Re: [PATCH v7 2/3] btrfs: always uses root memcgroup for filemap_add_folio()

[Qu Wenruo <wqu@suse.com>]

在 2024/7/22 17:04, Michal Hocko 写道:
> On Sat 20-07-24 07:41:19, Qu Wenruo wrote:
> [...]
>> So according to the trend, I'm pretty sure VFS people will reject such
>> new interface just to skip accounting.
> 
> I would just give it a try with your usecase described. If this is a
> nogo then the root cgroup workaround is still available.

I have submitted a patchset doing exactly that, and thankfully that's 
the series where I got all the helpful feedbacks:

https://lore.kernel.org/linux-btrfs/92dea37a395781ee4d5cf8b16307801ccd8a5700.1720572937.git.wqu@suse.com/

Unfortunately I haven't get any feedback from the VFS guys.

> 
>> Thus the GFP_NO_ACCOUNT solution looks more feasible.
> 
> So we have GFP_ACCOUNT to opt in for accounting and now we should be
> adding GFP_NO_ACCOUNT to override it? This doesn't sound like a good use
> of gfp flags (which we do not have infinitely) and it is also quite
> confusing TBH.

The problem is, for filemap_add_folio(), we didn't specify GFP_ACCOUNT 
(nor any other caller) but it is still doing the charge, due to the 
mostly-correct assumption that all filemap page caches are accessible to 
user space programs.

So one can argue that, cgroup is still charged even if no GFP_ACCOUNT is 
specified.

But I get your point, indeed it's not that a good idea to introduce 
GFP_NO_ACCOUNT.

Thanks,
Qu