linux-kernel - Re: [PATCH v7 5/8] xfs: Support FS_XFLAG

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Message-ID: <20241016005201.GH21836@frogsfrogsfrogs>
Date: Tue, 15 Oct 2024 17:52:01 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: John Garry <john.g.garry@...cle.com>
Cc: Christoph Hellwig <hch@....de>, axboe@...nel.dk, brauner@...nel.org,
	viro@...iv.linux.org.uk, jack@...e.cz, dchinner@...hat.com,
	cem@...nel.org, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, hare@...e.de,
	martin.petersen@...cle.com, catherine.hoang@...cle.com,
	mcgrof@...nel.org, ritesh.list@...il.com, ojaswin@...ux.ibm.com
Subject: Re: [PATCH v7 5/8] xfs: Support FS_XFLAG_ATOMICWRITES

On Sun, Oct 13, 2024 at 10:06:04PM +0100, John Garry wrote:
> On 07/10/2024 06:42, Christoph Hellwig wrote:
> > On Fri, Oct 04, 2024 at 02:07:05PM +0100, John Garry wrote:
> > > Sure, that is true (about being able to atomically write 1x FS block if the
> > > bdev support it).
> > > 
> > > But if we are going to add forcealign or similar later, then it would make
> > > sense (to me) to have FS_XFLAG_ATOMICWRITES (and its other flags) from the
> > > beginning. I mean, for example, if FS_XFLAG_FORCEALIGN were enabled and we
> > > want atomic writes, setting FS_XFLAG_ATOMICWRITES would be rejected if AG
> > > count is not aligned with extsize, or extsize is not a power-of-2, or
> > > extsize exceeds bdev limits. So FS_XFLAG_ATOMICWRITES could have some value
> > > there.
> > > 
> > > As such, it makes sense to have a consistent user experience and require
> > > FS_XFLAG_ATOMICWRITES from the beginning.
> > 
> > Well, even with forcealign we're not going to lose support for atomic
> > writes <= block size, are we?
> > 
> 
> forcealign would not be required for atomic writes <= FS block size.
> 
> How about this modified approach:
> 
> a. Drop FS_XFLAG_ATOMICWRITES support from this series, and so we can always
> atomic write 1x FS block (if the bdev supports it)
> 
> b. If we agree to support forcealign afterwards, then we can introduce 2x
> new flags:
> 	- FS_XFLAG_FORCEALIGN - as before
> 	- FS_XFLAG_BIG_ATOMICWRITES - this depends on  FS_XFLAG_FORCEALIGN being
> enabled per inode, and allows us to atomically write > 1 FS block
> 
> c. Later support writing < 1 FS block
> 	- this would not depend on forcealign
> 	- would require a real user, and I don't know one yet
> 
> better?

Sounds fine to /me/, but that's just my opinion. :)

--D