In last year's LSF/MM I talked about a VFS-like swap system. That is the pony that was chosen. However, I did not have much chance to go into details. This year, I would like to discuss what it takes to re-architect the whole swap back end from scratch? Let’s start from the requirements for the swap back end. 1) support the existing swap usage (not the implementation). Some other design goals:: 2) low per swap entry memory usage. 3) low io latency. What are the functions the swap system needs to support? At the device level. Swap systems need to support a list of swap files with a priority order. The same priority of swap device will do round robin writing on the swap device. The swap device type includes zswap, zram, SSD, spinning hard disk, swap file in a file system. At the swap entry level, here is the list of existing swap entry usage: * Swap entry allocation and free. Each swap entry needs to be associated with a location of the disk space in the swapfile. (offset of swap entry). * Each swap entry needs to track the map count of the entry. (swap_map) * Each swap entry needs to be able to find the associated memory cgroup. (swap_cgroup_ctrl->map) * Swap cache. Lookup folio/shadow from swap entry * Swap page writes through a swapfile in a file system other than a block device. (swap_extent) * Shadow entry. (store in swap cache) Any new swap back end might have different internal implementation, but needs to support the above usage. For example, using the existing file system as swap backend, per vma or per swap entry map to a file would mean it needs additional data structure to track the swap_cgroup_ctrl, combined with the size of the file inode. It would be challenging to meet the design goal 2) and 3) using another file system as it is.. I am considering grouping different swap entry data into one single struct and dynamically allocate it so no upfront allocation of swap_map. For the swap entry allocation.Current kernel support swap out 0 order or pmd order pages. There are some discussions and patches that add swap out for folio size in between (mTHP) https://lore.kernel.org/linux-mm/20231025144546.577640-1-ryan.roberts@xxxxxxx/ and swap in for mTHP: https://lore.kernel.org/all/20240229003753.134193-1-21cnbao@xxxxxxxxx/ The introduction of swapping different order of pages will further complicate the swap entry fragmentation issue. The swap back end has no way to predict the life cycle of the swap entries. Repeat allocate and free swap entry of different sizes will fragment the swap entries array. If we can’t allocate the contiguous swap entry for a mTHP, it will have to split the mTHP to a smaller size to perform the swap in and out. T Current swap only supports 4K pages or pmd size pages. When adding the other in between sizes, it greatly increases the chance of fragmenting the swap entry space. When no more continuous swap swap entry for mTHP, it will force the mTHP split into 4K pages. If we don’t solve the fragmentation issue. It will be a constant source of splitting the mTHP. Another limitation I would like to address is that swap_writepage can only write out IO in one contiguous chunk, not able to perform non-continuous IO. When the swapfile is close to full, it is likely the unused entry will spread across different locations. It would be nice to be able to read and write large folio using discontiguous disk IO locations. Some possible ideas for the fragmentation issue. a) buddy allocator for swap entities. Similar to the buddy allocator in memory. We can use a buddy allocator system for the swap entry to avoid the low order swap entry fragment too much of the high order swap entry. It should greatly reduce the fragmentation caused by allocate and free of the swap entry of different sizes. However the buddy allocator has its own limit as well. Unlike system memory, we can move and compact the memory. There is no rmap for swap entry, it is much harder to move a swap entry to another disk location. So the buddy allocator for swap will help, but not solve all the fragmentation issues. b) Large swap entries. Take file as an example, a file on the file system can write to a discontinuous disk location. The file system responsible for tracking how to map the file offset into disk location. A large swap entry can have a similar indirection array map out the disk location for different subpages within a folio. This allows a large folio to write out dis-continguos swap entries on the swap file. The array will need to store somewhere as part of the overhead.When allocating swap entries for the folio, we can allocate a batch of smaller 4k swap entries into an array. Use this array to read/write the large folio. There will be a lot of plumbing work to get it to work. Solution a) and b) can work together as well. Only use b) if not able to allocate swap entries from a). Chris
