On Fri, 5 Mar 2021, Shakeel Butt wrote: > On Fri, Mar 5, 2021 at 1:26 PM Shakeel Butt <shakeelb@xxxxxxxxxx> 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(). > > And of course I forgot to update the commit message. > > Andrew, please replace the third paragraph with the following para: > > To resolve the issue, this patch decouples the charging for swapin pages from > mem_cgroup_charge(). Two new functions are introduced, > mem_cgroup_swapin_charge_page() for just charging the swapin page and > mem_cgroup_swapin_uncharge_swap() for uncharging the swap slot once the > page has been successfully added to the swapcache. Lgtm Hugh
