Re: [PATCH RFC 2/2] btrfs: defrag: prepare defrag for larger data folio size

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jan 24, 2024 at 02:29:08PM +1030, Qu Wenruo wrote:
> Although we have migrated defrag to use the folio interface, we can
> still further enhance it for the future larger data folio size.

This patch is wrong.  Please drop it.

>  {
>  	struct btrfs_fs_info *fs_info = inode->root->fs_info;
>  	struct extent_changeset *data_reserved = NULL;
>  	const u64 start = target->start;
>  	const u64 len = target->len;
> -	unsigned long last_index = (start + len - 1) >> PAGE_SHIFT;
> -	unsigned long start_index = start >> PAGE_SHIFT;
> +	unsigned long last_index = (start + len - 1) >> fs_info->folio_shift;
> +	unsigned long start_index = start >> fs_info->folio_shift;

indices are always in multiples of PAGE_SIZE.

>  	unsigned long first_index = folios[0]->index;

... so if you've shifted a file position by some "folio_shift" and then
subtracted it from folio->index, you have garbage.