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Message-ID: <c209f283-5078-4969-a69f-bd6b4fd49274@suse.de>
Date: Mon, 3 Jun 2024 08:39:02 +0200
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Subject: Re: [PATCH v6 07/11] iomap: fix iomap_dio_zero() for fs bs > system
 page size
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To: "Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>, david@fromorbit.com,
 chandan.babu@oracle.com, akpm@linux-foundation.org, brauner@kernel.org,
 willy@infradead.org, djwong@kernel.org
Cc: linux-kernel@vger.kernel.org, john.g.garry@oracle.com,
 gost.dev@samsung.com, yang@os.amperecomputing.com, p.raghav@samsung.com,
 cl@os.amperecomputing.com, linux-xfs@vger.kernel.org, hch@lst.de,
 mcgrof@kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org
References: <20240529134509.120826-1-kernel@pankajraghav.com>
 <20240529134509.120826-8-kernel@pankajraghav.com>
From: Hannes Reinecke <hare@suse.de>
In-Reply-To: <20240529134509.120826-8-kernel@pankajraghav.com>
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On 5/29/24 15:45, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.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@samsung.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@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
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