Hello everyone, I would like to propose a topic for the upcoming LSF/MM/BPF in May 2023 about swap & zswap (hope I am not too late). ==================== Intro ==================== Currently, using zswap is dependent on swapfiles in an unnecessary way. To use zswap, you need a swapfile configured (even if the space will not be used) and zswap is restricted by its size. When pages reside in zswap, the corresponding swap entry in the swapfile cannot be used, and is essentially wasted. We also go through unnecessary code paths when using zswap, such as finding and allocating a swap entry on the swapout path, or readahead in the swapin path. I am proposing a swapping abstraction layer that would allow us to remove zswap's dependency on swapfiles. This can be done by introducing a data structure between the actual swapping implementation (swapfiles, zswap) and the rest of the MM code. ==================== Objective ==================== Enabling the use of zswap without a backing swapfile, which makes zswap useful for a wider variety of use cases. Also, when zswap is used with a swapfile, the pages in zswap do not use up space in the swapfile, so the overall swapping capacity increases. ==================== Idea ==================== Introduce a data structure, which I currently call a swap_desc, as an abstraction layer between swapping implementation and the rest of MM code. Page tables & page caches would store a swap id (encoded as a swp_entry_t) instead of directly storing the swap entry associated with the swapfile. This swap id maps to a struct swap_desc, which acts as our abstraction layer. All MM code not concerned with swapping details would operate in terms of swap descs. The swap_desc can point to either a normal swap entry (associated with a swapfile) or a zswap entry. It can also include all non-backend specific operations, such as the swapcache (which would be a simple pointer in swap_desc), swap counting, etc. It creates a clear, nice abstraction layer between MM code and the actual swapping implementation. ==================== Benefits ==================== This work enables using zswap without a backing swapfile and increases the swap capacity when zswap is used with a swapfile. It also creates a separation that allows us to skip code paths that don't make sense in the zswap path (e.g. readahead). We get to drop zswap's rbtree which might result in better performance (less lookups, less lock contention). The abstraction layer also opens the door for multiple cleanups (e.g. removing swapper address spaces, removing swap count continuation code, etc). Another nice cleanup that this work enables would be separating the overloaded swp_entry_t into two distinct types: one for things that are stored in page tables / caches, and for actual swap entries. In the future, we can potentially further optimize how we use the bits in the page tables instead of sticking everything into the current type/offset format. Another potential win here can be swapoff, which can be more practical by directly scanning all swap_desc's instead of going through page tables and shmem page caches. Overall zswap becomes more accessible and available to a wider range of use cases. ==================== Cost ==================== The obvious downside of this is added memory overhead, specifically for users that use swapfiles without zswap. Instead of paying one byte (swap_map) for every potential page in the swapfile (+ swap count continuation), we pay the size of the swap_desc for every page that is actually in the swapfile, which I am estimating can be roughly around 24 bytes or so, so maybe 0.6% of swapped out memory. The overhead only scales with pages actually swapped out. For zswap users, it should be a win (or at least even) because we get to drop a lot of fields from struct zswap_entry (e.g. rbtree, index, etc). Another potential concern is readahead. With this design, we have no way to get a swap_desc given a swap entry (type & offset). We would need to maintain a reverse mapping, adding a little bit more overhead, or search all swapped out pages instead :). A reverse mapping might pump the per-swapped page overhead to ~32 bytes (~0.8% of swapped out memory). ==================== Bottom Line ==================== It would be nice to discuss the potential here and the tradeoffs. I know that other folks using zswap (or interested in using it) may find this very useful. I am sure I am missing some context on why things are the way they are, and perhaps some obvious holes in my story. Looking forward to discussing this with anyone interested :) I think Johannes may be interested in attending this discussion, since a lot of ideas here are inspired by discussions I had with him :)
