On Thu, Dec 17, 2015 at 03:29:54PM +0300, Vladimir Davydov wrote: > In the legacy hierarchy we charge memsw, which is dubious, because: > > - memsw.limit must be >= memory.limit, so it is impossible to limit > swap usage less than memory usage. Taking into account the fact that > the primary limiting mechanism in the unified hierarchy is > memory.high while memory.limit is either left unset or set to a very > large value, moving memsw.limit knob to the unified hierarchy would > effectively make it impossible to limit swap usage according to the > user preference. > > - memsw.usage != memory.usage + swap.usage, because a page occupying > both swap entry and a swap cache page is charged only once to memsw > counter. As a result, it is possible to effectively eat up to > memory.limit of memory pages *and* memsw.limit of swap entries, which > looks unexpected. > > That said, we should provide a different swap limiting mechanism for > cgroup2. > > This patch adds mem_cgroup->swap counter, which charges the actual > number of swap entries used by a cgroup. It is only charged in the > unified hierarchy, while the legacy hierarchy memsw logic is left > intact. > > The swap usage can be monitored using new memory.swap.current file and > limited using memory.swap.max. > > Note, to charge swap resource properly in the unified hierarchy, we have > to make swap_entry_free uncharge swap only when ->usage reaches zero, > not just ->count, i.e. when all references to a swap entry, including > the one taken by swap cache, are gone. This is necessary, because > otherwise swap-in could result in uncharging swap even if the page is > still in swap cache and hence still occupies a swap entry. At the same > time, this shouldn't break memsw counter logic, where a page is never > charged twice for using both memory and swap, because in case of legacy > hierarchy we uncharge swap on commit (see mem_cgroup_commit_charge).
This was actually an oversight when rewriting swap accounting. It should have always been uncharged when the swap slot is released. > Signed-off-by: Vladimir Davydov <[email protected]> Acked-by: Johannes Weiner <[email protected]> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
