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Re: beginners project: RENAME_WHITEOUT

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: beginners project: RENAME_WHITEOUT
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Sun, 9 Nov 2014 10:42:32 +1100
Cc: xfs@xxxxxxxxxxx, Miklos Szeredi <miklos@xxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20141107190959.GB21021@xxxxxxxxxxxxx>
References: <20141107190959.GB21021@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Fri, Nov 07, 2014 at 11:09:59AM -0800, Christoph Hellwig wrote:
> The overlayfs merge introduces a new rename flag to create to whiteouts.
> Should be a fairly easy to implement.
> Miklos, do you have any good documentation and/or test cases for this?

So overlayfs uses some weird char dev hack to implement whiteout
inodes in directories?  Why do we need a whiteout inode on disk?
what information is actually stored in the whiteout inode that
overlayfs actually needs?  Only readdir and lookup care about
whiteouts, and AFAICT nothing of the inode is ever used except
checking the chrdev/whiteoutdev hack via ovl_is_whiteout(dentry).

Indeed, whatever happened to just storing the whiteout in the dirent
via DT_WHT and using that information on lookup in the lower
filesystem to mark the dentry returned appropriately without needing
to lookup a real inode?


Dave Chinner

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