Re: [RFC] Support for stackable file systems on top of nfs

["John T. Kohl" <jtk@us.ibm.com>]

>>>>> "Nikolai" == Nikolai Joukov <kolya@cs.sunysb.edu> writes:

Nikolai> Here is a kludge that works in many cases:

Nikolai> int stackable_readpage(file_t *file, page_t *page)
Nikolai> {
Nikolai>  ...
Nikolai>  page->mapping = lower_inode->i_mapping;
Nikolai>  err = lower_inode->i_mapping->a_ops->readpage(lower_file, page);
Nikolai>  page->mapping = inode->i_mapping;
Nikolai>  ...
Nikolai> }

What are the locking requirements to do this safely?  Are the proper
locks held (or safely grabbable during readpage()) to avoid a race with
another paging operation?
I see a troubling comment in do_generic_mapping_read():

		/* ... and start the actual read. The read will unlock the page. */
		error = mapping->a_ops->readpage(filp, page);

but I've not studied the page locking design to know for sure that this
example is race-free.

-- 
John Kohl
Senior Software Engineer
Rational Software
IBM Software Group
Lexington, Massachusetts, USA
jtk@us.ibm.com

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