linux-kernel - Re: [RESEND PATCH] dm-ebs: Mark full buffer dirty even on partial write

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Message-ID: <230baa83-cd79-f232-5fb8-1476115e1ae7@redhat.com>
Date: Tue, 18 Nov 2025 18:21:56 +0100 (CET)
From: Mikulas Patocka <mpatocka@...hat.com>
To: Benjamin Marzinski <bmarzins@...hat.com>
cc: Uladzislau Rezki <urezki@...il.com>, Alasdair Kergon <agk@...hat.com>, 
    DMML <dm-devel@...ts.linux.dev>, Andrew Morton <akpm@...ux-foundation.org>, 
    Mike Snitzer <snitzer@...hat.com>, Christoph Hellwig <hch@....de>, 
    LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH] dm-ebs: Mark full buffer dirty even on partial
 write



On Tue, 18 Nov 2025, Benjamin Marzinski wrote:

> On Tue, Nov 18, 2025 at 01:00:36PM +0100, Mikulas Patocka wrote:
> > 
> > 
> > On Tue, 18 Nov 2025, Uladzislau Rezki wrote:
> > 
> > > Hello, Mikulas!
> > > 
> > > > Hi
> > > > 
> > > > What is the logical_block_size of the underlying nvme device? - i.e. 
> > > > what's the content of this file 
> > > > /sys/block/nvme0n1/queue/logical_block_size in the virtual machine?
> > > > 
> > > It is 512. Whereas a physical is bigger, i.e. my device can not perform
> > > I/O by 512 granularity.
> > 
> > And what is physical block size? Is it 8192?
> > 
> > > As for virtual machine, i just simulated the problem so people can set
> > > it up and check. The commit message describes how it can be reproduced.
> > > 
> > > The dm-ebs target which i setup does ebs to ubs conversion, so the NVME
> > > driver gets BIOs are in size and aligned to ubs size. The ubs size
> > > corresponds to the underlying physical device I/O size.
> > > 
> > > So your patch does not work if logical < physical. Therefore it does
> > > not help my project.
> > 
> > Logical block size is the granularity at which the device can accept I/O. 
> > Physical block size is the block size on the medium.
> > 
> > If logical < physical, then the device performs read-modify-write cycle 
> > when writing blocks that are not aligned at physical block size.
> > 
> > So, your setup is broken, because it advertises logical block size 512, 
> > but it is not able to perform I/O at this granularity.
> 
> This emulated nvme device is broken, but the question still remains,
> "should dm-ebs enfore writing at the alignment that was specified in its
> table line?" If you don't specify a ubs in the table line, it defaults
> to the logical block size. So, if you do specify a ubs, it stands to
> reason that you want IO at that alignment, instead of the logical-block
> size (perhaps because your device is broken, and advertises the wrong
> logical block size).  
> 
> So, I think that Uladzislau's patch makes sense, in addition to your
> own.
> 
> -Ben

OK - I accepted Uladzislau's patch. As logical block size and physical 
block size seem to be unreliable, it's better to set the size in dm-ebs.

Mikulas