Re: page->mapping == 0

[Daniel Phillips <news-innominate.list.linux.kernel@innominate.de>]

Linus Torvalds wrote:
> Now, I don't think this is really a valid usage pattern, so it's most
> likely to be a result of a buggy application, but I can imagine having
> some strange kind of user-space VM memory management scheme that depends
> on SIGBUS to maintain a file length. I've never heard of such a thing, but
> I could imagine somebody doing some kind of persistent data object store
> in user space this way.
> 
> Does anybody actually know of an application that does something like
> this? Because I'm more and more inclined to just going with the half-fix
> for now. It would at least guarantee internal kernel data consistency (and
> no oopses, of course).
> 
> Don't get me wrong - we need to clean this part up, but as far as I can
> tell we have never done this "right", so in that sense it's not a new
> 2.4.x bug and it can't break existing applications.

Strace shows that the history really is updated with no locking:

  4030  open("/home/phillips/.bash_history", O_WRONLY|O_TRUNC) = 4
  4030  write(4, "l\nless README\ne ../*test8/fs/ext"..., 7546) = 7546
  4030  close(4)                          =
0                                                                                                

There's no reason to continue doing the read after you've detected the
page was pulled out from under you - clearly it's a user error state,
the only question is: return the error and break the buggy application? 
Or gloss over it.  And it seems like returning the error might actually
break bash - maybe it deserves it... but...

--
Daniel
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