Re: [PATCHv5 01/28] mm, proc: adjust PSS calculation

[Kirill A. Shutemov]

On Thu, May 14, 2015 at 04:12:29PM +0200, Vlastimil Babka wrote:
> On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
> >With new refcounting all subpages of the compound page are not nessessary
> >have the same mapcount. We need to take into account mapcount of every
> >sub-page.
> >
> >Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> >Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> 
> (some nitpicks below)
> 
> >---
> >  fs/proc/task_mmu.c | 43 ++++++++++++++++++++++---------------------
> >  1 file changed, 22 insertions(+), 21 deletions(-)
> >
> >diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> >index 956b75d61809..95bc384ee3f7 100644
> >--- a/fs/proc/task_mmu.c
> >+++ b/fs/proc/task_mmu.c
> >@@ -449,9 +449,10 @@ struct mem_size_stats {
> >  };
> >
> >  static void smaps_account(struct mem_size_stats *mss, struct page *page,
> >-		unsigned long size, bool young, bool dirty)
> >+		bool compound, bool young, bool dirty)
> >  {
> >-	int mapcount;
> >+	int i, nr = compound ? hpage_nr_pages(page) : 1;
> 
> Why not just HPAGE_PMD_NR instead of hpage_nr_pages(page)?

Okay, makes sense. Compiler is smart enough to optimize away HPAGE_PMD_NR
for THP=n. (HPAGE_PMD_NR is BUILD_BUG() for THP=n)

> We already came here through a pmd mapping. Even if the page stopped
> being a hugepage meanwhile (I'm not sure if any locking prevents that or
> not?),

We're under ptl here. PMD will not go away under us.

> it would be more accurate to continue assuming it's a hugepage,
> otherwise we account only the base page (formerly head) and skip the 511
> formerly tail pages?
> 
> Also, is there some shortcut way to tell us that we are the only one mapping
> the whole compound page, and nobody has any base pages, so we don't need to
> loop on each tail page? I guess not under the new design, right...

No, we don't have shortcut here.

> >+	unsigned long size = nr * PAGE_SIZE;
> >
> >  	if (PageAnon(page))
> >  		mss->anonymous += size;
> >@@ -460,23 +461,23 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
> >  	/* Accumulate the size in pages that have been accessed. */
> >  	if (young || PageReferenced(page))
> >  		mss->referenced += size;
> >-	mapcount = page_mapcount(page);
> >-	if (mapcount >= 2) {
> >-		u64 pss_delta;
> >
> >-		if (dirty || PageDirty(page))
> >-			mss->shared_dirty += size;
> >-		else
> >-			mss->shared_clean += size;
> >-		pss_delta = (u64)size << PSS_SHIFT;
> >-		do_div(pss_delta, mapcount);
> >-		mss->pss += pss_delta;
> >-	} else {
> >-		if (dirty || PageDirty(page))
> >-			mss->private_dirty += size;
> >-		else
> >-			mss->private_clean += size;
> >-		mss->pss += (u64)size << PSS_SHIFT;
> >+	for (i = 0; i < nr; i++) {
> >+		int mapcount = page_mapcount(page + i);
> >+
> >+		if (mapcount >= 2) {
> >+			if (dirty || PageDirty(page + i))
> >+				mss->shared_dirty += PAGE_SIZE;
> >+			else
> >+				mss->shared_clean += PAGE_SIZE;
> >+			mss->pss += (PAGE_SIZE << PSS_SHIFT) / mapcount;
> >+		} else {
> >+			if (dirty || PageDirty(page + i))
> >+				mss->private_dirty += PAGE_SIZE;
> >+			else
> >+				mss->private_clean += PAGE_SIZE;
> >+			mss->pss += PAGE_SIZE << PSS_SHIFT;
> >+		}
> 
> That's 3 instances of "page + i", why not just use page and do a page++ in
> the for loop?

Okay.

-- 
 Kirill A. Shutemov
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