linux-ext4 - Re: [PATCH 1/3] ext4: remove block bitmap initialization in ext4_new

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Message-ID: <CAOQ4uxhwTOvibaVEKZ9OONCtJcVbAmYMYmVMvOX8=8KheUF2HQ@mail.gmail.com>
Date:	Sat, 14 Jan 2012 20:41:50 +0200
From:	Amir Goldstein <amir73il@...il.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 1/3] ext4: remove block bitmap initialization in ext4_new_inode()

On Sat, Jan 14, 2012 at 7:02 AM, Andreas Dilger <adilger@...ger.ca> wrote:
>
> On 2012-01-13, at 2:30 PM, Theodore Ts'o wrote:
> > We don't need to initialize the block bitmap when we allocate a new
> > inode.
>
> The reason the block bitmap is initialized when an inode is allocated
> in that group is to indicate that the inode bitmap is in use, and the
> inode table blocks are also in use.

but the inode table blocks will not be from this block group when flex_bg
is used and if it's the first group of a flex_bg group, then it's bitmaps
are already initialized by mkfs/resize.

>
> > This is old code from the very early days that is just
> > confusing things, and also has the problem of modifying the block
> > group descriptor without obeying the ext4_journal_get_write_access() /
> > ext4_handle_dirty_metadata() modification protocols.
>
> The group descriptor was already modified earlier on when the inode was
> being allocated from the group, so the group descriptor block was already
> accounted for in the transaction credits after "repeat_in_this_group:"
>
> repeat_in_this_group:
>                ino = ext4_find_next_zero_bit((unsigned long *)
>                                              inode_bitmap_bh->b_data,
>                                              EXT4_INODES_PER_GROUP(sb), ino);
>
>                if (ino < EXT4_INODES_PER_GROUP(sb)) {
>
>                        BUFFER_TRACE(inode_bitmap_bh, "get_write_access");
>                        err = ext4_journal_get_write_access(handle,
>                                                            inode_bitmap_bh);
>                        if (err)
>                                goto fail;
>
>                        BUFFER_TRACE(group_desc_bh, "get_write_access");
> *****>>>>>              err = ext4_journal_get_write_access(handle,
>                                                            group_desc_bh);
>
> That is why ext4_handle_dirty_metadata() isn't called until after the group
> descriptor is modified during the block bitmap initialization.
>
>
> I'm not wholly against removing this code, but we have to do it with the
> clear understanding that we will have inodes in use for which the block
> bitmap is showing that the in-use blocks are free.  This doesn't seem like
> a great situation to me.
>
>
> > Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> > ---
> > fs/ext4/ialloc.c |   31 -------------------------------
> > 1 files changed, 0 insertions(+), 31 deletions(-)
> >
> > diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
> > index 72fc989..a4ce10f 100644
> > --- a/fs/ext4/ialloc.c
> > +++ b/fs/ext4/ialloc.c
> > @@ -807,37 +807,6 @@ repeat_in_this_group:
> >       goto out;
> >
> > got:
> > -     /* We may have to initialize the block bitmap if it isn't already */
> > -     if (EXT4_HAS_RO_COMPAT_FEATURE(sb, EXT4_FEATURE_RO_COMPAT_GDT_CSUM) &&
> > -         gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
> > -             struct buffer_head *block_bitmap_bh;
> > -
> > -             block_bitmap_bh = ext4_read_block_bitmap(sb, group);
> > -             BUFFER_TRACE(block_bitmap_bh, "get block bitmap access");
> > -             err = ext4_journal_get_write_access(handle, block_bitmap_bh);
> > -             if (err) {
> > -                     brelse(block_bitmap_bh);
> > -                     goto fail;
> > -             }
> > -
> > -             BUFFER_TRACE(block_bitmap_bh, "dirty block bitmap");
> > -             err = ext4_handle_dirty_metadata(handle, NULL, block_bitmap_bh);
> > -             brelse(block_bitmap_bh);
> > -
> > -             /* recheck and clear flag under lock if we still need to */
> > -             ext4_lock_group(sb, group);
> > -             if (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
> > -                     gdp->bg_flags &= cpu_to_le16(~EXT4_BG_BLOCK_UNINIT);
> > -                     ext4_free_group_clusters_set(sb, gdp,
> > -                             ext4_free_clusters_after_init(sb, group, gdp));
> > -                     gdp->bg_checksum = ext4_group_desc_csum(sbi, group,
> > -                                                             gdp);
> > -             }
> > -             ext4_unlock_group(sb, group);
> > -
> > -             if (err)
> > -                     goto fail;
> > -     }
> >       BUFFER_TRACE(group_desc_bh, "call ext4_handle_dirty_metadata");
> >       err = ext4_handle_dirty_metadata(handle, NULL, group_desc_bh);
> >       if (err)
> > --
> > 1.7.8.11.gefc1f.dirty
> >
> > --
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>
>
> Cheers, Andreas
>
>
>
>
>
> --
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