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.
