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Date: Fri, 25 Sep 2026 07:53:30 +0200
From: Christoph Hellwig <hch@lst.de>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, Carlos Maiolino <cem@kernel.org>,
	Jens Axboe <axboe@kernel.dk>,
	Christian Brauner <brauner@kernel.org>, linux-xfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 02/21] iomap: add support for data checksumming
Message-ID: <20260925055330.GA3193@lst.de>
References: <20260924100032.2733101-1-hch@lst.de> <20260924100032.2733101-3-hch@lst.de> <20260924213902.GD2705364@frogsfrogsfrogs>
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On Thu, Sep 24, 2026 at 02:39:02PM -0700, Darrick J. Wong wrote:
> > +void *iomap_csum_alloc(struct iomap_ioend *ioend, u8 csum_shift)
> > +{
> > +	ioend->io_csum_shift = csum_shift;
> > +	ioend->io_csum = __iomap_csum_alloc(ioend);
> > +	return ioend->io_csum;
> > +}
> > +EXPORT_SYMBOL_GPL(iomap_csum_alloc);
> 
> AFAICT, io_csum_inline is a small amount of memory in the ioend itself
> to store checksums for the blocks being written, and io_csum_alloc is a
> dynamically allocated blob if the checksums don't fit inline?

Yes.  And both can only exist for the initial ioend and not a split one.

> And io_csum points to wherever that data lives?
> 
> Hrm.  What if you want to split an ioend, I guess the child's io_csum
> points to somewhere inside io_parent->io_csum?

Yes.

> (Perhaps it would help to point this out in the struct definition?)

I'll find a good place to better document this, thanks.