Re: [PATCH 1/9] mm/rmap: recompute pgoff for huge page

[Andrew Morton]

On Thu, 28 Nov 2013 16:48:38 +0900 Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:

> We have to recompute pgoff if the given page is huge, since result based
> on HPAGE_SIZE is not approapriate for scanning the vma interval tree, as
> shown by commit 36e4f20af833 ("hugetlb: do not use vma_hugecache_offset()
> for vma_prio_tree_foreach") and commit 369a713e ("rmap: recompute pgoff
> for unmapping huge page").
> 
> ...
>
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1714,6 +1714,10 @@ static int rmap_walk_file(struct page *page, int (*rmap_one)(struct page *,
>  
>  	if (!mapping)
>  		return ret;
> +
> +	if (PageHuge(page))
> +		pgoff = page->index << compound_order(page);
> +
>  	mutex_lock(&mapping->i_mmap_mutex);
>  	vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
>  		unsigned long address = vma_address(page, vma);

a)  Can't we just do this?

--- a/mm/rmap.c~mm-rmap-recompute-pgoff-for-huge-page-fix
+++ a/mm/rmap.c
@@ -1708,16 +1708,13 @@ static int rmap_walk_file(struct page *p
 		struct vm_area_struct *, unsigned long, void *), void *arg)
 {
 	struct address_space *mapping = page->mapping;
-	pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
+	pgoff_t pgoff = page->index << compound_order(page);
 	struct vm_area_struct *vma;
 	int ret = SWAP_AGAIN;
 
 	if (!mapping)
 		return ret;
 
-	if (PageHuge(page))
-		pgoff = page->index << compound_order(page);
-
 	mutex_lock(&mapping->i_mmap_mutex);
 	vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
 		unsigned long address = vma_address(page, vma);

compound_order() does the right thing for all styles of page, yes?

b) If that PageHuge() test you added the correct thing to use?

/*
 * PageHuge() only returns true for hugetlbfs pages, but not for normal or
 * transparent huge pages.  See the PageTransHuge() documentation for more
 * details.
 */

   Obviously we won't be encountering transparent huge pages here,
   but what's the best future-safe approach?


I hate that PageHuge() oddity with a passion!  Maybe it would be better
if it was called PageHugetlbfs.
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