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.
