Re: [PATCH] AFS: Implement shared-writable mmap [try #2]

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David Howells wrote:
Nick Piggin <[email protected]> wrote:


You can drop the lock, do the invalidation,


Hmmm...  There's a danger of incurring a race by doing that.  Consider two
processes both trying to write to a dirty page for which writeback will be
rejected:

 (1) The first process gets EKEYREJECTED from the server, drops its lock and
     is then preempted.

 (2) The second process gets EKEYREJECTED from the server, drops its lock,
     truncates the page, reloads the page and modifies it.

 (3) The first process resumes and truncates the page, thereby splatting the
     second process's write.

Or:

 (1) The first process gets EKEYREJECTED from the server, clears the writeback
     information from the page, drops its lock and is then preempted.

 (2) The second process attaches its own writeback information to the page and
     modifies it.

 (3) The first process resumes and truncates the page, thereby splatting the
     second process's write.

If there are race issues involving concurrent invalidations, then you'd
fix that up by taking a filesystem specific lock to prevent them.

Generic write path should be holding i_mutex, but I don't think you can
take that from page_mkwrite... Just add one of your own.


Really, what I want to do is pass the page lock to truncate to deal with.
Better still, I want truncate to be selective, based on whether or not a page
is still associated with the rejected writeback.  I wonder if I should call
truncate_complete_page() or invalidate_complete_page() directly.

No, you shouldn't. We could theoretically introduce a new API for this,
but I think it would be preferable if you can fix the race in the fs.

--
SUSE Labs, Novell Inc.
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