From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 F3416846F for ; Tue, 25 Nov 2025 09:20:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764062405; cv=none; b=tYA+J7H1Hjum5OuuXuOLA/TY2KoI1OdXcElNFRI+H8sw7YQzhUmCWS5LDI3+UAg+ZmkjUmFBr/jIDlPD1W9PQC8IiP4Qz2cuEaynleAawRQluVFiyTH5D45pBXSYPBTaE+AQeEUvFiciS0AsMp7j/GGyi/tCjARWUKZlpB28P2M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764062405; c=relaxed/simple; bh=nVwlnoLibl3qsrLZN/1S21rj+mcRK/zv7JSEiwlrXuw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=VeqYlVN3UXjxvoBMneAZDj+qCsr/wJJRk0hPNup1hR3SWAx7m1q2R9xbQjVRX9wFaGf7UbX5CA99vkfxtpeujCoIHJhahrYRXH3MSXZWNR8zbyioJJ7guZZZF1uHNaAnGbUefFoyJdu+4R1xyujLLFipaeWnoY2C8Y6gZubombw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=d6N3pgVO; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="d6N3pgVO" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1764062401; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=e18+dlXOUuhyYDJ7hRKWocHc1eXAQtcNtVD1Eal52UI=; b=d6N3pgVOSaqaY+TZ7xpnJuaDggoKGGXuMt1sbpTAaLEjHV0+DhvqNZcIMsDs1o3xqhEQSL /pgCPFp0+u1smb/qi7aA9GWRbgSFah+SFZ/F+mv+0gnKGtfm+O8NSb+jrnoLhxYVt+sLCi PIZQGdNChj0qdE52cQFTC2PaP5BSKwM= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-642-UhNJNOjZMYWJOor55cxcZA-1; Tue, 25 Nov 2025 04:19:58 -0500 X-MC-Unique: UhNJNOjZMYWJOor55cxcZA-1 X-Mimecast-MFC-AGG-ID: UhNJNOjZMYWJOor55cxcZA_1764062397 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 245E51954B17; Tue, 25 Nov 2025 09:19:56 +0000 (UTC) Received: from fedora (unknown [10.72.116.210]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id B3F23180047F; Tue, 25 Nov 2025 09:19:50 +0000 (UTC) Date: Tue, 25 Nov 2025 17:19:44 +0800 From: Ming Lei To: Gao Xiang Cc: Christoph Hellwig , linux-block@vger.kernel.org, Mikulas Patocka , Zhaoyang Huang , Dave Chinner , linux-fsdevel@vger.kernel.org, Jens Axboe Subject: Re: calling into file systems directly from ->queue_rq, was Re: [PATCH V5 0/6] loop: improve loop aio perf by IOCB_NOWAIT Message-ID: References: <20251015110735.1361261-1-ming.lei@redhat.com> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 On Tue, Nov 25, 2025 at 03:26:39PM +0800, Gao Xiang wrote: > Hi Ming and Christoph, > > On 2025/11/25 11:00, Ming Lei wrote: > > On Mon, Nov 24, 2025 at 01:05:46AM -0800, Christoph Hellwig wrote: > > > On Mon, Nov 24, 2025 at 05:02:03PM +0800, Ming Lei wrote: > > > > On Sun, Nov 23, 2025 at 10:12:24PM -0800, Christoph Hellwig wrote: > > > > > FYI, with this series I'm seeing somewhat frequent stack overflows when > > > > > using loop on top of XFS on top of stacked block devices. > > > > > > > > Can you share your setting? > > > > > > > > BTW, there are one followup fix: > > > > > > > > https://lore.kernel.org/linux-block/20251120160722.3623884-1-ming.lei@redhat.com/ > > > > > > > > I just run 'xfstests -q quick' on loop on top of XFS on top of dm-stripe, > > > > not see stack overflow with the above fix against -next. > > > > > > This was with a development tree with lots of local code. So the > > > messages aren't applicable (and probably a hint I need to reduce my > > > stack usage). The observations is that we now stack through from block > > > submission context into the file system write path, which is bad for a > > > lot of reasons. journal_info being the most obvious one. > > > > > > > > In other words: I don't think issuing file system I/O from the > > > > > submission thread in loop can work, and we should drop this again. > > > > > > > > I don't object to drop it one more time. > > > > > > > > However, can we confirm if it is really a stack overflow because of > > > > calling into FS from ->queue_rq()? > > > > > > Yes. > > > > > > > If yes, it could be dead end to improve loop in this way, then I can give up. > > > > > > I think calling directly into the lower file system without a context > > > switch is very problematic, so IMHO yes, it is a dead end. > I've already explained the details in > https://lore.kernel.org/r/8c596737-95c1-4274-9834-1fe06558b431@linux.alibaba.com > > to zram folks why block devices act like this is very > risky (in brief, because virtual block devices don't > have any way (unlike the inner fs itself) to know enough > about whether the inner fs already did something without > context save (a.k.a side effect) so a new task context > is absolutely necessary for virtual block devices to > access backing fses for stacked usage. > > So whether a nested fs can success is intrinsic to > specific fses (because either they assure no complex > journal_info access or save all effected contexts before > transiting to the block layer. But that is not bdev can > do since they need to do any block fs. IMO, task stack overflow could be the biggest trouble. block layer has current->blk_plug/current->bio_list, which are dealt with in the following patches: https://lore.kernel.org/linux-block/20251120160722.3623884-4-ming.lei@redhat.com/ https://lore.kernel.org/linux-block/20251120160722.3623884-5-ming.lei@redhat.com/ I am curious why FS task context can't be saved/restored inside block layer when calling into new FS IO? Given it is just per-task info. Thanks, Ming