Re: [PATCH 2/5] mm/zswap: change dstmem size to one page

[Nhat Pham]

On Wed, Dec 13, 2023 at 3:34 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> On Tue, Dec 12, 2023 at 8:18 PM Chengming Zhou
> <zhouchengming@xxxxxxxxxxxxx> wrote:
> >
> > Change the dstmem size from 2 * PAGE_SIZE to only one page since
> > we only need at most one page when compress, and the "dlen" is also
> > PAGE_SIZE in acomp_request_set_params(). If the output size > PAGE_SIZE
> > we don't wanna store the output in zswap anyway.
> >
> > So change it to one page, and delete the stale comment.
>
> I couldn't find the history of why we needed 2 * PAGE_SIZE, it would
> be nice if someone has the context, perhaps one of the maintainers.

It'd be very nice indeed.

>
> One potential reason is that we used to store a zswap header
> containing the swap entry in the compressed page for writeback
> purposes, but we don't do that anymore. Maybe we wanted to be able to
> handle the case where an incompressible page would exceed PAGE_SIZE
> because of that?

It could be hmm. I didn't study the old zswap architecture too much,
but it has been 2 * PAGE_SIZE since the time zswap was first merged
last I checked.
I'm not 100% comfortable ACK-ing the undoing of something that looks
so intentional, but FTR, AFAICT, this looks correct to me.