linux-kernel - Re: [PATCH v6 07/11] iomap: fix iomap_dio

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Date: Mon, 3 Jun 2024 08:39:02 +0200
From: Hannes Reinecke <hare@...e.de>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>, david@...morbit.com,
 chandan.babu@...cle.com, akpm@...ux-foundation.org, brauner@...nel.org,
 willy@...radead.org, djwong@...nel.org
Cc: linux-kernel@...r.kernel.org, john.g.garry@...cle.com,
 gost.dev@...sung.com, yang@...amperecomputing.com, p.raghav@...sung.com,
 cl@...amperecomputing.com, linux-xfs@...r.kernel.org, hch@....de,
 mcgrof@...nel.org, linux-mm@...ck.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v6 07/11] iomap: fix iomap_dio_zero() for fs bs > system
 page size

On 5/29/24 15:45, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@...sung.com>
> 
> iomap_dio_zero() will pad a fs block with zeroes if the direct IO size
> < fs block size. iomap_dio_zero() has an implicit assumption that fs block
> size < page_size. This is true for most filesystems at the moment.
> 
> If the block size > page size, this will send the contents of the page
> next to zero page(as len > PAGE_SIZE) to the underlying block device,
> causing FS corruption.
> 
> iomap is a generic infrastructure and it should not make any assumptions
> about the fs block size and the page size of the system.
> 
> Signed-off-by: Pankaj Raghav <p.raghav@...sung.com>
> ---
> 
> After disucssing a bit in LSFMM about this, it was clear that using a
> PMD sized zero folio might not be a good idea[0], especially in platforms
> with 64k base page size, the huge zero folio can be as high as
> 512M just for zeroing small block sizes in the direct IO path.
> 
> The idea to use iomap_init to allocate 64k zero buffer was suggested by
> Dave Chinner as it gives decent tradeoff between memory usage and efficiency.
> 
> This is a good enough solution for now as moving beyond 64k block size
> in XFS might take a while. We can work on a more generic solution in the
> future to offer different sized zero folio that can go beyond 64k.
> 
> [0] https://lore.kernel.org/linux-fsdevel/ZkdcAsENj2mBHh91@casper.infradead.org/
> 
Reviewed-by: Hannes Reinecke <hare@...e.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich