Re: [PATCH v4 1/3] mm/filemap: add AS_KERNEL_FILE

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

On Thu, Aug 21, 2025 at 02:55:35PM -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.
> 
> We have two options for how to manage such file pages:
> 1. charge them to the root cgroup.
> 2. don't charge them to any cgroup at all.
> 
> 2. breaks the invariant that every mapped page has a cgroup. This is
> workable, but unnecessarily risky. Therefore, go with 1.
> 
> 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
> 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 to account
> that to the root cgroup as well.
> 
> Link: https://lore.kernel.org/linux-mm/b5fef5372ae454a7b6da4f2f75c427aeab6a07d6.1727498749.git.wqu@suse.com/
> Suggested-by: Qu Wenruo <wqu@suse.com>
> Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
> Tested-by: syzbot@syzkaller.appspotmail.com
> Signed-off-by: Boris Burkov <boris@bur.io>

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