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ovl: document lower modification caveats

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Some overlayfs optional features are incompatible with offline changes to
the lower tree and may result in -EXDEV, -EIO, or other errors.  Such
modification is not supported and the error behavior is intentionally not
specified.

Update the "Changes to underlying filesystems" section to note this
restriction.  Move the paragraph describing the offline behavior below the
online behavior so it is adjacent to the following 3 paragraphs describing
the NFS export offline modification behavior.

Link: https://lore.kernel.org/linux-unionfs/20200708142353.GA103536@redhat.com/
Link: https://lore.kernel.org/linux-unionfs/CAOQ4uxi23Zsmfb4rCed1n=On0NNA5KZD74jjjeyz+et32sk-gg@mail.gmail.com/
Link: https://lore.kernel.org/linux-unionfs/20200817135651.GA637139@redhat.com/
Link: https://lore.kernel.org/linux-unionfs/20200709153616.GE150543@redhat.com/
Link: https://lore.kernel.org/linux-unionfs/20200812135529.GA122370@kevinolos/
Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
This commit is contained in:
Kevin Locke 2020-08-22 20:22:57 -06:00 committed by Miklos Szeredi
parent 0a8d0b64dd
commit 13c6ad0f45
1 changed files with 7 additions and 3 deletions
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10
Documentation/filesystems/overlayfs.rst
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@ -469,14 +469,18 @@ summarized in the `Inode properties`_ table above.
Changes to underlying filesystems Changes to underlying filesystems
--------------------------------- ---------------------------------
Offline changes, when the overlay is not mounted, are allowed to either
the upper or the lower trees.
Changes to the underlying filesystems while part of a mounted overlay Changes to the underlying filesystems while part of a mounted overlay
filesystem are not allowed. If the underlying filesystem is changed, filesystem are not allowed. If the underlying filesystem is changed,
the behavior of the overlay is undefined, though it will not result in the behavior of the overlay is undefined, though it will not result in
a crash or deadlock. a crash or deadlock.
Offline changes, when the overlay is not mounted, are allowed to the
upper tree. Offline changes to the lower tree are only allowed if the
"metadata only copy up", "inode index", and "redirect_dir" features
have not been used. If the lower tree is modified and any of these
features has been used, the behavior of the overlay is undefined,
though it will not result in a crash or deadlock.
When the overlay NFS export feature is enabled, overlay filesystems When the overlay NFS export feature is enabled, overlay filesystems
behavior on offline changes of the underlying lower layer is different behavior on offline changes of the underlying lower layer is different
than the behavior when NFS export is disabled. than the behavior when NFS export is disabled.