Re: [GIT PULL] multi-layer support for overlay filesystem

From: Al Viro
Date: Thu Feb 19 2015 - 02:42:13 EST

On Wed, Feb 18, 2015 at 11:59:03AM +0100, Miklos Szeredi wrote:
> On Wed, Feb 11, 2015 at 10:30:39AM +0100, Miklos Szeredi wrote:
> > Al,
> >
> > Please pull from
> >
> > git:// overlayfs-next
> >
> > This adds support for multiple read-only layers to overlayfs. It also makes the
> > writable upper layer optional.
> >
> > This is a highly requested feature and has been in -next for the last cycle.
> Ping.

Pong. I'm still looking through a large pile of assorted stuff, that pull
request sitting in the queue. Should be done with that tomorrow (as in "pull
or reply with last-minute-found $SOMETHING_DIRE"; FWIW, the last time I looked
at multilayer stuff was a couple of weeks ago and nothing dire had been spotted,
so the former is more likely).

ObSomethingDire^H^H^H^HOddElsewhere: what happens if libfuse
fuse_lowlevel_notify_store() is called with SPLICE_F_MOVE in flags on an
inode that happens to be mmaped with MAP_SHARED? Cache coherency isn't the
main concern; I'm looking at
* This is a new and locked page, it shouldn't be mapped or
* have any special flags on it
if (WARN_ON(page_mapped(oldpage)))
goto out_fallback_unlock;
if (WARN_ON(page_has_private(oldpage)))
goto out_fallback_unlock;
if (WARN_ON(PageDirty(oldpage) || PageWriteback(oldpage)))
goto out_fallback_unlock;
if (WARN_ON(PageMlocked(oldpage)))
goto out_fallback_unlock;
in fuse_try_move_page() and I don't see anything to prevent those
WARN_ON getting triggered in that case. The call chain is
fuse_try_move_page() <- fuse_copy_page() <- fuse_notify_store() <-
fuse_notify() <- fuse_dev_do_write() <- fuse_dev_splice_write(),
oldpage is obtained by
page = find_or_create_page(mapping, index,
in the loop in fuse_notify_store() and that ought to be able to pick
the pages present in MAP_SHARED mappings...

What am I missing here? FWIW, I'd been trying to resurrect SPLICE_F_MOVE
for local filesystems, and with FUSE being the only place in the kernel
still trying to support it...
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