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

[Yosry Ahmed]

On Thu, Dec 14, 2023 at 5:33 AM Chengming Zhou
<zhouchengming@xxxxxxxxxxxxx> wrote:
>
> On 2023/12/14 08:18, Nhat Pham wrote:
> > 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.
>
> Right, there is no any history about the reason why we needed 2 pages.
> But obviously only one page is needed from the current code and no any
> problem found in the kernel build stress testing.

Could you try manually stressing the compression with data that
doesn't compress at all (i.e. dlen == PAGE_SIZE)? I want to make sure
that this case is specifically handled. I think using data from
/dev/random will do that but please double check that dlen ==
PAGE_SIZE.