Re: [f2fs-dev] [PATCH RESEND 3/5] f2fs: punch largest extent instead of dropping it entirely on overlap

On 6/15/26 8:05 PM, Chao Yu via Linux-f2fs-devel wrote:
> On 6/12/26 19:58, Yongpeng Yang wrote:
>> From: Yongpeng Yang <[email protected]>
>>
>> Previously, when an extent being inserted overlaps with the largest
>> extent, the largest extent is dropped entirely. This was done to handle
> 
> Please correct me if I missed anything, I remember that we will add largest
> extent in below path?
> 
> - __update_extent_tree_range
>   - __insert_extent_tree
>    - __try_update_largest_extent : update largest w/ right extent
>   - __try_update_largest_extent : update largest w/ left extent

The largest extent might not reside in the extent tree. If
__update_extent_tree_range invokes __drop_largest_extent, the length of
the largest extent will have been set to zero. As a result, the largest
extent updated inside __try_update_largest_extent can end up smaller
than the largest extent obtained after __punch_largest_extent completes
its punch operation.

Thanks
Yongpeng,

> 
> Thanks,
> 
>> the case where the largest extent is not in memory, avoiding
>> inconsistency between the largest extent and the extent tree.
>>
>> This patch changes the semantics of __drop_largest_extent (renamed to
>> __punch_largest_extent): instead of discarding the entire largest
>> extent when any overlap is detected, keep the larger remaining portion
>> (left or right) after the punch. This preserves extent cache coverage
>> for truncate and overwrite operations that only partially overlap the
>> largest extent.
>>
>> Signed-off-by: Yongpeng Yang <[email protected]>
>> ---
>>   fs/f2fs/extent_cache.c | 31 ++++++++++++++++++++++++-------
>>   1 file changed, 24 insertions(+), 7 deletions(-)
>>
>> diff --git a/fs/f2fs/extent_cache.c b/fs/f2fs/extent_cache.c
>> index f8d94db60dc6..82d84c4e98b2 100644
>> --- a/fs/f2fs/extent_cache.c
>> +++ b/fs/f2fs/extent_cache.c
>> @@ -397,14 +397,31 @@ static unsigned int __free_extent_tree(struct f2fs_sb_info *sbi,
>>   	return count;
>>   }
>>   
>> -static void __drop_largest_extent(struct extent_tree *et,
>> +static void __punch_largest_extent(struct extent_tree *et,
>>   					pgoff_t fofs, unsigned int len)
>>   {
>> -	if (fofs < (pgoff_t)et->largest.fofs + et->largest.len &&
>> -			fofs + len > et->largest.fofs) {
>> -		et->largest.len = 0;
>> -		et->largest_updated = true;
>> +	unsigned int largest_end, punch_end;
>> +	unsigned int left_len, right_len;
>> +
>> +	if (fofs >= (pgoff_t)et->largest.fofs + et->largest.len ||
>> +			fofs + len <= et->largest.fofs)
>> +		return;
>> +
>> +	/* Punch [fofs, fofs + len) from largest extent. */
>> +	largest_end = et->largest.fofs + et->largest.len;
>> +	punch_end = fofs + len;
>> +
>> +	left_len = fofs > et->largest.fofs ? fofs - et->largest.fofs : 0;
>> +	right_len = largest_end > punch_end ? largest_end - punch_end : 0;
>> +
>> +	if (left_len >= right_len) {
>> +		et->largest.len = left_len;
>> +	} else {
>> +		et->largest.blk += punch_end - et->largest.fofs;
>> +		et->largest.fofs = punch_end;
>> +		et->largest.len = right_len;
>>   	}
>> +	et->largest_updated = true;
>>   }
>>   
>>   void f2fs_init_read_extent_tree(struct inode *inode, struct folio *ifolio)
>> @@ -680,10 +697,10 @@ static void __update_extent_tree_range(struct inode *inode,
>>   		dei.len = 0;
>>   
>>   		/*
>> -		 * drop largest extent before lookup, in case it's already
>> +		 * punch largest extent before lookup, in case it's already
>>   		 * been shrunk from extent tree
>>   		 */
>> -		__drop_largest_extent(et, fofs, len);
>> +		__punch_largest_extent(et, fofs, len);
>>   	}
>>   
>>   	if (et->largest.len != 0 &&
> 
> 
> 
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