Re: Memdesc wiki pages

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, May 15, 2024 at 5:21 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> For those who couldn't read the text on the wiki page earlier:
>
> https://kernelnewbies.org/MatthewWilcox/Memdescs
> https://kernelnewbies.org/MatthewWilcox/FolioAlloc
> https://kernelnewbies.org/MatthewWilcox/BuddyAllocator
> https://kernelnewbies.org/MatthewWilcox/Memdescs/Path
>
> That fourth page is freshly created as a result of the discussions in
> the room.  I'll continue to refine these pages as my understanding of
> various issues improves.
>

I took a look again at folio->memcg_data.

We have three types of pointers that we can put in there:
(a) struct mem_cgroup pointer (no flags set, for anon & file folios).
(b) struct obj_cgroup pointer (MEMCG_DATA_KMEM set, for non-slab
accounted allocations).
(c) a vector of struct obj_cgroup pointers (MEMCG_DATA_OBJCGS set, for
slab allocations).

I believe we should be able to get rid of MEMCG_DATA_OBJCGS now and
use a struct obj_cgroup ** directly in struct slab. I see a few places
where we change struct slab to struct folio or vice versa, but I don't
think they miss with the memcg stuff.

The only exception I can find is print_page_owner_memcg(), where we
use this flag when printing the memcg of a page to print that it is a
slab page (and hence no memcg will be printed). I believe a PageSlab()
check may be racy, but I am not sure, and I don't know if we really
care.

Now for (a) and (b). I believe once struct folio only represents anon
and file folios (or if they have separate types), we should be able to
use a struct mem_cgroup pointer. This will take care of (a).

Things like PerCPU and PageTable fit into category (b) and will have a
struct obj_cgroup pointer. The problem is other category (b)
allocations that fit into Misc and do not have associated metadata.
These allocations should have an associated obj_cgroup pointer if they
are accounted, but as far as I can tell we have no place to store it
according to the current plan.