Re: [PATCH 14/19] mm: Introduce a cgroup for pinned memory

[Alistair Popple <apopple@nvidia.com>]

Jason Gunthorpe <jgg@nvidia.com> writes:

> On Wed, Feb 22, 2023 at 10:38:25PM +1100, Alistair Popple wrote:
>> When a driver unpins a page we scan the pinners list and assign
>> ownership to the next driver pinning the page by updating memcg_data and
>> removing the vm_account from the list.
>
> I don't see how this works with just the data structure you outlined??
> Every unique page needs its own list_head in the vm_account, it is
> doable just incredibly costly.

The idea was every driver already needs to allocate a pages array to
pass to pin_user_pages(), and by necessity drivers have to keep a
reference to the contents of that in one form or another. So
conceptually the equivalent of:

struct vm_account {
       struct list_head possible_pinners;
       struct mem_cgroup *memcg;
       struct pages **pages;
       [...]
};

Unpinnig involves finding a new owner by traversing the list of
page->memcg_data->possible_pinners and iterating over *pages[] to figure
out if that vm_account actually has this page pinned or not and could
own it.

Agree this is costly though. And I don't think all drivers keep the
array around so "iterating over *pages[]" may need to be a callback.

> Jason