linux-ext4 - Inquiry about ext4 atomic write

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Message-ID: <CAPbN7U669+wCDWDuxuY9xAg_PxhXR7_vvVXRmmD-DeyM_PGSMA@mail.gmail.com>
Date: Wed, 5 Feb 2025 20:43:31 +0800
From: 磊李 <lilei.777@...edance.com>
To: Ritesh Harjani <ritesh.list@...il.com>
Cc: "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Inquiry about ext4 atomic write

Hi  Harjani,

I tend to enable ext4 16kb atomic write on my X86 machine, and read your RFC.
https://lwn.net/ml/linux-kernel/cover.1709356594.git.ritesh.list@gmail.com/

>From the cover patch, it seems that this feature will be successfully enabled
if I enable bigalloc on ext4, and the underlying device also supports
16kb write union.
However, after checking statx results, I found that
atomic_write_unit_max was always 4096.

This snippet below limits s_awu_max to 4096(bs) which is the max value
on my platform.

```
static void ext4_atomic_write_init(struct super_block *sb)
{
      ...
      sbi->s_awu_min = max(sb->s_blocksize,
                        bdev_atomic_write_unit_min_bytes(bdev));
      sbi->s_awu_max = min(sb->s_blocksize,
                        bdev_atomic_write_unit_max_bytes(bdev));
      ...
}
```

I am wondering if I missed something? Or if there are any other ways
that I could enable 16kb
atomic write on my platform?

Thanks!
Li Lei