Re: [PATCH 1/3] [FIX] hugetlb: Fix broken fs quota management

[Adam Litke <agl@us.ibm.com>]

On Wed, 2007-10-24 at 11:43 -0700, Dave Hansen wrote:
> On Wed, 2007-10-24 at 06:23 -0700, Adam Litke wrote:
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -685,7 +685,17 @@ void __unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start,
> >  	flush_tlb_range(vma, start, end);
> >  	list_for_each_entry_safe(page, tmp, &page_list, lru) {
> >  		list_del(&page->lru);
> > -		put_page(page);
> > +		if (put_page_testzero(page)) {
> > +			/*
> > +			 * When releasing the last reference to a page we must
> > +			 * credit the quota.  For MAP_PRIVATE pages this occurs
> > +			 * when the last PTE is cleared, for MAP_SHARED pages
> > +			 * this occurs when the file is truncated.
> > +			 */
> > +			VM_BUG_ON(PageMapping(page));
> > +			hugetlb_put_quota(vma->vm_file->f_mapping);
> > +			free_huge_page(page);
> > +		}
> >  	}
> >  }
> 
> That's a pretty good mechanism to use.   
> 
> This particular nugget is for MAP_PRIVATE pages only, right?  The shared
> ones should have another ref out on them for the 'mapping' too, so won't
> get released at unmap, right?

Yep that's right.  Shared pages are released by truncate_hugepages()
when the ref for the mapping is dropped.

> It isn't obvious from the context here, but how did free_huge_page() get
> called for these pages before?

put_page() calls put_compound_page() which looks up the page dtor
(free_huge_page) and calls it.

> Can you use free_pages_check() here to get some more comprehensive page
> tests?  Just a cursory look at it makes me think that it should work.  

Yeah, that should work but I am a little hesitant to use
free_pages_check() because it may happen to "just work."  None of the
other hugetlb code concerns itself with all of those page flags so it
seems a little out of place to start caring only here.

-- 
Adam Litke - (agl at us.ibm.com)
IBM Linux Technology Center

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