On Thu, Mar 21, 2024 at 5:29 PM Chris Li <chrisl@xxxxxxxxxx> wrote: > > On Thu, Mar 21, 2024 at 12:58 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > > > On Thu, Mar 21, 2024 at 12:29 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > > > > > On Thu, Mar 21, 2024 at 06:44:54PM +0000, Yosry Ahmed wrote: > > > > > In some cases where the current objcg is not "correct", the testcases in > > > > > test_zswap.c may break? Maybe we can use swap_cgroup info to charge the > > > > > stats to the correct memcg? Not sure if this is feasible. > > > > > > > > For cgroup v1, swap_cgroup will be cleared from > > > > mem_cgroup_swapin_uncharge_swap() before the zswap load. > > > > > > > > I think the current objcg will remain correct as long as swapin happens > > > > from the same memcg as swapout (or if swapin happens from the parent > > > > memcg and the swapout memcg was offlined). > > > > > > Swap readahead will pull in physically adjacent entries that may > > > belong to somebody unrelated. > > > > Right. For those as well the current objcg would be correct if they > > are readahead from the same memcg as the one they were swapped out > > from, but I understand your point that readahead makes that more > > likely to not be the case. > > > > I am slightly nervous about using the current objcg tbh, even though > > it only affects the stats. It's just less straightforward this way. I > > think I prefer either: > > (a) Only supporting zero-filled pages and storing the objcg directly > > in the xarray. > > We can add the zero-filled page as a special case to speed it up. I > don't think we should remove non zero values of the same fill pages > from zswap though. We can never declare the non zero same filled page > is not going to happen. In that case, the current same fill is still > better than going through the zsmalloc. If we do this, we still need to support same-filled entries in zswap_entry, which means we still cannot separate the logic completely. Also, it would be weird to have zero-filled pages and same-filled pages handled differently. How common do we think same-filled but not zero-filled pages are? Are there known user space usages or patterns that lead to this? The only thing I can think of is if user space initializes a large array to the same non-zero value, and then it gets swapped out before it is modified. If it is not common, perhaps it's not worth optimizing it.
