linux-ext4 - Re: [RFC 2/6] ext4: Implement ext4_group_block

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Date:   Sat, 5 Feb 2022 16:13:46 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        "Theodore Ts'o" <tytso@....edu>,
        Harshad Shirwadkar <harshadshirwadkar@...il.com>
Subject: Re: [RFC 2/6] ext4: Implement ext4_group_block_valid() as common
 function

On 22/02/04 12:49PM, Jan Kara wrote:
> On Fri 04-02-22 15:38:44, Ritesh Harjani wrote:
> > On 22/02/01 12:34PM, Jan Kara wrote:
> > > On Mon 31-01-22 20:46:51, Ritesh Harjani wrote:
> > > > This patch implements ext4_group_block_valid() check functionality,
> > > > and refactors all the callers to use this common function instead.
> > > >
> > > > Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>
> > > ...
> > >
> > > > diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> > > > index 8d23108cf9d7..60d32d3d8dc4 100644
> > > > --- a/fs/ext4/mballoc.c
> > > > +++ b/fs/ext4/mballoc.c
> > > > @@ -6001,13 +6001,7 @@ void ext4_free_blocks(handle_t *handle, struct inode *inode,
> > > >  		goto error_return;
> > > >  	}
> > > >
> > > > -	if (in_range(ext4_block_bitmap(sb, gdp), block, count) ||
> > > > -	    in_range(ext4_inode_bitmap(sb, gdp), block, count) ||
> > > > -	    in_range(block, ext4_inode_table(sb, gdp),
> > > > -		     sbi->s_itb_per_group) ||
> > > > -	    in_range(block + count - 1, ext4_inode_table(sb, gdp),
> > > > -		     sbi->s_itb_per_group)) {
> > > > -
> > > > +	if (!ext4_group_block_valid(sb, block_group, block, count)) {
> > > >  		ext4_error(sb, "Freeing blocks in system zone - "
> > > >  			   "Block = %llu, count = %lu", block, count);
> > > >  		/* err = 0. ext4_std_error should be a no op */
> > >
> > > When doing this, why not rather directly use ext4_inode_block_valid() here?
> >
> > This is because while freeing these blocks we have their's corresponding block
> > group too. So there is little point in checking FS Metadata of all block groups
> > v/s FS Metadata of just this block group, no?
> >
> > Also, I am not sure if we changing this to check against system-zone's blocks
> > (which has FS Metadata blocks from all block groups), can add any additional
> > penalty?
>
> I agree the check will be somewhat more costly (rbtree lookup). OTOH with
> more complex fs structure (like flexbg which is default for quite some
> time), this is by far not checking the only metadata blocks, that can
> overlap the freed range. Also this is not checking for freeing journal
> blocks. So I'd either got for no check (if we really want performance) or
> full check (if we care more about detecting fs errors early). Because these
> half-baked checks do not bring much value these days...

Agreed. Thanks for putting out your points.
I am making these suggested changes to add stricter checking via
ext4_inode_block_valid() and will be sending out v1 soon.

-ritesh