[PATCH] proc: fix pagemap_read() error case (was Re: [PATCH] proc: put check_mem_permission before __get_free_page in mem_read)

[KOSAKI Motohiro]

> Hi High,
> 
> > On Sun, 17 Apr 2011, bookjovi@xxxxxxxxx wrote:
> > > From: Jovi Zhang <bookjovi@xxxxxxxxx>
> > > 
> > > It should be better if put check_mem_permission before __get_free_page
> > > in mem_read, to be same as function mem_write.
> > > 
> > > Signed-off-by: Jovi Zhang <bookjovi@xxxxxxxxx>
> > 
> > Sorry to be contrary, but I disagree with this.  I'm all for consistency,
> > but is there a particular reason why you think the mem_write ordering is
> > right and mem_read wrong?
> > 
> > My reason for preferring the current mem_read ordering is this:
> > 
> > check_mem_permission gets a reference to the mm.  If we __get_free_page
> > after check_mem_permission, imagine what happens if the system is out
> > of memory, and the mm we're looking at is selected for killing by the
> > OOM killer: while we wait in __get_free_page for more memory, no memory
> > is freed from the selected mm because it cannot reach exit_mmap while
> > we hold that reference.
> 
> Right.
> 
> sorry for that. I missed this point.
> 
> 
> > (I may be overstating the case: a little memory may be freed from the
> > exiting task's stack, and kswapd should still be able to pick some pages
> > off the mm.  But nonetheless, we would do better to let this mm go.)
> > 
> > No doubt there are plenty of other places in /proc which try to
> > allocate memory after taking a reference on an mm; but I think
> > we should be working to eliminate those rather than add to them.
> 
> then, Should we change mem_write instead?

I've finished audit other /proc allocation callsite. If my understand
is correct, only pagemap_read() has the same issue.

fixed.