Re: [PATCH] mm: Exclude HugeTLB pages from THP page_mapped logic

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 29 Mar 2016 17:39:41 +0100 Steve Capper <steve.capper@arm.com> wrote:

> HugeTLB pages cannot be split, thus use the compound_mapcount to
> track rmaps.
> 
> Currently the page_mapped function will check the compound_mapcount, but

s/the page_mapped function/page_mapped()/.  It's so much simpler!

> will also go through the constituent pages of a THP compound page and
> query the individual _mapcount's too.
> 
> Unfortunately, the page_mapped function does not distinguish between
> HugeTLB and THP compound pages and assumes that a compound page always
> needs to have HPAGE_PMD_NR pages querying.
> 
> For most cases when dealing with HugeTLB this is just inefficient, but
> for scenarios where the HugeTLB page size is less than the pmd block
> size (e.g. when using contiguous bit on ARM) this can lead to crashes.
> 
> This patch adjusts the page_mapped function such that we skip the
> unnecessary THP reference checks for HugeTLB pages.
> 
> Fixes: e1534ae95004 ("mm: differentiate page_mapped() from page_mapcount() for compound pages")
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Signed-off-by: Steve Capper <steve.capper@arm.com>
> ---
> 
> Hi,
> 
> This patch is my approach to fixing a problem that unearthed with
> HugeTLB pages on arm64. We ran with PAGE_SIZE=64KB and placed down 32
> contiguous ptes to create 2MB HugeTLB pages. (We can provide hints to
> the MMU that page table entries are contiguous thus larger TLB entries
> can be used to represent them).

So which kernel version(s) need this patch?  I think both 4.4 and 4.5
will crash in this manner?  Should we backport the fix into 4.4.x and
4.5.x?

>
> ...
>
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1031,6 +1031,8 @@ static inline bool page_mapped(struct page *page)
>  	page = compound_head(page);
>  	if (atomic_read(compound_mapcount_ptr(page)) >= 0)
>  		return true;
> +	if (PageHuge(page))
> +		return false;
>  	for (i = 0; i < hpage_nr_pages(page); i++) {
>  		if (atomic_read(&page[i]._mapcount) >= 0)
>  			return true;

page_mapped() is moronically huge.  Uninlining it saves 206 bytes per
callsite. It has 40+ callsites.




btw, is anyone else seeing this `make M=' breakage?

akpm3:/usr/src/25> make M=mm
Makefile:679: Cannot use CONFIG_KCOV: -fsanitize-coverage=trace-pc is not supported by compiler

  WARNING: Symbol version dump ./Module.symvers
           is missing; modules will have no dependencies and modversions.

make[1]: *** No rule to make target `mm/filemap.o', needed by `mm/built-in.o'.  Stop.
make: *** [_module_mm] Error 2

It's a post-4.5 thing.



From: Andrew Morton <akpm@linux-foundation.org>
Subject: mm: uninline page_mapped()

It's huge.  Uninlining it saves 206 bytes per callsite.  Shaves 4924 bytes
from the x86_64 allmodconfig vmlinux.

Cc: Steve Capper <steve.capper@arm.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/mm.h |   21 +--------------------
 mm/util.c          |   22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+), 20 deletions(-)

diff -puN include/linux/mm.h~mm-uninline-page_mapped include/linux/mm.h
--- a/include/linux/mm.h~mm-uninline-page_mapped
+++ a/include/linux/mm.h
@@ -1019,26 +1019,7 @@ static inline pgoff_t page_file_index(st
 	return page->index;
 }
 
-/*
- * Return true if this page is mapped into pagetables.
- * For compound page it returns true if any subpage of compound page is mapped.
- */
-static inline bool page_mapped(struct page *page)
-{
-	int i;
-	if (likely(!PageCompound(page)))
-		return atomic_read(&page->_mapcount) >= 0;
-	page = compound_head(page);
-	if (atomic_read(compound_mapcount_ptr(page)) >= 0)
-		return true;
-	if (PageHuge(page))
-		return false;
-	for (i = 0; i < hpage_nr_pages(page); i++) {
-		if (atomic_read(&page[i]._mapcount) >= 0)
-			return true;
-	}
-	return false;
-}
+bool page_mapped(struct page *page);
 
 /*
  * Return true only if the page has been allocated with
diff -puN mm/util.c~mm-uninline-page_mapped mm/util.c
--- a/mm/util.c~mm-uninline-page_mapped
+++ a/mm/util.c
@@ -346,6 +346,28 @@ void *page_rmapping(struct page *page)
 	return __page_rmapping(page);
 }
 
+/*
+ * Return true if this page is mapped into pagetables.
+ * For compound page it returns true if any subpage of compound page is mapped.
+ */
+bool page_mapped(struct page *page)
+{
+	int i;
+	if (likely(!PageCompound(page)))
+		return atomic_read(&page->_mapcount) >= 0;
+	page = compound_head(page);
+	if (atomic_read(compound_mapcount_ptr(page)) >= 0)
+		return true;
+	if (PageHuge(page))
+		return false;
+	for (i = 0; i < hpage_nr_pages(page); i++) {
+		if (atomic_read(&page[i]._mapcount) >= 0)
+			return true;
+	}
+	return false;
+}
+EXPORT_SYMBOL(page_mapped);
+
 struct anon_vma *page_anon_vma(struct page *page)
 {
 	unsigned long mapping;
_

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