Re: [rfc][patch] mm: madvise(WILLNEED) for anonymous memory

[Peter Zijlstra]

On Thu, 2007-12-20 at 14:09 +0000, Hugh Dickins wrote:
> On Thu, 20 Dec 2007, Peter Zijlstra wrote:
> > 
> > Lennart asked for madvise(WILLNEED) to work on anonymous pages, he plans
> > to use this to pre-fault pages. He currently uses: mlock/munlock for
> > this purpose.
> 
> I certainly agree with this in principle: it just seems an unnecessary
> and surprising restriction to refuse on anonymous vmas; I guess the only
> reason for not adding this was not having anyone asking for it until now.
> Though, does Lennart realize he could use MAP_POPULATE in the mmap?

I think he's trying to get his data swapped-in.

> > 
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > ---
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 93ee375..eff60ce 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -100,6 +100,24 @@ out:
> >  	return error;
> >  }
> >  
> > +static long madvice_willneed_anon(struct vm_area_struct *vma,
> > +				  struct vm_area_struct **prev,
> > +				  unsigned long start, unsigned long end)
> 
> mavise.c uses "madvise_" rather than " madvice_" throughout,
> so please go with the flow.

Ah, quite. I hadn't noticed this, will fix.

> > +{
> > +	int ret, len;
> > +
> > +	*prev = vma;
> > +	if (end > vma->vm_end)
> > +		end = vma->vm_end;
> 
> Please check, but I think the upper level ensures end is within range.

It certainly looks like it, but I since the file case did this check I
thought it prudent to also do it. I guess I might as well remove both.

> > +
> > +	len = end - start;
> > +	ret = get_user_pages(current, current->mm, start, len,
> > +			0, 0, NULL, NULL);
> > +	if (ret < 0)
> > +		return ret;
> > +	return ret == len ? 0 : -1;
> 
> It's not good to return -1 as an alternative to a real errno:
> it'll look like -EPERM.  If you copied that from somewhere, better
> send a patch to fix the somewhere!  Ah, yes, make_pages_present: it
> happens that nobody is interested in its return value, so we could
> make it a void; but that'd just be a cleanup.  What to do here if
> non-negative ret less than len?  Oh, just return 0, that's good
> enough in this case (the file case always returns 0).

ok, return 0; it is.

> Hmm, might it be better to use make_pages_present itself,
> fixing its retval, rather than using get_user_pages directly?
> (I'd hope the caching makes its repeat of find_vma not an overhead.)
> 
> Interesting divergence: make_pages_present faults in writable pages
> in a writable vma, whereas the file case's force_page_cache_readahead
> doesn't even insert the pages into the mm.

Yeah, the find_vma and write fault thing are the reason I didn't use
make_pages_present.

I had noticed the difference in pte population between
force_page_cache_readahead and make_pages_present, but it seemed to me
that writing a function to walk the page tables and populate the
swapcache but not populate the ptes wasn't worth the effort.

> > +}
> > +
> >  /*
> >   * Schedule all required I/O operations.  Do not wait for completion.
> >   */
> > @@ -110,7 +128,7 @@ static long madvise_willneed(struct vm_area_struct * vma,
> >  	struct file *file = vma->vm_file;
> >  
> >  	if (!file)
> > -		return -EBADF;
> > +		return madvice_willneed_anon(vma, prev, start, end);
> >  
> >  	if (file->f_mapping->a_ops->get_xip_page) {
> >  		/* no bad return value, but ignore advice */
> 
> And there's a correctly invisible hunk to the patch too: this
> extension of MADV_WILLNEED also does not require down_write of
> mmap_sem, so madvise_need_mmap_write can remain unchanged.

Indeed, I did check that :-)



Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
diff --git a/mm/madvise.c b/mm/madvise.c
index 93ee375..563bf00 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -100,6 +100,21 @@ out:
 	return error;
 }
 
+static long madvise_willneed_anon(struct vm_area_struct *vma,
+				  struct vm_area_struct **prev,
+				  unsigned long start, unsigned long end)
+{
+	int ret;
+
+	*prev = vma;
+	ret = get_user_pages(current, current->mm, start, end - start,
+			0, 0, NULL, NULL);
+	if (ret < 0)
+		return ret;
+
+	return 0;
+}
+
 /*
  * Schedule all required I/O operations.  Do not wait for completion.
  */
@@ -110,7 +125,7 @@ static long madvise_willneed(struct vm_area_struct * vma,
 	struct file *file = vma->vm_file;
 
 	if (!file)
-		return -EBADF;
+		return madvise_willneed_anon(vma, prev, start, end);
 
 	if (file->f_mapping->a_ops->get_xip_page) {
 		/* no bad return value, but ignore advice */
@@ -119,8 +134,6 @@ static long madvise_willneed(struct vm_area_struct * vma,
 
 	*prev = vma;
 	start = ((start - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
-	if (end > vma->vm_end)
-		end = vma->vm_end;
 	end = ((end - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
 
 	force_page_cache_readahead(file->f_mapping,


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