From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F3DB5313552; Sat, 18 Jul 2026 13:27:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784381268; cv=none; b=VJWWkIhr64h9ZF/gs0ZjGr9/8/XM+cv1sDpTbFkVUMjcHkmA2DGwaCONQyhwYLwsjnxYxsXMswjXjqpAlkKyq+WY8uREa3X9ritV+r8EgaBzaP9XuIOZIS4dHx2m0udEh1nN2K8niph+ELQRculKf0AVQCbllZJ0SaUXc9pHg4g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784381268; c=relaxed/simple; bh=wJQn9dt/OTGA/Bw8LXVw+ivM5XP2+IHKZ+skbqGQeQA=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=i63GVfY0oTnGxbEPHa44T4mpYK6G5SiPWINEG1+9Z9tpBar5aZ5fyy0pWykyx33+ta3O2c921UDwhSGwegLL5UfJ4S/rDK79ibOQXKoTbmZPvwS/q72TmpDy30L6KF93KXF/JpPU+T/gOQZjwiR1ZbHCwJJu8i1HnropDHJTDyI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=T9TQTS4a; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="T9TQTS4a" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0EA551F000E9; Sat, 18 Jul 2026 13:27:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784381266; bh=qrvvzcx4gjJ7GXHnj2buieV/FlsGxdd0zXqNqfrlm3Q=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=T9TQTS4aCCjs0iNjgCzLmsWQ4BBRWOF54QPV9qcNv1WvCTeaa0SkMDpsEQmfJwbW1 88V4ciul4cVJD/PYFfy8TIqO1G33O6BP7o6rXG45EAYGw114LYj3AEmhqHE/pXZ+1n oq/BKG9TECPNCtQU3riHfrRnlyEaUAUi4RtypVoY3ghWTKM7I8NfFZt3tLw/nlgPN3 wK73ySaJ6t0fDLld7GfrBIjww88jW8MWDvdjSTyyy1RPPZY/FTq/C9R8OYNKi25r7z S2OCzF98JZWWLmWGI08p/3gs8lsaCmtTwECj6jcA3TB9+zraUAGnvCIPlsrm+imj7q kZfTZW9sWGNqA== Message-ID: <022ba61d615e08d031fa63d396801ab4bfa44bc2.camel@kernel.org> Subject: Re: [PATCH 3/4] btrfs: handle ENOMEM from btrfs_insert_dir_item() without aborting From: Jeff Layton To: Boris Burkov Cc: Chris Mason , David Sterba , linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Date: Sat, 18 Jul 2026 09:27:44 -0400 In-Reply-To: <20260718001309.GA420230@zen.localdomain> References: <20260717-btrfs-enomem-v1-0-cdc9c0e265d0@kernel.org> <20260717-btrfs-enomem-v1-3-cdc9c0e265d0@kernel.org> <20260717201805.GB142812@zen.localdomain> <20260717230455.GA251596@zen.localdomain> <223c999d96a6a69f5824f145cabe3bec891c5956.camel@kernel.org> <20260718001309.GA420230@zen.localdomain> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw 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Evolution 3.60.2 (3.60.2-1.fc44) Precedence: bulk X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2026-07-17 at 17:13 -0700, Boris Burkov wrote: > On Fri, Jul 17, 2026 at 07:55:35PM -0400, Jeff Layton wrote: > > On Fri, 2026-07-17 at 16:04 -0700, Boris Burkov wrote: > > > On Fri, Jul 17, 2026 at 06:40:42PM -0400, Jeff Layton wrote: > > > > On Fri, 2026-07-17 at 13:18 -0700, Boris Burkov wrote: > > > > > On Fri, Jul 17, 2026 at 12:52:38PM -0400, Jeff Layton wrote: > > > > > > Now that btrfs_insert_dir_item() returns -ENOMEM before modifyi= ng the > > > > > > btree (thanks to delayed dir index pre-allocation), callers can= handle > > > > > > ENOMEM gracefully instead of aborting the transaction. > > > > > >=20 > > > > > > In btrfs_add_link(), add -ENOMEM to the set of recoverable erro= rs > > > > > > alongside -EEXIST and -EOVERFLOW. The fail_dir_item cleanup pat= h > > > > > > unwinds the inode_ref/root_ref and returns the error to userspa= ce. > > > > > >=20 > > > > > > In btrfs_create_new_inode(), when btrfs_add_link() fails with -= ENOMEM, > > > > > > convert the newly-created inode into an orphan instead of abort= ing. > > > > > > This is done by clearing nlink and adding an orphan item, which= ensures > > > > > btrfs_evict_inode() will delete the INODE_ITEM and INODE= _REF, and > > > > > > crash-recovery will clean it up via orphan processing. If > > > > > > btrfs_orphan_add() itself fails, we fall back to aborting. > > > > > >=20 > > > > > > This turns a filesystem-killing transaction abort into a gracef= ul > > > > > > -ENOMEM return to userspace for create(), mkdir(), mknod(), sym= link(), > > > > > > and link() operations under memory pressure. > > > > > >=20 > > > > > > Assisted-by: LLM > > > > > > Signed-off-by: Jeff Layton > > > > > > --- > > > > > > fs/btrfs/inode.c | 17 +++++++++++++++-- > > > > > > 1 file changed, 15 insertions(+), 2 deletions(-) > > > > > >=20 > > > > > > diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c > > > > > > index b7b4e6177135..4d9947ae08f7 100644 > > > > > > --- a/fs/btrfs/inode.c > > > > > > +++ b/fs/btrfs/inode.c > > > > > > @@ -6676,7 +6676,20 @@ int btrfs_create_new_inode(struct btrfs_= trans_handle *trans, > > > > > > } else { > > > > > > ret =3D btrfs_add_link(trans, BTRFS_I(dir), BTRFS_I(inode), = name, > > > > > > false, BTRFS_I(inode)->dir_index); > > > > > > - if (unlikely(ret)) { > > > > > > + if (ret =3D=3D -ENOMEM) { >=20 > So we are now handling ENOMEM from btrfs_add_link() >=20 > > > > > > + /* > > > > > > + * The ENOMEM came before the DIR_ITEM was inserted, > > > > > > + * so the btree has our INODE_ITEM + INODE_REF but no > > > > > > + * directory entry. Convert this into an orphan so > > > > > > + * eviction (or crash-recovery) cleans up the inode. > > > > > > + */ > > > > > > + clear_nlink(inode); > > > > > > + ret =3D btrfs_orphan_add(trans, BTRFS_I(inode)); >=20 > And when we get ENOMEM, we call btrfs_orphan_add() isntead of > unilaterally aborting. I contend that this call is now likely to ENOMEM >=20 > > > > > > + if (unlikely(ret)) > > > > > > + btrfs_abort_transaction(trans, ret); >=20 > And abort here. >=20 > > > > >=20 > > > > > I feel like the crux of this series to me is whether you have pra= ctical > > > > > conditions where the allocation of the delayed_node is failing, b= ut the > > > > > allocations involved in btrfs_orphan_add() succeed. It allocates = a > > > > > btrfs_path and has to walk the btree which might have to read the= node > > > > > at every level which might need to allocate 16k extent buffers an= d > > > > > extent buffer objects and xarray storage for each one. For size > > > > > reference, on my build (maybe debug..?) a delayed_node is 552 byt= es, > > > > > while a btrfs_path is 112 and an extent_buffer is 432. So they ar= e > > > > > pretty similar in size (not to mention the 16k of node file backe= d > > > > > memory we are sort of likely to have to allocate if we are under > > > > > reclaim) > > > > >=20 > > > > > Were you able to reproduce this issue and help in practice or is = this a > > > > > theoretical / structural improvement? > > > > >=20 > > > >=20 > > > > I didn't really try to reproduce this in earnest. We only see it in= our > > > > fleet under heavy memory pressure, and even then at such low freque= ncy, > > > > I doubt our chances of hitting this on anything other than a huge s= et > > > > of machines. > > > >=20 > > > > So, theoretical / structural, but we have record of filesystem abor= ts > > > > where the stack indicates that this would have prevented it. Userla= nd > > > > would have gotten an -ENOMEM back but the fs wouldn't have aborted. > > > >=20 > > >=20 > > > My concern is not that we don't hit ENOMEM in btrfs_add_link(), since > > > like you said we can observe that in abort logs. I am worried that ev= en > > > if we try to handle it gracefully, we will just ENOMEM in > > > btrfs_orphan_add() and abort anyway. That is why I was wanting to see > > > some more concrete evidence this actually helps to make it worth the > > > complexity. > > >=20 > >=20 > > In the case where we handle this gracefully, we won't hit that because > > it will have returned -ENOMEM before that point. But, you do have a > > good point that we could allocate these objects successfully, and then > > hit an error in btrfs_orphan_add() anyway. >=20 > I am still a bit confused. I made comments inline closer to the the code. > Sorry if I am being dense or missing the point! >=20 Nope, you're correct. I missed which bit you were talking about. > >=20 > > One thought: It looks like the main allocation in that codepath is > > btrfs_alloc_path()? We could consider preallocating that too -- maybe > > stash it in a new pointer in btrfs_trans_handle? > >=20 > > Thanks! This is good food for thought. >=20 > Take a peek at https://lore.kernel.org/linux-btrfs/cover.1782249000.git.b= oris@bur.io/ >=20 > which I need to RESEND :) >=20 >=20 Interesting. Between that approach and Qu's suggestion we should be able fix this up. I'll look at doing that for v2. Thanks again for the review! --=20 Jeff Layton