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

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Date: Thu, 14 Dec 2023 09:58:34 +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.
> 
> Signed-off-by: Ye Bin <yebin10@...wei.com>

OK, I agree that there is this behavioral difference. However is that a
practical problem? I mean I would not expect the range to be particularly
small, rather something like 1GB and then these boundary conditions don't
really matter. This is also sensible so that we can properly track whether
the whole block group was trimmed or not. Finally I'd also argue that
trimming outside of specified range might be unexpected for the user. So a
*fix* for this in my opinion lays in userspace which needs to select
sensible ranges to use for trimming.

								Honza

> ---
>  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