On Thu, Mar 4, 2021 at 7:48 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > On Wed, Mar 03, 2021 at 05:42:29PM -0800, Shakeel Butt wrote: > > Currently the kernel adds the page, allocated for swapin, to the > > swapcache before charging the page. This is fine but now we want a > > per-memcg swapcache stat which is essential for folks who wants to > > transparently migrate from cgroup v1's memsw to cgroup v2's memory and > > swap counters. In addition charging a page before exposing it to other > > parts of the kernel is a step in the right direction. > > > > To correctly maintain the per-memcg swapcache stat, this patch has > > adopted to charge the page before adding it to swapcache. One > > challenge in this option is the failure case of add_to_swap_cache() on > > which we need to undo the mem_cgroup_charge(). Specifically undoing > > mem_cgroup_uncharge_swap() is not simple. > > > > To resolve the issue, this patch introduces transaction like interface > > to charge a page for swapin. The function mem_cgroup_charge_swapin_page() > > initiates the charging of the page and mem_cgroup_finish_swapin_page() > > completes the charging process. So, the kernel starts the charging > > process of the page for swapin with mem_cgroup_charge_swapin_page(), > > adds the page to the swapcache and on success completes the charging > > process with mem_cgroup_finish_swapin_page(). > > > > Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx> > > The patch looks good to me, I have just a minor documentation nit > below. But with that addressed, please add: > > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> Thanks. > [...] > > It's possible somebody later needs to change things around in the > swapin path and it's not immediately obvious when exactly these two > functions need to be called in the swapin sequence. > > Maybe add here and above that charge_swapin_page needs to be called > before we try adding the page to the swapcache, and finish_swapin_page > needs to be called when swapcache insertion has been successful? I will update the comments and send v4 after a day or so to see if someone else has any comments.
