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?