[PATCH v1] erofs-utils: mkfs: Implement 'dsunit' alignment on blobdev

[Gao Xiang]

Hi Friendly,

On 2025/8/20 15:23, Friendy Su wrote:
> Set proper 'dsunit' to let file body align on huge page on blobdev,
> 
> where 'dsunit' * 'blocksize' = huge page size (2M).
> 
> When do mmap() a file mounted with dax=always, aligning on huge page
> makes kernel map huge page(2M) per page fault exception, compared with
> mapping normal page(4K) per page fault.
> 
> This greatly improves mmap() performance by reducing times of page
> fault being triggered.
> 
> Signed-off-by: Friendy Su <friendy.su at sony.com>
> Reviewed-by: Yuezhang Mo <Yuezhang.Mo at sony.com>
> Reviewed-by: Daniel Palmer <daniel.palmer at sony.com>

Sigh, thanks for the patch! Actually the blob chunk implementation
(this file) is now an implementation debt since:

  1) In principle, chunks (and deduplicated pclusters) should
     across blobs (considering the main device is also a blob);

  2) Each blob should have its own space allocation context
     which is independent to the in-memory chunk records...

My mid-term plan tends to use the current metabox way
(i.e. use buffer management for all extra blobs too..)

> ---
>   lib/blobchunk.c | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
> 
> diff --git a/lib/blobchunk.c b/lib/blobchunk.c
> index bbc69cf..e47afb5 100644
> --- a/lib/blobchunk.c
> +++ b/lib/blobchunk.c
> @@ -309,6 +309,21 @@ int erofs_blob_write_chunked_file(struct erofs_inode *inode, int fd,
>   	minextblks = BLK_ROUND_UP(sbi, inode->i_size);
>   	interval_start = 0;
>   
> +	/* Align file on 'dsunit' */
> +	if (sbi->bmgr->dsunit > 1) {
> +		off_t off = lseek(blobfile, 0, SEEK_CUR);
> +
> +		erofs_dbg("Try to round up 0x%llx to align on %d blocks (dsunit)",
> +				off, sbi->bmgr->dsunit);
> +		off = roundup(off, sbi->bmgr->dsunit * erofs_blksiz(sbi));
> +		if (lseek(blobfile, off, SEEK_SET) != off) {
> +			ret = -errno;
> +			erofs_err("lseek to blobdev 0x%llx error", off);
> +			goto err;
> +		}
> +		erofs_dbg("Aligned on 0x%llx", off);
> +	}

But since you have use case on the current chunk
approach, so...

As for this patch, what if the inode itself is
chunk-deduplicated, could we apply this if the inode
only has one new chunk instead at least for now?

Thanks,
Gao Xiang