Re: [PATCH 15/18] mm: memcontrol: make swap tracking an integral part of memory control

[Joonsoo Kim]

On Mon, Apr 20, 2020 at 06:11:23PM -0400, Johannes Weiner wrote:
> Without swap page tracking, users that are otherwise memory controlled
> can easily escape their containment and allocate significant amounts
> of memory that they're not being charged for. That's because swap does
> readahead, but without the cgroup records of who owned the page at
> swapout, readahead pages don't get charged until somebody actually
> faults them into their page table and we can identify an owner task.
> This can be maliciously exploited with MADV_WILLNEED, which triggers
> arbitrary readahead allocations without charging the pages.
> 
> Make swap swap page tracking an integral part of memcg and remove the
> Kconfig options. In the first place, it was only made configurable to
> allow users to save some memory. But the overhead of tracking cgroup
> ownership per swap page is minimal - 2 byte per page, or 512k per 1G
> of swap, or 0.04%. Saving that at the expense of broken containment
> semantics is not something we should present as a coequal option.
> 
> The swapaccount=0 boot option will continue to exist, and it will
> eliminate the page_counter overhead and hide the swap control files,
> but it won't disable swap slot ownership tracking.
> 
> This patch makes sure we always have the cgroup records at swapin
> time; the next patch will fix the actual bug by charging readahead
> swap pages at swapin time rather than at fault time.
> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Reviewed-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>