linux-ext4 - Re: e2fsck: set dir_nlink feature if large dir exists

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Message-Id: <6FF8BA29-74A3-483D-A194-C5AD3A4E3BF2@dilger.ca>
Date:   Thu, 31 Aug 2017 12:17:59 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: e2fsck: set dir_nlink feature if large dir exists

On Aug 30, 2017, at 9:13 AM, Theodore Ts'o <tytso@....edu> wrote:
> 
> On Wed, Aug 30, 2017 at 01:43:30AM -0600, Andreas Dilger wrote:
>> On a related note, I was trying to implement the optimization for
>> more efficient ext2fs_link() insertion, but this has proven to be
>> much more complex than I expected.  I was thinking I would just
>> call the link_proc() function directly to handle insertion of the
>> new name, using block number of the previous caller.  However, this
>> function depends on the directory leaf buffers having been read
>> from disk, and being passed a dirent pointer to the free space after
>> being called through a series of callback indirections.
> 
> I think it's more trouble than it's worth to preseve the existing
> link_proc() function.  I'd recommend out to be just what is necessary
> to do the directory entry insert, and make that a helper function
> which is called by link_proc() and the shortcut handling code in
> ext2fs_link().

I looked at implementing it this way, but unfortunately that doesn't
appear workable since ext2fs_link() never gets the block numbers to
be able to do this.  I'd essentially have to reimplement all of the
block and directory leaf processing in ext2fs_dir_iterate2() to get
figure out where to store the first entries in the directory.

What I'm looking at now is add a new ext2fs_dir_iterate3() function
that passes *blocknr and blockcnt to the link_proc() and unlink_proc()
callback functions so they can be stored in struct_ext2_filsys, and
used to shortcut the iteration in ext2fs_link() for multiple inserts.
It is added to unlink_proc() so that we can reset the saved state if
entries are being deleted.

Cheers, Andreas

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