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. Best Regards, Huang, Ying
