRU.
> - In zswap_load, we take the entry out of the xarray and check if it's tagged.
>   - If it is tagged, then instead of decompressing, we just copy the page's
>     contents to the newly allocated page.
> - (More details about how to teach vmscan / page_io / list iterators how to
>   handle this, but we can gloss over those details for now)
>
> I have a working version, but have been holding off because I have only been
> seeing regressions. I wasn't really sure where they were coming from, but
> after going through some perf traces with Nhat, found out that the regressions
> come from the associated page faults that come from initially unmapping the
> page, and then re-allocating it for every load. This causes (1) more memcg
> flushing, and (2) extra allocations ==> more pressure ==> more reclaim, even
> though we only temporarily keep the extra page.

Also, double check your benchmark :) If you only cycle through the
pages only in the LRU, or if you will need the pages soon, by
definition you cannot do better than the status quo.

The gains will only materialize when you have longer workload, which
can (and often will) cycle the incompressible pages through the LRU
multiple times, recompressing (and failing) them multiple times with
zswap, etc.