linux-kernel - Re: [PATCH v2] fs/buffer: Remove redundant assignment to err

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Message-ID: <20230403-oxymoron-unease-ca8a023e45c7@brauner>
Date:   Mon, 3 Apr 2023 18:40:06 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Jiapeng Chong <jiapeng.chong@...ux.alibaba.com>,
        viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Abaci Robot <abaci@...ux.alibaba.com>
Subject: Re: [PATCH v2] fs/buffer: Remove redundant assignment to err

On Mon, Apr 03, 2023 at 06:10:43PM +0200, Jan Kara wrote:
> On Thu 23-03-23 10:32:59, Jiapeng Chong wrote:
> > Variable 'err' set but not used.
> > 
> > fs/buffer.c:2613:2: warning: Value stored to 'err' is never read.
> > 
> > Reported-by: Abaci Robot <abaci@...ux.alibaba.com>
> > Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=4589
> > Signed-off-by: Jiapeng Chong <jiapeng.chong@...ux.alibaba.com>
> 
> I don't think the patch is quite correct (Christian, please drop it if I'm
> correct). See below:

Thank you for taking a look at this!

> 
> > diff --git a/fs/buffer.c b/fs/buffer.c
> > index d759b105c1e7..b3eb905f87d6 100644
> > --- a/fs/buffer.c
> > +++ b/fs/buffer.c
> > @@ -2580,7 +2580,7 @@ int block_truncate_page(struct address_space *mapping,
> >  	struct inode *inode = mapping->host;
> >  	struct page *page;
> >  	struct buffer_head *bh;
> > -	int err;
> > +	int err = 0;
> >  
> >  	blocksize = i_blocksize(inode);
> >  	length = offset & (blocksize - 1);
> > @@ -2593,9 +2593,8 @@ int block_truncate_page(struct address_space *mapping,
> >  	iblock = (sector_t)index << (PAGE_SHIFT - inode->i_blkbits);
> >  	
> >  	page = grab_cache_page(mapping, index);
> > -	err = -ENOMEM;
> >  	if (!page)
> > -		goto out;
> > +		return -ENOMEM;
> >  
> >  	if (!page_has_buffers(page))
> >  		create_empty_buffers(page, blocksize, 0);
> > @@ -2609,7 +2608,6 @@ int block_truncate_page(struct address_space *mapping,
> >  		pos += blocksize;
> >  	}
> >  
> > -	err = 0;
> >  	if (!buffer_mapped(bh)) {
> >  		WARN_ON(bh->b_size != blocksize);
> >  		err = get_block(inode, iblock, bh, 0);
> > @@ -2633,12 +2631,11 @@ int block_truncate_page(struct address_space *mapping,
> >  
> >  	zero_user(page, offset, length);
> >  	mark_buffer_dirty(bh);
> > -	err = 0;
> 
> There is:
> 
>         if (!buffer_uptodate(bh) && !buffer_delay(bh) && !buffer_unwritten(bh))
>                 err = -EIO;
> 
> above this assignment. So now we'll be returning -EIO if
> block_truncate_page() needs to read the block AFAICT. Did this pass fstests
> with some filesystem exercising this code (ext2 driver comes to mind)?

Hm, the code in current mainline is:

        if (!buffer_uptodate(bh) && !buffer_delay(bh) && !buffer_unwritten(bh)) {
                err = bh_read(bh, 0);
                /* Uhhuh. Read error. Complain and punt. */
                if (err < 0)
                        goto unlock;
        }

Before e7ea1129afab ("fs/buffer: replace ll_rw_block()") that code really was

        if (!buffer_uptodate(bh) && !buffer_delay(bh) && !buffer_unwritten(bh)) {
                err = -EIO;
                ll_rw_block(REQ_OP_READ, 1, &bh);
                wait_on_buffer(bh);
                /* Uhhuh. Read error. Complain and punt. */
                if (!buffer_uptodate(bh))
                        goto unlock;
        }

which would've indeed caused this change to return -EIO.
Is this still an issue now? Sorry if I'm being dense here.