linux-ext4 - Re: [PATCH] ext4: attempt to shrink directory on dentry removal

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Message-ID: <20190826031926.GC4918@mit.edu>
Date:   Sun, 25 Aug 2019 23:19:26 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     harshad shirwadkar <harshadshirwadkar@...il.com>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: attempt to shrink directory on dentry removal

On Sun, Aug 25, 2019 at 07:46:50PM -0700, harshad shirwadkar wrote:
> If last is an intermediate dx node, is there a way to find out if it
> actually is an intermediate dx node? Because an empty dirent block and
> an intermediate dx block look the same. Unless we do dx_probe() there
> is no way to know if a block is an intermediate dx block. Is that
> right or am I missing something?

You can simply look at the first hash value in the intermediate
dx_node (remember, an empty intermediate node is not allowed), and
then do a dx_probe to search from the root and validate that we find
our way back to the last block.

> Looking at your following comment, if metadata_csum feature is
> enabled, then we can distinguish if a block is an empty dirent block
> or an index block based on dentry->rec_len. If metadata csum is
> enabled, then for index blocks, fake_dentry->rec_len is set to
> blocksize while for a dirent not dentry->rec_len is set to blocksize -
> sizeof(ext4_dir_entry_tail). Is my understanding correct?

Yes.  Although even if the metadata_csum feature is enabled, it's a
good idea to search from the root to make sure this really is the
intermediate dx node block that you are looking for.  You need to do
the dx_probe() to find its parent block anyway, in order to update it.
And if you don't find a pointer to that intermediate node, then it
must not be an a dx node --- or the htree pointers are corrupted.  In
the case of metadata_csum and a dx_tail, that sequence should never
occur normally, so if you don't find an entry for that block when
doing a dx_probe(), it's likely the directory structures have gotten
corrupted.

						- Ted