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Date:   Thu, 30 Dec 2021 14:04:46 -0500
From:   "Theodore Ts'o" <tytso@mit.edu>
To:     Hans Montero <hjm2133@columbia.edu>
Cc:     linux-fsdevel@vger.kernel.org, Tal Zussman <tz2294@columbia.edu>,
        Xijiao Li <xl2950@columbia.edu>,
        OS-TA <cucs4118-tas@googlegroups.com>,
        Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: Question about `generic_write_checks()` FIXME comment
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On Thu, Dec 30, 2021 at 03:07:25AM -0500, Hans Montero wrote:
> We noticed the FIXME comment and we weren't sure exactly what it meant so we
> kept tracing through older versions of `generic_write_checks()`, going as far as
> Linux 2.5.75, before it was implemented for `write_iter()` usage:
> 
>   if (!isblk) {
>           /* FIXME: this is for backwards compatibility with 2.4 */
>           if (file->f_flags & O_APPEND)
>                   *pos = inode->i_size;
>           ...
>   }

I was able to trace it back farther, to v2.4.14.8 -> v2.4.14.9.  And it
looks like the mysterious FIXME is about the block device:

commit 1040c54c3b98ac4f8d91bc313cdc9d6669481da3
Author: Linus Torvalds <torvalds@athlon.transmeta.com>
Date:   Mon Feb 4 20:33:54 2002 -0800
	...
@@ -2765,7 +2876,8 @@ generic_file_write(struct file *file,const char *buf,size_t count, loff_t *ppos)
 
 	written = 0;
 
-	if (file->f_flags & O_APPEND)
+	/* FIXME: this is for backwards compatibility with 2.4 */
+	if (!S_ISBLK(inode->i_mode) && file->f_flags & O_APPEND)
 		pos = inode->i_size;
 
 	/*

 But as Al Viro pointed out, O_APPEND really can't have any real
 meaning for a block device.  It would be neat if we could magically
 make a 10TB HDD to become as 12TB HDD by writing 2TB using
 O_APPEND, but reality doesn't work that way.  :-)

It probably makes sense just to remove the FIXME from the current
kernel sources so that future people don't get confused, asking the
same questions you have.

Cheers,

						- Ted