linux-kernel - Re: [PATCH RFC] statx.2: Add stx_atomic_write_unit_max

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Message-ID: <20250320141200.GC10939@lst.de>
Date: Thu, 20 Mar 2025 15:12:00 +0100
From: Christoph Hellwig <hch@....de>
To: John Garry <john.g.garry@...cle.com>
Cc: Christoph Hellwig <hch@....de>, alx@...nel.org, brauner@...nel.org,
	djwong@...nel.org, dchinner@...hat.com, linux-man@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, ojaswin@...ux.ibm.com,
	ritesh.list@...il.com, martin.petersen@...cle.com
Subject: Re: [PATCH RFC] statx.2: Add stx_atomic_write_unit_max_opt

On Thu, Mar 20, 2025 at 09:19:40AM +0000, John Garry wrote:
> But is there value in reporting this limit? I am not sure. I am not sure 
> what the user would do with this info.

Align their data structures to it, e.g. size the log buffers to it.

> Maybe, for example, they want to write 1K consecutive 16K pages, each 
> atomically, and decide to do a big 16M atomic write but find that it is 
> slow as bdev atomic limit is < 16M.
>
> Maybe I should just update the documentation to mention that for XFS they 
> should check the mounted bdev atomic limits.

For something working on files having to figure out the underlying
block device (which is non-trivial given the various methods of
multi-device support) and then looking into block sysfs is a no-go.

So if we have any sort of use case for it we should expose the limit.