On Tue, Apr 4, 2023 at 1:12 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > > Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes: > > > 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. > > This isn't a big issue for me. For file systems, there are duplicated > functionality in different file system implementation, such as free > block space management. Instead, I hope we can design better swap > implementation in the future. > > > 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. > > My main concern for the initial swap_desc proposal is that the zswap > code is put in swap core instead of zswap implementation per my > understanding. So zswap isn't another swap implementation encapsulated > with a common interface. Please correct me if my understanding isn't > correct. > > If so, the flexibility of the swap system is the cost. For example, > zswap may be always at the highest priority among all swap devices. We > can move the cold page from zswap to some swap device. But we cannot > move the cold page from some swap device to zswap. Not really. In the swap_desc proposal, I intended to have struct swap_desc contain either a swap device entry (swp_entry_t) or a frontswap entry (a pointer). zswap implementation would not be in the swap core, instead, we would have two swap implementations: swap devices and frontswap/zswap -- each of which implement a common swap API. We can use one of the free bits to distinguish the type of the underlying entry (swp_entry_t or pointer to frontswap/zswap entry). We can start by only supporting moving pages from frontswap/zswap to swap devices, but I don't see why the same design would not support pages moving in the other direction if the need arises. The number of free bits in swp_entry_t and pointers is limited (2 bits on 32-bit systems, 3 bits on 64-bit systems), so there are only a handful of different swap types we can support with the swap_desc design, but we only need two to begin with. If in the future we need more, we can add an indirection layer then or expand swap_desc -- or we can encode the data within the swap device itself (how it compares to frontswap/zswap). In summary, the swap_desc proposal does NOT involve moving zswap code to core swap, it involves a generic swap API with two implementations: swap devices and frontswap/zswap. The only problems I see with the swap_desc design are: - Extra overhead for users using swapfiles only. - A bigger leap from what we have today than other ideas proposed (e.g. virtual swap device for zswap). > > > Maybe compression is always faster than any other swap devices, so we > will never need the flexibility. Maybe the cost to hide zswap behind a > common interface is unacceptable. I'm open to these. But please > provide the evidence, and maybe data. > > Best Regards, > Huang, Ying > > > (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? > >
