Re: [RFC][PATCH 2/2] shmem: fix recovery on rename failures

[Christian Brauner]

On Sun, Dec 14, 2025 at 03:30:49AM +0000, Al Viro wrote:
> maple_tree insertions can fail if we are seriously short on memory;
> simple_offset_rename() does not recover well if it runs into that.
> The same goes for simple_offset_rename_exchange().
> 
> Moreover, shmem_whiteout() expects that if it succeeds, the caller will
> progress to d_move(), i.e. that shmem_rename2() won't fail past the
> successful call of shmem_whiteout().
> 
> Not hard to fix, fortunately - mtree_store() can't fail if the index we
> are trying to store into is already present in the tree as a singleton.
> 
> For simple_offset_rename_exchange() that's enough - we just need to be
> careful about the order of operations.
> 
> For simple_offset_rename() solution is to preinsert the target into the
> tree for new_dir; the rest can be done without any potentially failing
> operations.
> 
> That preinsertion has to be done in shmem_rename2() rather than in
> simple_offset_rename() itself - otherwise we'd need to deal with the
> possibility of failure after successful shmem_whiteout().
> 
> Fixes: a2e459555c5f ("shmem: stable directory offsets")
> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> ---

Looks good to me,
Reviewed-by: Christian Brauner <brauner@xxxxxxxxxx>