linux-ext4 - Re: [PATCH 12/37] e2fsck: insert a missing dirent tail for checksums if possible

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Message-ID: <20140505231645.GL8434@birch.djwong.org>
Date:	Mon, 5 May 2014 16:16:45 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Lukáš Czerner <lczerner@...hat.com>
Cc:	tytso@....edu, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 12/37] e2fsck: insert a missing dirent tail for checksums
 if possible

On Fri, May 02, 2014 at 02:54:08PM +0200, Lukáš Czerner wrote:
> On Thu, 1 May 2014, Darrick J. Wong wrote:
> 
> > Date: Thu, 01 May 2014 16:13:41 -0700
> > From: Darrick J. Wong <darrick.wong@...cle.com>
> > To: tytso@....edu, darrick.wong@...cle.com
> > Cc: linux-ext4@...r.kernel.org
> > Subject: [PATCH 12/37] e2fsck: insert a missing dirent tail for checksums if
> >     possible
> > 
> > If e2fsck is writing a block of directory entries to disk, it should
> > adjust the dirents to add the dirent tail if one is missing.  It's not
> > a big deal if there's no space to do this since rehash (pass 3A) will
> > reconstruct directories for us.  However, we may as well avoid
> > unnecessary work.
> 
> I am sorry for the stupid questions, but in what case can be the
> dirent tail missing ? It's not immediately obvious to me.

Primarily the "dirent tail missing" case happens if the user runs tune2fs to
add checksums to a FS and it encounters a directory block that doesn't have
enough space to store the directory block checksum field.  When this happens,
tune2fs advises the user to run e2fsck -D to rebuild the directories.  The -D
switch isn't strictly necessary.

The particular sub-case that this patch tries to capture is where the user
ignores the "e2fsck -D" request, deletes entries out of the dir block, and some
time later runs e2fsck.  In that case, we can skip a full rebuild and just fix
the block.

--D
> 
> Thanks!
> -Lukas
> 
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> > ---
> >  e2fsck/pass2.c |   40 ++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 40 insertions(+)
> > 
> > 
> > diff --git a/e2fsck/pass2.c b/e2fsck/pass2.c
> > index 5488c73..95f51b7 100644
> > --- a/e2fsck/pass2.c
> > +++ b/e2fsck/pass2.c
> > @@ -739,6 +739,41 @@ static int is_last_entry(ext2_filsys fs, int inline_data_size,
> >  		return (offset < fs->blocksize - csum_size);
> >  }
> >  
> > +static errcode_t insert_dirent_tail(ext2_filsys fs, void *dirbuf)
> > +{
> > +	struct ext2_dir_entry *d;
> > +	void *top;
> > +	struct ext2_dir_entry_tail *t;
> > +	unsigned int rec_len;
> > +
> > +	d = dirbuf;
> > +	top = EXT2_DIRENT_TAIL(dirbuf, fs->blocksize);
> > +
> > +	rec_len = d->rec_len;
> > +	while (rec_len && !(rec_len & 0x3)) {
> > +		d = (struct ext2_dir_entry *)(((char *)d) + rec_len);
> > +		if (((void *)d) + d->rec_len >= top)
> > +			break;
> > +		rec_len = d->rec_len;
> > +	}
> > +
> > +	if (d != top) {
> > +		size_t min_size = EXT2_DIR_REC_LEN(
> > +				ext2fs_dirent_name_len(dirbuf));
> > +		if (min_size > d->rec_len - sizeof(struct ext2_dir_entry_tail))
> > +			return EXT2_ET_DIR_NO_SPACE_FOR_CSUM;
> > +		d->rec_len -= sizeof(struct ext2_dir_entry_tail);
> > +	}
> > +
> > +	t = (struct ext2_dir_entry_tail *)top;
> > +	if (t->det_reserved_zero1 ||
> > +	    t->det_rec_len != sizeof(struct ext2_dir_entry_tail) ||
> > +	    t->det_reserved_name_len != EXT2_DIR_NAME_LEN_CSUM)
> > +		ext2fs_initialize_dirent_tail(fs, t);
> > +
> > +	return 0;
> > +}
> > +
> >  static int check_dir_block(ext2_filsys fs,
> >  			   struct ext2_db_entry2 *db,
> >  			   void *priv_data)
> > @@ -1275,8 +1310,13 @@ skip_checksum:
> >  		if (EXT2_HAS_RO_COMPAT_FEATURE(fs->super,
> >  				EXT4_FEATURE_RO_COMPAT_METADATA_CSUM) &&
> >  		    is_leaf &&
> > +		    !inline_data_size &&
> >  		    !ext2fs_dirent_has_tail(fs, (struct ext2_dir_entry *)buf))
> > +		{
> > +			if (insert_dirent_tail(fs, buf) == 0)
> > +				goto write_and_fix;
> >  			e2fsck_rehash_dir_later(ctx, ino);
> > +		}
> >  
> >  write_and_fix:
> >  		if (e2fsck_dir_will_be_rehashed(ctx, ino))
> > 
> > --
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> > 
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