From: Andreas Dilger <adilger@sun.com>
Subject: Re: Question on fallocate/ftruncate sequence (and flags)
Date: Thu, 23 Jul 2009 15:53:57 -0600
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Cc: Frank Mayhar <fmayhar@google.com>,
	Curt Wohlgemuth <curtw@google.com>,
	ext4 development <linux-ext4@vger.kernel.org>
To: Eric Sandeen <sandeen@redhat.com>
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On Jul 23, 2009  15:37 -0500, Eric Sandeen wrote:
> Frank Mayhar wrote:
> > On Tue, 2009-07-21 at 15:54 -0600, Andreas Dilger wrote:
> >> That said, we might need to have some kind of flag in the on-disk
> >> inode to indicate that it was preallocated beyond EOF.  Otherwise,
> >> e2fsck will try and extend the file size to match the block count,
> >> which isn't correct.  We could also use this flag to determine if
> >> truncate needs to be run on the inode even if the new size is the
> >> same.
> > 
> > As it happens there's already a flag, FS_FALLOC_FL, set by ext2 in
> > fallocate().  Unfortunately ext4 is using that bit (0x00040000) for
> > EXT4_HUGE_FILE_FL.  (Ext4 is using another bit as well, 0x00100000, for
> > EXT4_EXT_MIGRATE_FL when fs.h defines it as FS_DIRECTIO_FL.)  I really
> > want to use the FS_FALLOC_FL bit for this purpose but that means
> > reallocating HUGE_FILE_FL to some other big.  Objections?
> 
> I'm confused (again?) :).  I don't see FS_FALLOC_FL in the latest kernel
> source, and ext2 (well, my ext2 anyway) can't do fallocate().  Google
> (well, my google search) can't find it either.  Is this something in
> your tree?

I think I recall Google working on a patch for fallocate on ext2, but
it was vetoed from upstream inclusion because we don't want to flog
a dead horse.  Hence the Google patch to run ext4 w/o a journal.

> As for:
> 
> #define EXT4_EXT_MIGRATE                0x00100000 /* Inode is migrating */
> 
> this is not in the mask that FS_IOC_GETFLAGS can see ... and I don't
> think anyone else uses FS_DIRECTIO_FL.
> 
> I'm not sure if the flags not in FS_FL_USER_VISIBLE are supposed to be
> fs-unique.

Well, they are stored on disk, so we shouldn't have conflicts between
ext2 and ext4 for sure.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.