details:

 We want, for now, to xor all chars in a page just before it is written to
 disk 
 (swap), with, say 'a' and xor all the chars in a page just after it is read
 
 from disk (swap).
 
 We do the xor's before writing in rw_page_swap_base() only if we came from 
 rw_page_swap().
 
 We do the xor's after readin in end_buffer_io_async() only if the page is 
 marked with a flag, which we raised in rw_page_swap(). 
 
 If we do it for specific pages - say pages that are all 'a's - it works.
 
 We can even change all-'a's pages to all-'b's pages during write, and change
 
 the 'b's to 'c's during reading.

 When we do it to all pages, however, it crashes...
 
 We suspect problems with the swap cache... If there is more then one
 reference 
 to a page in the swap cache, and this page is being written to disk, and we
 
 changed it and start io - what happens if other processes try to touch it?
 
 Would really appriciate help....
 We're quite desperate here, actually...
 

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