On Tue, Mar 28, 2023 at 6:33 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > > Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes: > > > On Tue, Mar 28, 2023 at 2:32 PM Chris Li <chrisl@xxxxxxxxxx> wrote: > >> > >> On Tue, Mar 28, 2023 at 02:01:09PM -0700, Yosry Ahmed wrote: > >> > On Tue, Mar 28, 2023 at 1:50 PM Chris Li <chrisl@xxxxxxxxxx> wrote: > >> > > > >> > > On Tue, Mar 28, 2023 at 12:59:31AM -0700, Yosry Ahmed wrote: > >> > > > > > I don't have a problem with this approach, it is not really clean as > >> > > > > > we still treat zswap as a swapfile and have to deal with a lot of > >> > > > > > unnecessary code like swap slots handling and whatnot. > >> > > > > > >> > > > > These are existing code? > >> > > > >> > > Yes. The ghost swap file are existing code used in Google for many years. > >> > > > >> > > > I was referring to the fact that today with zswap being tied to > >> > > > swapfiles we do some necessary work such as searching for swap slots > >> > > > during swapout. The initial swap_desc approach aimed to avoid that. > >> > > > With this minimal ghost swapfile approach we retain this unfavorable > >> > > > behavior. > >> > > > >> > > Can you explain how you can avoid the free swap entry search > >> > > in the swap descriptor world? > >> > > >> > For zswap, in the swap descriptor world, you just need to allocate a > >> > struct zswap_entry and have the swap descriptor point to it. No need > >> > for swap slot management since we are not tied to a swapfile and pages > >> > in zswap do not have a specific position. > >> > >> Your swap descriptor will be using one swp_entry_t, which get from the PTE > >> to lookup, right? That is the swap entry I am talking about. You just > >> substitute zswap swap entry with the swap descriptor swap entry. > >> You still need to allocate from the free swap entry space at least once. > > > > Oh, you mean the swap ID space. We just need to find an unused ID, we > > can simply use an allocating xarray > > (https://docs.kernel.org/core-api/xarray.html#allocating-xarrays). > > This is simpler than keeping track of swap slots in a swapfile. > > If we want to implement the swap entry management inside the zswap > implementation (instead of reusing swap_map[]), then the allocating > xarray can be used too. Some per-entry data (such as swap count, etc.) > can be stored there. I understanding that this isn't perfect (one more > xarray looking up, one more data structure, etc.), but this is a choice > too. My main concern here would be having two separate swap counting implementations -- although it might not be the end of the world. It would be useful to consider all the options. So far, I think we have been discussing 3 alternatives: (a) The initial swap_desc proposal. (b) Add an optional indirection layer that can move swap entries between swap devices and add a virtual swap device for zswap in the kernel. (c) Add an optional indirection layer that can move entries between different swap backends. Swap backends would be zswap & swap devices for now. Zswap needs to implement swap entry management, swap counting, etc. Does this accurately summarize what we have discussed so far? > > Best Regards, > Huang, Ying >
