Hi Ying, On Tue, Apr 23, 2024 at 7:26 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > > Hi, Matthew, > > Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: > > > On Mon, Apr 22, 2024 at 03:54:58PM +0800, Huang, Ying wrote: > >> Is it possible to add "start_offset" support in xarray, so "index" > >> will subtract "start_offset" before looking up / inserting? > > > > We kind of have that with XA_FLAGS_ZERO_BUSY which is used for > > XA_FLAGS_ALLOC1. But that's just one bit for the entry at 0. We could > > generalise it, but then we'd have to store that somewhere and there's > > no obvious good place to store it that wouldn't enlarge struct xarray, > > which I'd be reluctant to do. > > > >> Is it possible to use multiple range locks to protect one xarray to > >> improve the lock scalability? This is why we have multiple "struct > >> address_space" for one swap device. And, we may have same lock > >> contention issue for large files too. > > > > It's something I've considered. The issue is search marks. If we delete > > an entry, we may have to walk all the way up the xarray clearing bits as > > we go and I'd rather not grab a lock at each level. There's a convenient > > 4 byte hole between nr_values and parent where we could put it. > > > > Oh, another issue is that we use i_pages.xa_lock to synchronise > > address_space.nrpages, so I'm not sure that a per-node lock will help. > > Thanks for looking at this. > > > But I'm conscious that there are workloads which show contention on > > xa_lock as their limiting factor, so I'm open to ideas to improve all > > these things. > > I have no idea so far because my very limited knowledge about xarray. For the swap file usage, I have been considering an idea to remove the index part of the xarray from swap cache. Swap cache is different from file cache in a few aspects. For one if we want to have a folio equivalent of "large swap entry". Then the natural alignment of those swap offset on does not make sense. Ideally we should be able to write the folio to un-aligned swap file locations. The other aspect for swap files is that, we already have different data structures organized around swap offset, swap_map and swap_cgroup. If we group the swap related data structure together. We can add a pointer to a union of folio or a shadow swap entry. We can use atomic updates on the swap struct member or breakdown the access lock by ranges just like swap cluster does. I want to discuss those ideas in the upcoming LSF/MM meet up as well. Chris
