Re: [f2fs-dev] [PATCH v2] f2fs: fix age extent cache insertion skip on counter overflow

[Chao Yu via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>]

Jaegeuk,

Just in case, you may missed this patch.

On 10/27/25 17:23, Xiaole He wrote:
> The age extent cache uses last_blocks (derived from
> allocated_data_blocks) to determine data age. However, there's a
> conflict between the deletion
> marker (last_blocks=0) and legitimate last_blocks=0 cases when
> allocated_data_blocks overflows to 0 after reaching ULLONG_MAX.
> 
> In this case, valid extents are incorrectly skipped due to the
> "if (!tei->last_blocks)" check in __update_extent_tree_range().
> 
> This patch fixes the issue by:
> 1. Reserving ULLONG_MAX as an invalid/deletion marker
> 2. Limiting allocated_data_blocks to range [0, ULLONG_MAX-1]
> 3. Using F2FS_EXTENT_AGE_INVALID for deletion scenarios
> 4. Adjusting overflow age calculation from ULLONG_MAX to (ULLONG_MAX-1)
> 
> Reproducer (using a patched kernel with allocated_data_blocks
> initialized to ULLONG_MAX - 3 for quick testing):
> 
> Step 1: Mount and check initial state
>   # dd if=/dev/zero of=/tmp/test.img bs=1M count=100
>   # mkfs.f2fs -f /tmp/test.img
>   # mkdir -p /mnt/f2fs_test
>   # mount -t f2fs -o loop,age_extent_cache /tmp/test.img /mnt/f2fs_test
>   # cat /sys/kernel/debug/f2fs/status | grep -A 4 "Block Age"
>   Allocated Data Blocks: 18446744073709551612 # ULLONG_MAX - 3
>   Inner Struct Count: tree: 1(0), node: 0
> 
> Step 2: Create files and write data to trigger overflow
>   # touch /mnt/f2fs_test/{1,2,3,4}.txt; sync
>   # cat /sys/kernel/debug/f2fs/status | grep -A 4 "Block Age"
>   Allocated Data Blocks: 18446744073709551613 # ULLONG_MAX - 2
>   Inner Struct Count: tree: 5(0), node: 1
> 
>   # dd if=/dev/urandom of=/mnt/f2fs_test/1.txt bs=4K count=1; sync
>   # cat /sys/kernel/debug/f2fs/status | grep -A 4 "Block Age"
>   Allocated Data Blocks: 18446744073709551614 # ULLONG_MAX - 1
>   Inner Struct Count: tree: 5(0), node: 2
> 
>   # dd if=/dev/urandom of=/mnt/f2fs_test/2.txt bs=4K count=1; sync
>   # cat /sys/kernel/debug/f2fs/status | grep -A 4 "Block Age"
>   Allocated Data Blocks: 18446744073709551615 # ULLONG_MAX
>   Inner Struct Count: tree: 5(0), node: 3
> 
>   # dd if=/dev/urandom of=/mnt/f2fs_test/3.txt bs=4K count=1; sync
>   # cat /sys/kernel/debug/f2fs/status | grep -A 4 "Block Age"
>   Allocated Data Blocks: 0 # Counter overflowed!
>   Inner Struct Count: tree: 5(0), node: 4
> 
> Step 3: Trigger the bug - next write should create node but gets skipped
>   # dd if=/dev/urandom of=/mnt/f2fs_test/4.txt bs=4K count=1; sync
>   # cat /sys/kernel/debug/f2fs/status | grep -A 4 "Block Age"
>   Allocated Data Blocks: 1
>   Inner Struct Count: tree: 5(0), node: 4
> 
>   Expected: node: 5 (new extent node for 4.txt)
>   Actual: node: 4 (extent insertion was incorrectly skipped due to
>   last_blocks = allocated_data_blocks = 0 in __get_new_block_age)
> 
> After this fix, the extent node is correctly inserted and node count
> becomes 5 as expected.
> 
> Fixes: 71644dff4811 ("f2fs: add block_age-based extent cache")
> Cc: stable@kernel.org
> Signed-off-by: Xiaole He <hexiaole1994@126.com>
> ---
> Changes in v2:
> - Added Fixes tag and Cc stable
> - Updated reproducer to use 'grep -A 4' for better output clarity
> ---
>  fs/f2fs/extent_cache.c | 5 +++--
>  fs/f2fs/f2fs.h         | 6 ++++++
>  fs/f2fs/segment.c      | 9 +++++++--
>  3 files changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/f2fs/extent_cache.c b/fs/f2fs/extent_cache.c
> index 33e09c453c70..0ed84cc065a7 100644
> --- a/fs/f2fs/extent_cache.c
> +++ b/fs/f2fs/extent_cache.c
> @@ -808,7 +808,7 @@ static void __update_extent_tree_range(struct inode *inode,
>  	}
>  	goto out_read_extent_cache;
>  update_age_extent_cache:
> -	if (!tei->last_blocks)
> +	if (tei->last_blocks == F2FS_EXTENT_AGE_INVALID)
>  		goto out_read_extent_cache;
>  
>  	__set_extent_info(&ei, fofs, len, 0, false,
> @@ -912,7 +912,7 @@ static int __get_new_block_age(struct inode *inode, struct extent_info *ei,
>  			cur_age = cur_blocks - tei.last_blocks;
>  		else
>  			/* allocated_data_blocks overflow */
> -			cur_age = ULLONG_MAX - tei.last_blocks + cur_blocks;
> +			cur_age = (ULLONG_MAX - 1) - tei.last_blocks + cur_blocks;
>  
>  		if (tei.age)
>  			ei->age = __calculate_block_age(sbi, cur_age, tei.age);
> @@ -1114,6 +1114,7 @@ void f2fs_update_age_extent_cache_range(struct dnode_of_data *dn,
>  	struct extent_info ei = {
>  		.fofs = fofs,
>  		.len = len,
> +		.last_blocks = F2FS_EXTENT_AGE_INVALID,
>  	};
>  
>  	if (!__may_extent_tree(dn->inode, EX_BLOCK_AGE))
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index 5b4e9548a231..fa3c676adc30 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -707,6 +707,12 @@ enum extent_type {
>  	NR_EXTENT_CACHES,
>  };
>  
> +/*
> + * Reserved value to mark invalid age extents, hence valid block range
> + * from 0 to ULLONG_MAX-1
> + */
> +#define F2FS_EXTENT_AGE_INVALID	ULLONG_MAX
> +
>  struct extent_info {
>  	unsigned int fofs;		/* start offset in a file */
>  	unsigned int len;		/* length of the extent */
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index b45eace879d7..a473cd1fb37d 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -3863,8 +3863,13 @@ int f2fs_allocate_data_block(struct f2fs_sb_info *sbi, struct folio *folio,
>  	locate_dirty_segment(sbi, GET_SEGNO(sbi, old_blkaddr));
>  	locate_dirty_segment(sbi, GET_SEGNO(sbi, *new_blkaddr));
>  
> -	if (IS_DATASEG(curseg->seg_type))
> -		atomic64_inc(&sbi->allocated_data_blocks);
> +	if (IS_DATASEG(curseg->seg_type)) {
> +		unsigned long long new_val;
> +
> +		new_val = atomic64_inc_return(&sbi->allocated_data_blocks);
> +		if (unlikely(new_val == ULLONG_MAX))
> +			atomic64_set(&sbi->allocated_data_blocks, 0);
> +	}
>  
>  	up_write(&sit_i->sentry_lock);
>  



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