Re: [PATCH] generic/747: handle ENOSPC gracefully during write/delete cycles

[Christoph Hellwig <hch@infradead.org>]

On Tue, Mar 24, 2026 at 04:39:43PM -0700, Leo Martins wrote:
> > The test fills a filesystem to 95% then does mixed write/delete cycles,
> > using statfs to decide whether to write or delete. However, statfs
> > f_bavail may overestimate the actual available space. On btrfs, the
> > statfs implementation documents its estimate as "a close approximation"
> > (fs/btrfs/super.c). At high fill levels the discrepancy between what
> > statfs reports and what the filesystem can actually allocate becomes
> > significant, causing dd to hit ENOSPC even though statfs indicated
> > there was room.
> > This is not a filesystem bug. The filesystem correctly rejects the
> > write when it cannot reserve space. The test's purpose is to stress
> > garbage collection through write/delete churn, not to validate space
> > accounting.

f_bavail overestimating is a btrfs implementation bug that needs fixing.
It can be wrong, but it needs to be too low in doubt.  Without this
you're going to cause unexpected ENOSPC for real life applictions as
well (and btrfs does have a bit of a reputation for that, so fixing
this properly will benefit your users!)