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

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Message-ID: <bcd929c2-88b0-36e9-0e22-4bb0293300b5@redhat.com>
Date: Wed, 19 Nov 2025 18:26:13 +0100 (CET)
From: Mikulas Patocka <mpatocka@...hat.com>
To: Christoph Hellwig <hch@....de>
cc: Benjamin Marzinski <bmarzins@...hat.com>, 
    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>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH] dm-ebs: Mark full buffer dirty even on partial
 write

On Wed, 19 Nov 2025, Christoph Hellwig wrote:

> On Tue, Nov 18, 2025 at 06:21:56PM +0100, Mikulas Patocka wrote:
> > 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.
> 
> logical and physical block size are reliable.  Uladzislau just seems
> to have a completely broken device that needs fixing, because it will

He created a qemu-emulated NVMe device with physical and logical block 
size 8192 in a virtual machine. And logical block size was reported as 512 
in the guest kernel - so it is either a qemu bug or a kernel bug.

> run into all kinds of other problems otherwise.

Hmm, Linux may get it wrong too. See this piece of code in 
include/linux/blkdev.h:

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
/*
 * We should strive for 1 << (PAGE_SHIFT + MAX_PAGECACHE_ORDER)
 * however we constrain this to what we can validate and test.
 */
#define BLK_MAX_BLOCK_SIZE      SZ_64K
#else
#define BLK_MAX_BLOCK_SIZE      PAGE_SIZE
#endif

/* blk_validate_limits() validates bsize, so drivers don't usually need to */
static inline int blk_validate_block_size(unsigned long bsize)
{
        if (bsize < 512 || bsize > BLK_MAX_BLOCK_SIZE || !is_power_of_2(bsize))
                return -EINVAL;

        return 0;
}

in nvme_update_disk_info there is this piece of code:
        if (blk_validate_block_size(bs)) {
                bs = (1 << 9);
                valid = false;
        }

So, the valid block size depends on whether CONFIG_TRANSPARENT_HUGEPAGE is 
defined, which is quite weird.

Mikulas