Re: [PATCH] swapfile : fix the wrong return value

[Hugh Dickins <hugh.dickins@tiscali.co.uk>]

On Thu, 4 Mar 2010, Huang Shijie wrote:
> > 
> > swap_duplicate()'s loop appears to miss out on returning the error code
> > from __swap_duplicate(), except when that's -ENOMEM.  In fact this is
> > intentional: prior to -ENOMEM for swap_count_continuation, swap_duplicate()
> > was void (and the case only occurs when copy_one_pte() hits a corrupt pte).
> >    
> only?
> 
> There are several paths calling the try_to_unmap(), Could you sure that
> the swap entries are valid in all the paths ?

Yes.  Well, we are debating the likelihoods of corruption in different memory
areas here.  I answer "Yes" because the swap entry involved in try_to_unmap_one()
comes from page->private when PageSwapCache is set (and the page is locked):
it requires either an mm bug, or corruption of struct page, for that swap entry
to be invalid for duplication.  Memory corruption of entries in a user page
table seems to have been a more common case, whether because of single-bit memory
errors, or use-after-free bugs: that's the case which copy_one_pte() might meet.
 
> 
> For the sake of the stability of the system, I perfer to export all the error
> value, and check it carefully.

But we were happy with void swap_duplicate() for many years.
If I wanted to make a further change, it would rather be to remove those
error returns from __swap_duplicate() which are not actually made use of.

Hugh

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