Re: [PATCH v2 1/3] mm/filemap: add AS_UNCHARGED

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Aug 15, 2025 at 04:40:31PM -0700, Boris Burkov wrote:
> Btrfs currently tracks its metadata pages in the page cache, using a
> fake inode (fs_info->btree_inode) with offsets corresponding to where
> the metadata is stored in the filesystem's full logical address space.
> 
> A consequence of this is that when btrfs uses filemap_add_folio(), this
> usage is charged to the cgroup of whichever task happens to be running
> at the time. These folios don't belong to any particular user cgroup, so
> I don't think it makes much sense for them to be charged in that way.
> Some negative consequences as a result:
> - A task can be holding some important btrfs locks, then need to lookup
>   some metadata and go into reclaim, extending the duration it holds
>   that lock for, and unfairly pushing its own reclaim pain onto other
>   cgroups.
> - If that cgroup goes into reclaim, it might reclaim these folios a
>   different non-reclaiming cgroup might need soon. This is naturally
>   offset by LRU reclaim, but still.
> 
> A very similar proposal to use the root cgroup was previously made by
> Qu, where he eventually proposed the idea of setting it per
> address_space. This makes good sense for the btrfs use case, as the
> uncharged behavior should apply to all use of the address_space, not
> select allocations. I.e., if someone adds another filemap_add_folio()
> call using btrfs's btree_inode, we would almost certainly want the
> uncharged behavior.
> 
> Link: https://lore.kernel.org/linux-mm/b5fef5372ae454a7b6da4f2f75c427aeab6a07d6.1727498749.git.wqu@suse.com/
> Suggested-by: Qu Wenruo <wqu@suse.com>
> Signed-off-by: Boris Burkov <boris@bur.io>

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>