Re: [patch][rfc] mm: hold page lock over page_mkwrite

[Zach Brown <zach.brown@oracle.com>]

Nick Piggin wrote:
> I want to have the page be protected by page lock between page_mkwrite
> notification to the filesystem, and the actual setting of the page
> dirty. Do this by holding the page lock over page_mkwrite, and keep it
> held until after set_page_dirty.

I fear that this will end up creating lock inversions with file systems
who grab cross-node locks, which are ordered outside of the page lock,
inside their ->page_mkwrite().  See ocfs2's call of ocfs2_inode_lock()
from ocfs2_page_mkwrite().

In a sense, it's prepare_write() all over again.  Please don't call into
file systems after having acquired the page lock.

Instead, can we get a helper that the file system would call after
having acquired its locks and the page lock?  Something like that.

- z

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