linux-kernel - Re: Maybe update the minextblks in wrong way?

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Message-ID: <b6d8f4eb-e1ab-4a83-b552-0b58ec120299@huawei.com>
Date: Thu, 24 Apr 2025 16:11:56 +0800
From: Hongbo Li <lihongbo22@...wei.com>
To: Sandeep Dhavale <dhavale@...gle.com>
CC: Gao Xiang <hsiangkao@...ux.alibaba.com>, LKML
	<linux-kernel@...r.kernel.org>, linux-erofs mailing list
	<linux-erofs@...ts.ozlabs.org>
Subject: Re: Maybe update the minextblks in wrong way?



On 2025/4/24 14:51, Sandeep Dhavale wrote:
> On Wed, Apr 23, 2025 at 6:50 PM Hongbo Li <lihongbo22@...wei.com> wrote:
>>
>> Hi Sandeep,
>>     The consecutive chunks will be merged if possible, but after commit
>> 545988a65131 ("erofs-utils: lib: Fix calculation of minextblks when
>> working with sparse files"), the @minextblks will be updated into a
>> smaller value even the chunks are consecutive by blobchunks.c:379. I
>> think maybe the last operation that updates @minextblks is unnecessary,
>> since this value would have already been adjusted earlier when handling
>> discontinuous chunks. Likes:
>>
>> ```
>> --- a/lib/blobchunk.c
>> +++ b/lib/blobchunk.c
>> @@ -376,7 +376,6 @@ int erofs_blob_write_chunked_file(struct erofs_inode
>> *inode, int fd,
>>                   *(void **)idx++ = chunk;
>>                   lastch = chunk;
>>           }
>> -       erofs_update_minextblks(sbi, interval_start, pos, &minextblks);
>>           inode->datalayout = EROFS_INODE_CHUNK_BASED;
>>           free(chunkdata);
>>           return erofs_blob_mergechunks(inode, chunkbits,
>>
>> ```
>> This way can reduces the chunk index array's size. And what about your
>> opinion?
>>
>> Thanks,
>> Hongbo
> 
> Hi Hongbo,
> I think the last call is necessary to handle the tail end which is not
> handled in the for loop. But I understand that if the file is
> contiguous then the last call can reduce minextblks.

Ok, also should update is_contiguous in another place when we found the 
dedup chunk for non-sparse file?

By the way, why we should use the value of lowbit instead of the (end - 
start) >> sbi->blkszbits to update minextblks? The lowbit will get the 
smaller value.

Thanks,
Hongbo
> 
> Does the below patch address your concern to conditionally call the
> last erofs_update_minextblks()?
> 
> Thanks,
> Sandeep.
> 
> diff --git a/lib/blobchunk.c b/lib/blobchunk.c
> index de9150f..47fe923 100644
> --- a/lib/blobchunk.c
> +++ b/lib/blobchunk.c
> @@ -303,6 +303,7 @@ int erofs_blob_write_chunked_file(struct
> erofs_inode *inode, int fd,
>          lastch = NULL;
>          minextblks = BLK_ROUND_UP(sbi, inode->i_size);
>          interval_start = 0;
> +       bool is_contiguous = true;
> 
>          for (pos = 0; pos < inode->i_size; pos += len) {
>   #ifdef SEEK_DATA
> @@ -332,6 +333,7 @@ int erofs_blob_write_chunked_file(struct
> erofs_inode *inode, int fd,
>                                  erofs_update_minextblks(sbi, interval_start,
>                                                          pos, &minextblks);
>                                  interval_start = pos;
> +                               is_contiguous = false;
>                          }
>                          do {
>                                  *(void **)idx++ = &erofs_holechunk;
> @@ -365,7 +367,8 @@ int erofs_blob_write_chunked_file(struct
> erofs_inode *inode, int fd,
>                  *(void **)idx++ = chunk;
>                  lastch = chunk;
>          }
> -       erofs_update_minextblks(sbi, interval_start, pos, &minextblks);
> +       if (!is_contiguous)
> +               erofs_update_minextblks(sbi, interval_start, pos, &minextblks);
>          inode->datalayout = EROFS_INODE_CHUNK_BASED;
>          free(chunkdata);
>          return erofs_blob_mergechunks(inode, chunkbits,
>