Re: [PATCH] Bug #3054 madvise(MADV_WILLNEED,...) fix for exceeding rlimit rss

[Matt Keenan <matthew.keenan@ntlworld.com>]

Chris Wright wrote:

> * Matt Keenan (matthew.keenan@ntlworld.com) wrote:
>  
>
> > --- linux-2.6.11.7/mm/madvise.c 2005-04-12 15:58:30.000000000 +0100
> > +++ linux/mm/madvise.c  2005-06-19 17:20:56.000000000 +0100
> > @@ -61,6 +61,7 @@ static long madvise_willneed(struct vm_a
> >                            unsigned long start, unsigned long end)
> > {
> >       struct file *file = vma->vm_file;
> > +       struct task_struct *tsk = current;
> >    
>
> Looks like you've got some tab/whitespace damage going on here.
>
>  
Damn mailer! I might have to go back to mutt.

> >       if (!file)
> >               return -EBADF;
> > @@ -70,6 +71,28 @@ static long madvise_willneed(struct vm_a
> >               end = vma->vm_end;
> >       end = ((end - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
> >
> > +       /*
> > +        * This code below checks to see if mapping the requested
> > +        * readahead would make the task's rss exceed the task's
> > +        * rlimit rss.
> > +        *
> > +        * This doesn't account for pages that may already be mapped
> > +        * due to readahead, but since this is merely a hint to the
> > +        * kernel no harm should be done, it won't unmap anything
> > +        * already mapped if it fails. N.B. This won't affect the
> > +        * kernel's internal automatic readahead which doesn't check
> > +        * (or honour) rlimit rss.
> > +        */
> > +
> > +       spin_lock(&tsk->mm->page_table_lock);
> > +       if (((max_sane_readahead(end-start) << PAGE_SHIFT) +
> > +           tsk->mm->_rss) > tsk->signal->rlim[RLIMIT_RSS].rlim_cur)
> >    
>
> I doubt this one would overflow, but we recenly made changes in similar
> tests to use page count rather than byte count.  I belive this should
> use get_mm_counter().  Isn't _rss counting pages rather than bytes,
> so I think the math is off here.  Something like:
>
> 	if ((max_sane_readahead(end - start) + get_mm_counter(tsk->mm, rss)) >
> 	    tsk->signal->rlim[RLIMIT_RSS].rlim_cur >> PAGE_SHIFT)
>
>  
Ok, here is the patch again with the suggested fixes. I have attached 
the patch (yes yes i know!), that will probably screw up people's mail 
-> patch generators, but the whitespace issue should be fixed. Hopefully 
this is ok, I'm going to bang on it for an hour or so here to make sure.

Signed-off-by: Matthew Keenan <matthew.keenan@ntlworld.com>


--- linux-2.6.11.7/mm/madvise.c	2005-04-12 15:58:30.000000000 +0100
+++ linux/mm/madvise.c	2005-06-19 21:39:57.000000000 +0100
@@ -61,6 +61,7 @@ static long madvise_willneed(struct vm_a
 			     unsigned long start, unsigned long end)
 {
 	struct file *file = vma->vm_file;
+	struct task_struct *tsk = current;
 
 	if (!file)
 		return -EBADF;
@@ -70,6 +71,28 @@ static long madvise_willneed(struct vm_a
 		end = vma->vm_end;
 	end = ((end - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
 
+	/*
+	 * This code below checks to see if mapping the requested
+	 * readahead would make the task's rss exceed the task's
+	 * rlimit rss.
+	 *
+	 * This doesn't account for pages that may already be mapped
+	 * due to readahead, but since this is merely a hint to the
+	 * kernel no harm should be done, it won't unmap anything
+	 * already mapped if it fails. N.B. This won't affect the
+	 * kernel's internal automatic readahead which doesn't check
+	 * (or honour) rlimit rss.
+	 */
+
+	spin_lock(&tsk->mm->page_table_lock);
+	if ((max_sane_readahead(end - start) + get_mm_counter(tsk->mm, rss)) >
+	    (tsk->signal->rlim[RLIMIT_RSS].rlim_cur >> PAGE_SHIFT))
+	{
+		spin_unlock(&tsk->mm->page_table_lock);
+		return -EIO;
+	}
+	spin_unlock(&tsk->mm->page_table_lock);
+
 	force_page_cache_readahead(file->f_mapping,
 			file, start, max_sane_readahead(end - start));
 	return 0;
@@ -170,6 +193,8 @@ static long madvise_vma(struct vm_area_s
  *  -ENOMEM - addresses in the specified range are not currently
  *		mapped, or are outside the AS of the process.
  *  -EIO    - an I/O error occurred while paging in data.
+ *          - MADV_WILLNEED would map in pages that would make the task's
+ *              rss exceed rlimit rss.
  *  -EBADF  - map exists, but area maps something that isn't a file.
  *  -EAGAIN - a kernel resource was temporarily unavailable.
  */