Re: Maybe update the minextblks in wrong way?

[Hongbo Li]

On 2025/4/24 14:51, Sandeep Dhavale wrote:
On Wed, Apr 23, 2025 at 6:50 PM Hongbo Li <lihongb...@huawei.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,