linux-ext4 - Re: [PATCH] ext4: fix inconsistent between segment fstrim and full fstrim

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Date: Fri, 15 Dec 2023 12:41:35 +0100
From: Jan Kara <jack@...e.cz>
To: Ye Bin <yebin10@...wei.com>
Cc: tytso@....edu, adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org, jack@...e.cz
Subject: Re: [PATCH] ext4: fix inconsistent between segment fstrim and full
 fstrim

On Thu 14-12-23 14:46:35, Ye Bin wrote:
> There will not issue discard cmd when do segment fstrim for ext4 fs, however,
> if full fstrim for the same fs will issue discard cmd.
> Above issue may happens as follows:
> Precondition:
> 1. Fstrim range [0, 15] and [16, 31];
> 2. Discard granularity is 16;
>             Range1          Range2
>       1111000000000000 0000111010101011
> There's no free space length large or equal than 16 in 'Range1' or 'Range2'.
> As ext4_try_to_trim_range() only search free space among range which user
> specified. However, there's maximum free space length 16 in 'Range1'+ 'Range2'.
> To solve above issue, we need to find the longest free space to discard.

The patch looks good so feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

I'd just rephrase the changelog to make it a bit easier to read:

Suppose we issue two FITRIM ioctls for ranges [0,15] and [16,31] with
mininum length of trimmed range set to 8 blocks. If we have say a range of
blocks 10-22 free, this range will not be trimmed because it straddles the
boundary of the two FITRIM ranges and neither part is big enough. This is a
bit surprising to some users that call FITRIM on smaller ranges of blocks
to limit impact on the system. Also XFS trims all free space extents that
overlap with the specified range so we are inconsistent among filesystems.
Let's change ext4_try_to_trim_range() to consider for trimming the whole
free space extent that straddles the end of specified range, not just the
part of it within the range.

								Honza
 
> Signed-off-by: Ye Bin <yebin10@...wei.com>
> ---
>  fs/ext4/mballoc.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index d72b5e3c92ec..d195461123d8 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -6753,13 +6753,15 @@ static int ext4_try_to_trim_range(struct super_block *sb,
>  __acquires(ext4_group_lock_ptr(sb, e4b->bd_group))
>  __releases(ext4_group_lock_ptr(sb, e4b->bd_group))
>  {
> -	ext4_grpblk_t next, count, free_count;
> +	ext4_grpblk_t next, count, free_count, last, origin_start;
>  	bool set_trimmed = false;
>  	void *bitmap;
>  
> +	last = ext4_last_grp_cluster(sb, e4b->bd_group);
>  	bitmap = e4b->bd_bitmap;
> -	if (start == 0 && max >= ext4_last_grp_cluster(sb, e4b->bd_group))
> +	if (start == 0 && max >= last)
>  		set_trimmed = true;
> +	origin_start = start;
>  	start = max(e4b->bd_info->bb_first_free, start);
>  	count = 0;
>  	free_count = 0;
> @@ -6768,7 +6770,10 @@ __releases(ext4_group_lock_ptr(sb, e4b->bd_group))
>  		start = mb_find_next_zero_bit(bitmap, max + 1, start);
>  		if (start > max)
>  			break;
> -		next = mb_find_next_bit(bitmap, max + 1, start);
> +
> +		next = mb_find_next_bit(bitmap, last + 1, start);
> +		if (origin_start == 0 && next >= last)
> +			set_trimmed = true;
>  
>  		if ((next - start) >= minblocks) {
>  			int ret = ext4_trim_extent(sb, start, next - start, e4b);
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR