Re: [RFC] Making memcg track ownership per address_space or anon_vma

Tejun Heo
2015-02-06T14:17:46Z

Hello, Greg.

On Thu, Feb 05, 2015 at 04:03:34PM -0800, Greg Thelen wrote:
> So this is a system which charges all cgroups using a shared inode
> (recharge on read) for all resident pages of that shared inode. There's
> only one copy of the page in memory on just one LRU, but the page may be
> charged to multiple container's (shared_)usage.

Yeap.

> Perhaps I missed it, but what happens when a child's limit is
> insufficient to accept all pages shared by its siblings? Example
> starting with 2M cached of a shared file:
>
> A
> +-B (usage=2M lim=3M hosted_usage=2M)
> +-C (usage=0 lim=2M shared_usage=2M)
> +-D (usage=0 lim=2M shared_usage=2M)
> \-E (usage=0 lim=1M shared_usage=0)
>
> If E faults in a new 4K page within the shared file, then E is a sharing
> participant so it'd be charged the 2M+4K, which pushes E over it's
> limit.

OOM? It shouldn't be participating in sharing of an inode if it can't
match others' protection on the inode, I think. What we're doing now
w/ page based charging is kinda unfair because in the situations like
above the one under pressure can end up siphoning off of the larger
cgroups' protection if they actually use overlapping areas; however,
for disjoint areas, per-page charging would behave correctly.

So, this part comes down to the same question - whether multiple
cgroups accessing disjoint areas of a single inode is an important
enough use case. If we say yes to that, we better make writeback
support that too.

Thanks.