Hi,
On 25/01/2023 11:52, Michal Hocko wrote:
On Tue 24-01-23 19:46:28, Shakeel Butt wrote:
On Tue, Jan 24, 2023 at 03:59:58PM +0100, Michal Hocko wrote:
On Mon 23-01-23 19:17:23, T.J. Mercier wrote:
When a buffer is exported to userspace, use memcg to attribute the
buffer to the allocating cgroup until all buffer references are
released.
Is there any reason why this memory cannot be charged during the
allocation (__GFP_ACCOUNT used)?
Also you do charge and account the memory but underlying pages do not
know about their memcg (this is normally done with commit_charge for
user mapped pages). This would become a problem if the memory is
migrated for example.
I don't think this is movable memory.
This also means that you have to maintain memcg
reference outside of the memcg proper which is not really nice either.
This mimicks tcp kmem limit implementation which I really have to say I
am not a great fan of and this pattern shouldn't be coppied.
I think we should keep the discussion on technical merits instead of
personal perference. To me using skmem like interface is totally fine
but the pros/cons need to be very explicit and the clear reasons to
select that option should be included.
I do agree with that. I didn't want sound to be personal wrt tcp kmem
accounting but the overall code maintenance cost is higher because
of how tcp take on accounting differs from anything else in the memcg
proper. I would prefer to not grow another example like that.
To me there are two options:
1. Using skmem like interface as this patch series:
The main pros of this option is that it is very simple. Let me list down
the cons of this approach:
a. There is time window between the actual memory allocation/free and
the charge and uncharge and [un]charge happen when the whole memory is
allocated or freed. I think for the charge path that might not be a big
issue but on the uncharge, this can cause issues. The application and
the potential shrinkers have freed some of this dmabuf memory but until
the whole dmabuf is freed, the memcg uncharge will not happen. This can
consequences on reclaim and oom behavior of the application.
b. Due to the usage model i.e. a central daemon allocating the dmabuf
memory upfront, there is a requirement to have a memcg charge transfer
functionality to transfer the charge from the central daemon to the
client applications. This does introduce complexity and avenues of weird
reclaim and oom behavior.
2. Allocate and charge the memory on page fault by actual user
In this approach, the memory is not allocated upfront by the central
daemon but rather on the page fault by the client application and the
memcg charge happen at the same time.
The only cons I can think of is this approach is more involved and may
need some clever tricks to track the page on the free patch i.e. we to
decrement the dmabuf memcg stat on free path. Maybe a page flag.
The pros of this approach is there is no need have a charge transfer
functionality and the charge/uncharge being closely tied to the actual
memory allocation and free.
Personally I would prefer the second approach but I don't want to just
block this work if the dmabuf folks are ok with the cons mentioned of
the first approach.
I am not familiar with dmabuf internals to judge complexity on their end
but I fully agree that charge-when-used is much more easier to reason
about and it should have less subtle surprises.
Disclaimer that I don't seem to see patches 3&4 on dri-devel so maybe I
am missing something, but in principle yes, I agree that the 2nd option
(charge the user, not exporter) should be preferred. Thing being that at
export time there may not be any backing store allocated, plus if the
series is restricting the charge transfer to just Android clients then
it seems it has the potential to miss many other use cases. At least
needs to outline a description on how the feature will be useful outside
Android.
Also stepping back for a moment - is a new memory category really
needed, versus perhaps attempting to charge the actual backing store
memory to the correct client? (There might have been many past
discussions on this so it's okay to point me towards something in the
archives.)
Regards,
Tvrtko
