On Mon, Sep 30, 2024 at 4:29 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote: > > On Mon, Sep 30, 2024 at 4:20 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > > > On Mon, Sep 30, 2024 at 4:11 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote: > > > > I suggested this in a previous version, and Kanchana faced some > > complexities implementing it: > > https://lore.kernel.org/lkml/SJ0PR11MB56785027ED6FCF673A84CEE6C96A2@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > > Sorry, I missed that conversation. > > > > > Basically, if we batch get the refs after the store I think it's not > > safe, because once an entry is published to writeback it can be > > written back and freed, and a ref that we never acquired would be > > dropped. > > Hmmm. I don't think writeback could touch any individual subpage just yet, no? > > Before doing any work, zswap writeback would attempt to add the > subpage to the swap cache (via __read_swap_cache_async()). However, > all subpage will have already been added to swap cache, and point to > the (large) folio. So zswap_writeback_entry() should short circuit > here (the if (!page_allocated) case). If it's safe to take the refs after all calls to zswap_store_page() are successful, then yeah that should be possible, for both the pool and objcg. I didn't look closely though. Just to clarify, you mean grab one ref first, then do the compressions, then grab the remaining refs, right? The other thing I would be worried about is code clarity, not sure if it would be confusing to initialize the entries in zswap_store_page(), then grab the refs later in zswap_store(). > > > > > Getting refs before the store would work, but then if the store fails > > at an arbitrary page, we need to only drop refs on the pool for pages > > that were not added to the tree, as the cleanup loop with > > zswap_entry_free() at the end of zswap_store() will drop the ref for > > those that were added to the tree. > > > > We agreed to (potentially) do the batching for refcounts as a followup. > > But yeah no biggie. Not a dealbreaker for me tbh. I thought it was a > quick change (hence the fixlet suggestion), but if not then let's do > it as a follow-up. I don't feel strongly either way.
