Re: [PATCH v2 1/7] mm: memcontrol: charge swap to cgroup2

[Johannes Weiner]

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 <vdavydov@xxxxxxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
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