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

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Message-ID: <CAB=BE-R4uPFeBSt6Z4Khv6_OjAu9=WoJR-VGG8eG0spAaovE1w@mail.gmail.com>
Date: Wed, 23 Apr 2025 23:51:31 -0700
From: Sandeep Dhavale <dhavale@...gle.com>
To: Hongbo Li <lihongbo22@...wei.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 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.

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,