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

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Message-ID: <20251215-mythologisch-losung-f99a7be1c735@brauner>
Date: Mon, 15 Dec 2025 12:58:17 +0100
From: Christian Brauner <brauner@...nel.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Hugh Dickins <hughd@...gle.com>, Miklos Szeredi <miklos@...redi.hu>, 
	Andrew Morton <akpm@...ux-foundation.org>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Chuck Lever <chuck.lever@...cle.com>
Subject: Re: [RFC][PATCH 2/2] shmem: fix recovery on rename failures

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@...iv.linux.org.uk>
> ---

Looks good to me,
Reviewed-by: Christian Brauner <brauner@...nel.org>