Re: [PATCHv6 26/36] mm: rework mapcount accounting to enable 4k mapping of THPs

From: Vlastimil Babka
Date: Wed Jun 10 2015 - 09:47:23 EST

On 06/03/2015 07:05 PM, Kirill A. Shutemov wrote:
We're going to allow mapping of individual 4k pages of THP compound.
It means we need to track mapcount on per small page basis.

Straight-forward approach is to use ->_mapcount in all subpages to track
how many time this subpage is mapped with PMDs or PTEs combined. But
this is rather expensive: mapping or unmapping of a THP page with PMD
would require HPAGE_PMD_NR atomic operations instead of single we have
now.

The idea is to store separately how many times the page was mapped as
whole -- compound_mapcount. This frees up ->_mapcount in subpages to
track PTE mapcount.

We use the same approach as with compound page destructor and compound
order to store compound_mapcount: use space in first tail page,
->mapping this time.

Any time we map/unmap whole compound page (THP or hugetlb) -- we
increment/decrement compound_mapcount. When we map part of compound page
with PTE we operate on ->_mapcount of the subpage.

page_mapcount() counts both: PTE and PMD mappings of the page.

Basically, we have mapcount for a subpage spread over two counters.
It makes tricky to detect when last mapcount for a page goes away.

We introduced PageDoubleMap() for this. When we split THP PMD for the
first time and there's other PMD mapping left we offset up ->_mapcount
in all subpages by one and set PG_double_map on the compound page.
These additional references go away with last compound_mapcount.

This approach provides a way to detect when last mapcount goes away on
per small page basis without introducing new overhead for most common
cases.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
include/linux/mm.h | 26 +++++++++++-
include/linux/mm_types.h | 1 +
include/linux/page-flags.h | 37 +++++++++++++++++
include/linux/rmap.h | 4 +-
mm/debug.c | 5 ++-
mm/huge_memory.c | 2 +-
mm/hugetlb.c | 4 +-
mm/memory.c | 2 +-
mm/migrate.c | 2 +-
mm/page_alloc.c | 14 +++++--
mm/rmap.c | 98 +++++++++++++++++++++++++++++++++++-----------
11 files changed, 160 insertions(+), 35 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 31cd5be081cf..22cd540104ec 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -403,6 +403,19 @@ static inline int is_vmalloc_or_module_addr(const void *x)

extern void kvfree(const void *addr);

+static inline atomic_t *compound_mapcount_ptr(struct page *page)
+{
+ return &page[1].compound_mapcount;
+}
+
+static inline int compound_mapcount(struct page *page)
+{
+ if (!PageCompound(page))
+ return 0;
+ page = compound_head(page);
+ return atomic_read(compound_mapcount_ptr(page)) + 1;
+}
+
/*
* The atomic page->_mapcount, starts from -1: so that transitions
* both from it and to it can be tracked, using atomic_inc_and_test
@@ -415,8 +428,17 @@ static inline void page_mapcount_reset(struct page *page)

static inline int page_mapcount(struct page *page)
{
+ int ret;
VM_BUG_ON_PAGE(PageSlab(page), page);
- return atomic_read(&page->_mapcount) + 1;
+
+ ret = atomic_read(&page->_mapcount) + 1;
+ if (PageCompound(page)) {
+ page = compound_head(page);
+ ret += compound_mapcount(page);

compound_mapcount() means another PageCompound() and compound_head(), which you just did. I've tried this to see the effect on a function that "calls" (inlines) page_mapcount() once:

- ret += compound_mapcount(page);
+ ret += atomic_read(compound_mapcount_ptr(page)) + 1;

bloat-o-meter on compaction.o:
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-59 (-59)
function old new delta
isolate_migratepages_block 1769 1710 -59

+ if (PageDoubleMap(page))
+ ret--;
+ }
+ return ret;
}

static inline int page_count(struct page *page)
@@ -898,7 +920,7 @@ static inline pgoff_t page_file_index(struct page *page)
*/
static inline int page_mapped(struct page *page)
{
- return atomic_read(&(page)->_mapcount) >= 0;
+ return atomic_read(&(page)->_mapcount) + compound_mapcount(page) >= 0;
}

/*
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 4b51a59160ab..4d182cd14c1f 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -56,6 +56,7 @@ struct page {
* see PAGE_MAPPING_ANON below.
*/
void *s_mem; /* slab first object */
+ atomic_t compound_mapcount; /* first tail page */
};

/* Second double word */
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 74b7cece1dfa..a8d47c1edf6a 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -127,6 +127,9 @@ enum pageflags {

/* SLOB */
PG_slob_free = PG_private,
+
+ /* THP. Stored in first tail page's flags */
+ PG_double_map = PG_private_2,

Well, not just THP. Any user of compound pages must make sure not to use PG_private_2 on the first tail page. At least where the page is going to be user-mapped. And same thing about fields that are in union with compound_mapcount. Should that be documented more prominently somewhere? I guess there's no such user so far, right?

};

#ifndef __GENERATING_BOUNDS_H

[...]

@@ -1167,6 +1194,41 @@ out:
mem_cgroup_end_page_stat(memcg);
}

+static void page_remove_anon_compound_rmap(struct page *page)
+{
+ int i, nr;
+
+ if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
+ return;
+
+ /* Hugepages are not counted in NR_ANON_PAGES for now. */
+ if (unlikely(PageHuge(page)))
+ return;
+
+ if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
+ return;
+
+ __dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
+
+ if (PageDoubleMap(page)) {
+ nr = 0;
+ ClearPageDoubleMap(page);
+ /*
+ * Subpages can be mapped with PTEs too. Check how many of
+ * themi are still mapped.
+ */
+ for (i = 0; i < HPAGE_PMD_NR; i++) {
+ if (atomic_add_negative(-1, &page[i]._mapcount))
+ nr++;
+ }
+ } else {
+ nr = HPAGE_PMD_NR;
+ }
+
+ if (nr)
+ __mod_zone_page_state(page_zone(page), NR_ANON_PAGES, nr);

-nr as we discussed on IRC.

+}
+
/**
* page_remove_rmap - take down pte mapping from a page
* @page: page to remove mapping from
@@ -1176,33 +1238,25 @@ out:
*/
void page_remove_rmap(struct page *page, bool compound)
{
- int nr = compound ? hpage_nr_pages(page) : 1;
-
if (!PageAnon(page)) {
VM_BUG_ON_PAGE(compound && !PageHuge(page), page);
page_remove_file_rmap(page);
return;
}

+ if (compound)
+ return page_remove_anon_compound_rmap(page);
+
/* page still mapped by someone else? */
if (!atomic_add_negative(-1, &page->_mapcount))
return;

- /* Hugepages are not counted in NR_ANON_PAGES for now. */
- if (unlikely(PageHuge(page)))
- return;
-
/*
* We use the irq-unsafe __{inc|mod}_zone_page_stat because
* these counters are not modified in interrupt context, and
* pte lock(a spinlock) is held, which implies preemption disabled.
*/
- if (compound) {
- VM_BUG_ON_PAGE(!PageTransHuge(page), page);
- __dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
- }
-
- __mod_zone_page_state(page_zone(page), NR_ANON_PAGES, -nr);
+ __dec_zone_page_state(page, NR_ANON_PAGES);

if (unlikely(PageMlocked(page)))
clear_page_mlock(page);
@@ -1643,7 +1697,7 @@ void hugepage_add_anon_rmap(struct page *page,
BUG_ON(!PageLocked(page));
BUG_ON(!anon_vma);
/* address might be in next vma when migration races vma_adjust */
- first = atomic_inc_and_test(&page->_mapcount);
+ first = atomic_inc_and_test(compound_mapcount_ptr(page));
if (first)
__hugepage_set_anon_rmap(page, vma, address, 0);
}
@@ -1652,7 +1706,7 @@ void hugepage_add_new_anon_rmap(struct page *page,
struct vm_area_struct *vma, unsigned long address)
{
BUG_ON(address < vma->vm_start || address >= vma->vm_end);
- atomic_set(&page->_mapcount, 0);
+ atomic_set(compound_mapcount_ptr(page), 0);
__hugepage_set_anon_rmap(page, vma, address, 1);
}
#endif /* CONFIG_HUGETLB_PAGE */


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