Re: [PATCH v3 4/4] memcg: remove warning from folio_lruvec

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 19 Aug 2025 03:41:30 +0100 Matthew Wilcox <willy@infradead.org> wrote:

> On Mon, Aug 18, 2025 at 05:36:56PM -0700, Boris Burkov wrote:
> > Commit a4055888629bc ("mm/memcg: warning on !memcg after readahead page
> > charged") added the warning in folio_lruvec (older name was
> > mem_cgroup_page_lruvec) for !memcg when charging of readahead pages were
> > added to the kernel. Basically lru pages on a memcg enabled system were
> > always expected to be charged to a memcg.
> > 
> > However a recent functionality to allow metadata of btrfs, which is in
> > page cache, to be uncharged is added to the kernel. We can either change
> > the condition to only check anon pages or file pages which does not have
> > AS_UNCHARGED in their mapping. Instead of such complicated check, let's
> > just remove the warning as it is not really helpful anymore.
> 
> This has to go before patch 3 (and I'd put it before patch 1) in order
> to preserve bisectability..

Thanks, I'll move it to [2/4]

?  That requires changing the tenses in the
> commit message, but that's perfectly acceptable.

I'm not spottig this.  a4055888629bc was added in 2020 and the btrfs
change is in a preceding series.  I'll describe that by name, so

: Commit a4055888629bc ("mm/memcg: warning on !memcg after readahead page
: charged") added the warning in folio_lruvec (older name was
: mem_cgroup_page_lruvec) for !memcg when charging of readahead pages were
: added to the kernel.  Basically lru pages on a memcg enabled system were
: always expected to be charged to a memcg.
: 
: However a recent functionality to allow metadata of btrfs, which is in
: page cache, to be uncharged was added to the kernel
: ("btrfs-set-as_uncharged-on-the-btree_inode.patch").  We can either change
: the condition to only check anon pages or file pages which does not have
: AS_UNCHARGED in their mapping.  Instead of such complicated check, let's
: just remove the warning as it is not really helpful anymore.