linux-kernel - Re: [PATCH 3/4] xfs: use IOCB_DONTCACHE when falling back to buffered writes

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Message-ID: <20251104155213.GA651@lst.de>
Date: Tue, 4 Nov 2025 16:52:13 +0100
From: Christoph Hellwig <hch@....de>
To: "Nirjhar Roy (IBM)" <nirjhar.roy.lists@...il.com>
Cc: Christoph Hellwig <hch@....de>, Carlos Maiolino <cem@...nel.org>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-raid@...r.kernel.org,
	linux-block@...r.kernel.org
Subject: Re: [PATCH 3/4] xfs: use IOCB_DONTCACHE when falling back to
 buffered writes

On Tue, Nov 04, 2025 at 06:03:35PM +0530, Nirjhar Roy (IBM) wrote:
> > Doing sub-block direct writes to COW inodes is not supported by XFS,
> > because new blocks need to be allocated as a whole.  Such writes
>
> Okay, since allocation of new blocks involves whole lot of metatdata
> updates/transactions etc and that would consume a lot of time and in
> this large window the user buffer(for direct I/O) can be re-used/freed
> which would cause corruptions?

I don't understand what you're trying to say here.

> Just thinking out loud: What if we supported sub-block direct IO in XFS
> and indeed allocated new blocks+ update the metadata structures and then
> directly write the user data to the newly allocated blocks instead of
> using the page cache?
>
> Assuming the application doesn't modify the user data buffer - can we
> (at least theoritically) do such kind of sub-block DIO?

Regular XFS does that.  Zoned XFS or the always COW debug mode can't do
that (except maybe for appends) as it it requires a read-modify-write
cycle that is not implemented in iomap.  Yes, we could implement that,
but it's not going to perform any better than the fallback, and would
also require full serialization.