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Date: Fri, 16 Aug 2024 00:43:34 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>, Roman Gushchin
 <roman.gushchin@linux.dev>, Shakeel Butt <shakeel.butt@linux.dev>, Johannes
 Weiner <hannes@cmpxchg.org>, Muchun Song <muchun.song@linux.dev>,
 "T . J . Mercier" <tjmercier@google.com>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org, Meta kernel team <kernel-team@meta.com>,
 cgroups@vger.kernel.org
Subject: Re: [PATCH v2] memcg: replace memcg ID idr with xarray
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On Fri, 16 Aug 2024 09:19:58 +0200 Michal Hocko <mhocko@suse.com> wrote:

> On Thu 15-08-24 22:16:27, Matthew Wilcox wrote:
> > On Thu, Aug 15, 2024 at 07:31:43PM +0000, Roman Gushchin wrote:
> > > There is another subtle change here: xa_alloc() returns -EBUSY in the case
> > > of the address space exhaustion, while the old code returned -ENOSPC.
> > > It's unlikely a big practical problem.
> > 
> > I decided that EBUSY was the right errno for this situation;
> > 
> > #define EBUSY           16      /* Device or resource busy */
> > #define ENOSPC          28      /* No space left on device */
> > 
> > ENOSPC seemed wrong; the device isn't out of space.
> 
> The thing is that this is observable by userspace - mkdir would return a
> different and potentially unexpected errno. We can try and see whether
> anybody complains or just translate the error.

The mkdir(2) manpage doesn't list EBUSY.  Maybe ENOMEM is close enough.