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Date: Thu, 22 Jan 2026 23:05:37 -0800
From: "Darrick J. Wong" <djwong@kernel.org>
To: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>, Christian Brauner <brauner@kernel.org>,
	Carlos Maiolino <cem@kernel.org>, Qu Wenruo <wqu@suse.com>,
	Al Viro <viro@zeniv.linux.org.uk>, linux-block@vger.kernel.org,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 02/14] block: open code bio_add_page and fix handling of
 mismatching P2P ranges
Message-ID: <20260123070537.GR5945@frogsfrogsfrogs>
References: <20260119074425.4005867-1-hch@lst.de>
 <20260119074425.4005867-3-hch@lst.de>
 <20260122175908.GZ5945@frogsfrogsfrogs>
 <20260123054314.GA24902@lst.de>
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In-Reply-To: <20260123054314.GA24902@lst.de>

On Fri, Jan 23, 2026 at 06:43:14AM +0100, Christoph Hellwig wrote:
> On Thu, Jan 22, 2026 at 09:59:08AM -0800, Darrick J. Wong wrote:
> > On Mon, Jan 19, 2026 at 08:44:09AM +0100, Christoph Hellwig wrote:
> > > bio_add_page fails to add data to the bio when mixing P2P with non-P2P
> > > ranges, or ranges that map to different P2P providers.  In that case
> > > it will trigger that WARN_ON and return an error up the chain instead of
> > > simply starting a new bio as intended.  Fix this by open coding
> > 
> > AFAICT we've already done all the other checks in bio_add_page, so
> > calling __bio_add_page directly from within the loop is ok since you've
> > explicitly handled the !zone_device_pages_have_same_pgmap() case.
> > 
> > > bio_add_page and handling this case explicitly.  While doing so, stop
> > > merging physical contiguous data that belongs to multiple folios.  While
> > > this merge could lead to more efficient bio packing in some case,
> > > dropping will allow to remove handling of this corner case in other
> > > places and make the code more robust.
> > 
> > That does sound like a landmine waiting to go off...
> 
> What?  Removing the handling?

Urk, sorry.  I meant to say that the *old code* combining pages from
multiple folios sounded like a landmine waiting to go off.

--D