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). > > 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.
