Re: [PATCH v16 08/11] secretmem: add memcg accounting

[Shakeel Butt]

On Mon, Jan 25, 2021 at 1:35 PM Mike Rapoport <rppt@xxxxxxxxxx> wrote:
>
> On Mon, Jan 25, 2021 at 09:18:04AM -0800, Shakeel Butt wrote:
> > On Mon, Jan 25, 2021 at 8:20 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > >
> > > On Thu, Jan 21, 2021 at 02:27:20PM +0200, Mike Rapoport wrote:
> > > > From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > > >
> > > > Account memory consumed by secretmem to memcg. The accounting is updated
> > > > when the memory is actually allocated and freed.
>
> I though about doing per-page accounting, but then one would be able to
> create a lot of secretmem file descriptors, use only a page from each while
> actual memory consumption will be way higher.
>
> > > I think this is wrong.  It fails to account subsequent allocators from
> > > the same PMD.  If you want to track like this, you need separate pools
> > > per memcg.
> > >
> >
> > Are these secretmem pools shared between different jobs/memcgs?
>
> A secretmem pool is per anonymous file descriptor and this file descriptor
> can be shared only explicitly between several processes. So, the secretmem
> pool should not be shared between different jobs/memcg. Of course, it's
> possible to spread threads of a process across different memcgs, but in
> that case the accounting will be similar to what's happening today with
> sl*b.

I don't think memcg accounting for sl*b works like that.

> The first thread to cause kmalloc() will be charged for the
> allocation of the entire slab and subsequent allocations from that slab
> will not be accounted.

The latest kernel does object level memcg accounting. So, each
allocation from these threads will correctly charge their own memcgs.