[PATCH v2] ext4: correct best extent lstart adjustment logic

[PATCH v2] ext4: correct best extent lstart adjustment logic

[Baokun Li]

When yangerkun review commit 93cdf49f6eca ("ext4: Fix best extent lstart
adjustment logic in ext4_mb_new_inode_pa()"), it was found that the best
extent did not completely cover the original request after adjusting the
best extent lstart in ext4_mb_new_inode_pa() as follows:

  original request: 2/10(8)
  normalized request: 0/64(64)
  best extent: 0/9(9)

When we check if best ex can be kept at start of goal, ac_o_ex.fe_logical
is 2 less than the adjusted best extent logical end 9, so we think the
adjustment is done. But obviously 0/9(9) doesn't cover 2/10(8), so we
should determine here if the original request logical end is less than or
equal to the adjusted best extent logical end.

In addition, add a comment stating when adjusted best_ex will not cover
the original request, and remove the duplicate assertion because adjusting
lstart makes no change to b_ex.fe_len.

Link: https://lore.kernel.org/r/3630fa7f-b432-7afd-5f79-781bc3b2c5ea@huawei.com
Fixes: 93cdf49f6eca ("ext4: Fix best extent lstart adjustment logic in ext4_mb_new_inode_pa()")
Cc: stable@kernel.org
Signed-off-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
---
V1->V2:
	Remove the problematic BUG_ON and add some comments.

 fs/ext4/mballoc.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 1ea6491b6b00..20cad0676aab 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -5148,10 +5148,16 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
 			.fe_len = ac->ac_orig_goal_len,
 		};
 		loff_t orig_goal_end = extent_logical_end(sbi, &ex);
+		loff_t o_ex_end = extent_logical_end(sbi, &ac->ac_o_ex);
 
-		/* we can't allocate as much as normalizer wants.
-		 * so, found space must get proper lstart
-		 * to cover original request */
+		/*
+		 * We can't allocate as much as normalizer wants, so we try
+		 * to get proper lstart to cover the original request, except
+		 * when the goal doesn't cover the original request as below:
+		 *
+		 * orig_ex:2045/2055(10), isize:8417280 -> normalized:0/2048
+		 * best_ex:0/200(200) -> adjusted: 1848/2048(200)
+		 */
 		BUG_ON(ac->ac_g_ex.fe_logical > ac->ac_o_ex.fe_logical);
 		BUG_ON(ac->ac_g_ex.fe_len < ac->ac_o_ex.fe_len);
 
@@ -5163,7 +5169,7 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
 		 * 1. Check if best ex can be kept at end of goal (before
 		 *    cr_best_avail trimmed it) and still cover original start
 		 * 2. Else, check if best ex can be kept at start of goal and
-		 *    still cover original start
+		 *    still cover original end
 		 * 3. Else, keep the best ex at start of original request.
 		 */
 		ex.fe_len = ac->ac_b_ex.fe_len;
@@ -5173,7 +5179,7 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
 			goto adjust_bex;
 
 		ex.fe_logical = ac->ac_g_ex.fe_logical;
-		if (ac->ac_o_ex.fe_logical < extent_logical_end(sbi, &ex))
+		if (o_ex_end <= extent_logical_end(sbi, &ex))
 			goto adjust_bex;
 
 		ex.fe_logical = ac->ac_o_ex.fe_logical;
@@ -5181,7 +5187,6 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
 		ac->ac_b_ex.fe_logical = ex.fe_logical;
 
 		BUG_ON(ac->ac_o_ex.fe_logical < ac->ac_b_ex.fe_logical);
-		BUG_ON(ac->ac_o_ex.fe_len > ac->ac_b_ex.fe_len);
 		BUG_ON(extent_logical_end(sbi, &ex) > orig_goal_end);
 	}
 
-- 
2.31.1

Re: [PATCH v2] ext4: correct best extent lstart adjustment logic

[Ojaswin Mujoo]

Hey Baokun, thanks for the v2.

On Thu, Feb 01, 2024 at 10:18:45PM +0800, Baokun Li wrote:
> When yangerkun review commit 93cdf49f6eca ("ext4: Fix best extent lstart
> adjustment logic in ext4_mb_new_inode_pa()"), it was found that the best
> extent did not completely cover the original request after adjusting the
> best extent lstart in ext4_mb_new_inode_pa() as follows:
> 
>   original request: 2/10(8)
>   normalized request: 0/64(64)
>   best extent: 0/9(9)
> 
> When we check if best ex can be kept at start of goal, ac_o_ex.fe_logical
> is 2 less than the adjusted best extent logical end 9, so we think the
> adjustment is done. But obviously 0/9(9) doesn't cover 2/10(8), so we
> should determine here if the original request logical end is less than or
> equal to the adjusted best extent logical end.
> 
> In addition, add a comment stating when adjusted best_ex will not cover
> the original request, and remove the duplicate assertion because adjusting
> lstart makes no change to b_ex.fe_len.
> 
> Link: https://lore.kernel.org/r/3630fa7f-b432-7afd-5f79-781bc3b2c5ea@huawei.com
> Fixes: 93cdf49f6eca ("ext4: Fix best extent lstart adjustment logic in ext4_mb_new_inode_pa()")
> Cc: stable@kernel.org

So I'm not sure if this is actually fixing a BUG and if we need to
backport this to stable but I guess Ted can take a call on that. 

Other than that, the patch looks good. Feel free to add:

Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>


Regards,
ojaswin

> Signed-off-by: yangerkun <yangerkun@huawei.com>
> Signed-off-by: Baokun Li <libaokun1@huawei.com>

Re: [PATCH v2] ext4: correct best extent lstart adjustment logic

[Jan Kara]

On Thu 01-02-24 22:18:45, Baokun Li wrote:
> When yangerkun review commit 93cdf49f6eca ("ext4: Fix best extent lstart
> adjustment logic in ext4_mb_new_inode_pa()"), it was found that the best
> extent did not completely cover the original request after adjusting the
> best extent lstart in ext4_mb_new_inode_pa() as follows:
> 
>   original request: 2/10(8)
>   normalized request: 0/64(64)
>   best extent: 0/9(9)
> 
> When we check if best ex can be kept at start of goal, ac_o_ex.fe_logical
> is 2 less than the adjusted best extent logical end 9, so we think the
> adjustment is done. But obviously 0/9(9) doesn't cover 2/10(8), so we
> should determine here if the original request logical end is less than or
> equal to the adjusted best extent logical end.
> 
> In addition, add a comment stating when adjusted best_ex will not cover
> the original request, and remove the duplicate assertion because adjusting
> lstart makes no change to b_ex.fe_len.
> 
> Link: https://lore.kernel.org/r/3630fa7f-b432-7afd-5f79-781bc3b2c5ea@huawei.com
> Fixes: 93cdf49f6eca ("ext4: Fix best extent lstart adjustment logic in ext4_mb_new_inode_pa()")
> Cc: stable@kernel.org
> Signed-off-by: yangerkun <yangerkun@huawei.com>
> Signed-off-by: Baokun Li <libaokun1@huawei.com>

Looks good. Feel free to add:

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

								Honza

> ---
> V1->V2:
> 	Remove the problematic BUG_ON and add some comments.
> 
>  fs/ext4/mballoc.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 1ea6491b6b00..20cad0676aab 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -5148,10 +5148,16 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
>  			.fe_len = ac->ac_orig_goal_len,
>  		};
>  		loff_t orig_goal_end = extent_logical_end(sbi, &ex);
> +		loff_t o_ex_end = extent_logical_end(sbi, &ac->ac_o_ex);
>  
> -		/* we can't allocate as much as normalizer wants.
> -		 * so, found space must get proper lstart
> -		 * to cover original request */
> +		/*
> +		 * We can't allocate as much as normalizer wants, so we try
> +		 * to get proper lstart to cover the original request, except
> +		 * when the goal doesn't cover the original request as below:
> +		 *
> +		 * orig_ex:2045/2055(10), isize:8417280 -> normalized:0/2048
> +		 * best_ex:0/200(200) -> adjusted: 1848/2048(200)
> +		 */
>  		BUG_ON(ac->ac_g_ex.fe_logical > ac->ac_o_ex.fe_logical);
>  		BUG_ON(ac->ac_g_ex.fe_len < ac->ac_o_ex.fe_len);
>  
> @@ -5163,7 +5169,7 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
>  		 * 1. Check if best ex can be kept at end of goal (before
>  		 *    cr_best_avail trimmed it) and still cover original start
>  		 * 2. Else, check if best ex can be kept at start of goal and
> -		 *    still cover original start
> +		 *    still cover original end
>  		 * 3. Else, keep the best ex at start of original request.
>  		 */
>  		ex.fe_len = ac->ac_b_ex.fe_len;
> @@ -5173,7 +5179,7 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
>  			goto adjust_bex;
>  
>  		ex.fe_logical = ac->ac_g_ex.fe_logical;
> -		if (ac->ac_o_ex.fe_logical < extent_logical_end(sbi, &ex))
> +		if (o_ex_end <= extent_logical_end(sbi, &ex))
>  			goto adjust_bex;
>  
>  		ex.fe_logical = ac->ac_o_ex.fe_logical;
> @@ -5181,7 +5187,6 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
>  		ac->ac_b_ex.fe_logical = ex.fe_logical;
>  
>  		BUG_ON(ac->ac_o_ex.fe_logical < ac->ac_b_ex.fe_logical);
> -		BUG_ON(ac->ac_o_ex.fe_len > ac->ac_b_ex.fe_len);
>  		BUG_ON(extent_logical_end(sbi, &ex) > orig_goal_end);
>  	}
>  
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH v2] ext4: correct best extent lstart adjustment logic

[Theodore Ts'o]

On Thu, 01 Feb 2024 22:18:45 +0800, Baokun Li wrote:
> When yangerkun review commit 93cdf49f6eca ("ext4: Fix best extent lstart
> adjustment logic in ext4_mb_new_inode_pa()"), it was found that the best
> extent did not completely cover the original request after adjusting the
> best extent lstart in ext4_mb_new_inode_pa() as follows:
> 
>   original request: 2/10(8)
>   normalized request: 0/64(64)
>   best extent: 0/9(9)
> 
> [...]

Applied, thanks!

[1/1] ext4: correct best extent lstart adjustment logic
      commit: 4fbf8bc733d14bceb16dda46a3f5e19c6a9621c5

Best regards,
-- 
Theodore Ts'o <tytso@mit.edu>