Chris Li <chrisl@xxxxxxxxxx> writes: > On Mon, Mar 20, 2023 at 10:55:03AM +0800, Huang, Ying wrote: >> > >> > How so? With the indirection enabled, the page tables & page cache >> > have the swap id (or swap_desc index), which can point to a swap entry >> > or a zswap entry -- which can change when the page is moved between >> > zswap & swapfiles. How is xarray (a) indexed by the swap entry in this >> > case? Shouldn't be indexed by the abstract swap id so that the >> > writeback from zswap is transparent? >> >> In my mind, >> >> - swap core will define a abstract interface to swap implementations >> (zswap, swap device/file, maybe more in the future), like VFS. > > I like your idea very much. Thanks! >> >> - zswap will be a special swap implementation (compressing instead of >> writing to disk). > > Agree. > >> >> - swap core will manage the indirection layer and swap cache. > > Agree, those are very good points. > >> >> - swap core can move swap pages between swap implementations (e.g., from >> zswap to a swap device, or from one swap device to another swap >> device) with the help of the indirection layer. > > We need to carefully design the swap cache that, when moving between > swap implementaions, there will be one shared swap cache. The current > swap cache belongs to swap devices, so two devices will have the same > page in two swap caches. We can remove a page from the swap cache for the swap device A, then insert the page into the swap cache for the swap device B. The swap entry will be changed too. >> In this design, the writeback from zswap becomes moving swapped pages >> from zswap to a swap device. > > Ack. > >> >> If my understanding were correct, your suggestion is kind of moving >> zswap logic to the swap core? And zswap will be always at a higher >> layer on top of swap device/file? > > It seems that way to me. I will let Yosry confirm that. > >> > I am not sure how this works with zswap. Currently swap_map[] >> > implementation is specific for swapfiles, it does not work for zswap >> > unless we implement separate swap counting logic for zswap & >> > swapfiles. Same for the swapcache, it currently supports being indexed >> > by a swap entry, it would need to support being indexed by a swap id, >> > or have a separate swap cache for zswap. Having separate >> > implementation would add complexity, and we would need to perform >> > handoffs of the swap count/cache when a page is moved from zswap to a >> > swapfile. >> >> We can allocate a swap entry for each swapped page in zswap. > > One thing to consider when moving page from zswap to swap file, is the > zswap swap entry the same entry as the swap file entry. I think that the swap entry will be changed after moving. Swap entry is kind of local to a swap device. While the swap desc ID isn't changed, that is why we need the indirection layer. >> > I think for this proposal, there are only 2 hardcoded tiers. Zswap is >> > fast, swapfile is slow. In the future, we can support more dynamic >> > tiering if the need arises. >> >> We can start from a simple implementation. And I think that it's better >> to consider the general design too. Try not to make it impossible now. > > In my mind there are a few usage cases: > 1) using only swap file. > 2) using only zswap, no swap file. > 3) Using zswap + swap file (SSD). > > The swap core should handle both 3 cases well with minial memory waste. Yes. Agree. Best Regards, Huang, Ying
