[PATCH 1/3] mm,hugetlb: use folio fields in second tail page

From: Hugh Dickins
Date: Wed Nov 02 2022 - 21:48:57 EST

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We want to declare one more int in the first tail of a compound page:
that first tail page being valuable property, since every compound page
has a first tail, but perhaps no more than that.

No problem on 64-bit: there is already space for it. No problem with
32-bit THPs: 5.18 commit 5232c63f46fd ("mm: Make compound_pincount always
available") kindly cleared the space for it, apparently not realizing
that only 64-bit architectures enable CONFIG_THP_SWAP (whose use of tail
page->private might conflict) - but make sure of that in its Kconfig.

But hugetlb pages use tail page->private of the first tail page for a
subpool pointer, which will conflict; and they also use page->private
of the 2nd, 3rd and 4th tails.

Undo "mm: add private field of first tail to struct page and struct
folio"'s recent addition of private_1 to the folio tail: instead add
hugetlb_subpool, hugetlb_cgroup, hugetlb_cgroup_rsvd, hugetlb_hwpoison
to a second tail page of the folio: THP has long been using several
fields of that tail, so make better use of it for hugetlb too.
This is not how a generic folio should be declared in future,
but it is an effective transitional way to make use of it.

Delete the SUBPAGE_INDEX stuff, but keep __NR_USED_SUBPAGE: now 3.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
include/linux/hugetlb.h | 23 +++--------
include/linux/hugetlb_cgroup.h | 31 +++++----------
include/linux/mm_types.h | 72 ++++++++++++++++++++++------------
mm/Kconfig | 2 +-
mm/memory-failure.c | 5 +--
5 files changed, 65 insertions(+), 68 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 65ea34022aa2..03ecf1c5e46f 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -33,22 +33,9 @@ typedef struct { unsigned long pd; } hugepd_t;
/*
* For HugeTLB page, there are more metadata to save in the struct page. But
* the head struct page cannot meet our needs, so we have to abuse other tail
- * struct page to store the metadata. In order to avoid conflicts caused by
- * subsequent use of more tail struct pages, we gather these discrete indexes
- * of tail struct page here.
+ * struct page to store the metadata.
*/
-enum {
- SUBPAGE_INDEX_SUBPOOL = 1, /* reuse page->private */
-#ifdef CONFIG_CGROUP_HUGETLB
- SUBPAGE_INDEX_CGROUP, /* reuse page->private */
- SUBPAGE_INDEX_CGROUP_RSVD, /* reuse page->private */
- __MAX_CGROUP_SUBPAGE_INDEX = SUBPAGE_INDEX_CGROUP_RSVD,
-#endif
-#ifdef CONFIG_MEMORY_FAILURE
- SUBPAGE_INDEX_HWPOISON,
-#endif
- __NR_USED_SUBPAGE,
-};
+#define __NR_USED_SUBPAGE 3

struct hugepage_subpool {
spinlock_t lock;
@@ -722,11 +709,11 @@ extern unsigned int default_hstate_idx;

static inline struct hugepage_subpool *hugetlb_folio_subpool(struct folio *folio)
{
- return (void *)folio_get_private_1(folio);
+ return folio->_hugetlb_subpool;
}

/*
- * hugetlb page subpool pointer located in hpage[1].private
+ * hugetlb page subpool pointer located in hpage[2].hugetlb_subpool
*/
static inline struct hugepage_subpool *hugetlb_page_subpool(struct page *hpage)
{
@@ -736,7 +723,7 @@ static inline struct hugepage_subpool *hugetlb_page_subpool(struct page *hpage)
static inline void hugetlb_set_folio_subpool(struct folio *folio,
struct hugepage_subpool *subpool)
{
- folio_set_private_1(folio, (unsigned long)subpool);
+ folio->_hugetlb_subpool = subpool;
}

static inline void hugetlb_set_page_subpool(struct page *hpage,
diff --git a/include/linux/hugetlb_cgroup.h b/include/linux/hugetlb_cgroup.h
index c70f92fe493e..f706626a8063 100644
--- a/include/linux/hugetlb_cgroup.h
+++ b/include/linux/hugetlb_cgroup.h
@@ -24,12 +24,10 @@ struct file_region;
#ifdef CONFIG_CGROUP_HUGETLB
/*
* Minimum page order trackable by hugetlb cgroup.
- * At least 4 pages are necessary for all the tracking information.
- * The second tail page (hpage[SUBPAGE_INDEX_CGROUP]) is the fault
- * usage cgroup. The third tail page (hpage[SUBPAGE_INDEX_CGROUP_RSVD])
- * is the reservation usage cgroup.
+ * At least 3 pages are necessary for all the tracking information.
+ * The second tail page contains all of the hugetlb-specific fields.
*/
-#define HUGETLB_CGROUP_MIN_ORDER order_base_2(__MAX_CGROUP_SUBPAGE_INDEX + 1)
+#define HUGETLB_CGROUP_MIN_ORDER order_base_2(__NR_USED_SUBPAGE)

enum hugetlb_memory_event {
HUGETLB_MAX,
@@ -69,21 +67,13 @@ struct hugetlb_cgroup {
static inline struct hugetlb_cgroup *
__hugetlb_cgroup_from_folio(struct folio *folio, bool rsvd)
{
- struct page *tail;
-
VM_BUG_ON_FOLIO(!folio_test_hugetlb(folio), folio);
if (folio_order(folio) < HUGETLB_CGROUP_MIN_ORDER)
return NULL;
-
- if (rsvd) {
- tail = folio_page(folio, SUBPAGE_INDEX_CGROUP_RSVD);
- return (void *)page_private(tail);
- }
-
- else {
- tail = folio_page(folio, SUBPAGE_INDEX_CGROUP);
- return (void *)page_private(tail);
- }
+ if (rsvd)
+ return folio->_hugetlb_cgroup_rsvd;
+ else
+ return folio->_hugetlb_cgroup;
}

static inline struct hugetlb_cgroup *hugetlb_cgroup_from_folio(struct folio *folio)
@@ -101,15 +91,12 @@ static inline void __set_hugetlb_cgroup(struct folio *folio,
struct hugetlb_cgroup *h_cg, bool rsvd)
{
VM_BUG_ON_FOLIO(!folio_test_hugetlb(folio), folio);
-
if (folio_order(folio) < HUGETLB_CGROUP_MIN_ORDER)
return;
if (rsvd)
- set_page_private(folio_page(folio, SUBPAGE_INDEX_CGROUP_RSVD),
- (unsigned long)h_cg);
+ folio->_hugetlb_cgroup_rsvd = h_cg;
else
- set_page_private(folio_page(folio, SUBPAGE_INDEX_CGROUP),
- (unsigned long)h_cg);
+ folio->_hugetlb_cgroup = h_cg;
}

static inline void set_hugetlb_cgroup(struct folio *folio,
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 834022721bc6..728eb6089bba 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -145,15 +145,22 @@ struct page {
atomic_t compound_pincount;
#ifdef CONFIG_64BIT
unsigned int compound_nr; /* 1 << compound_order */
- unsigned long _private_1;
#endif
};
- struct { /* Second tail page of compound page */
+ struct { /* Second tail page of transparent huge page */
unsigned long _compound_pad_1; /* compound_head */
unsigned long _compound_pad_2;
/* For both global and memcg */
struct list_head deferred_list;
};
+ struct { /* Second tail page of hugetlb page */
+ unsigned long _hugetlb_pad_1; /* compound_head */
+ void *hugetlb_subpool;
+ void *hugetlb_cgroup;
+ void *hugetlb_cgroup_rsvd;
+ void *hugetlb_hwpoison;
+ /* No more space on 32-bit: use third tail if more */
+ };
struct { /* Page table pages */
unsigned long _pt_pad_1; /* compound_head */
pgtable_t pmd_huge_pte; /* protected by page->ptl */
@@ -260,13 +267,16 @@ struct page {
* to find how many references there are to this folio.
* @memcg_data: Memory Control Group data.
* @_flags_1: For large folios, additional page flags.
- * @__head: Points to the folio. Do not use.
+ * @_head_1: Points to the folio. Do not use.
* @_folio_dtor: Which destructor to use for this folio.
* @_folio_order: Do not use directly, call folio_order().
* @_total_mapcount: Do not use directly, call folio_entire_mapcount().
* @_pincount: Do not use directly, call folio_maybe_dma_pinned().
* @_folio_nr_pages: Do not use directly, call folio_nr_pages().
- * @_private_1: Do not use directly, call folio_get_private_1().
+ * @_hugetlb_subpool: Do not use directly, use accessor in hugetlb.h.
+ * @_hugetlb_cgroup: Do not use directly, use accessor in hugetlb_cgroup.h.
+ * @_hugetlb_cgroup_rsvd: Do not use directly, use accessor in hugetlb_cgroup.h.
+ * @_hugetlb_hwpoison: Do not use directly, call raw_hwp_list_head().
*
* A folio is a physically, virtually and logically contiguous set
* of bytes. It is a power-of-two in size, and it is aligned to that
@@ -305,16 +315,31 @@ struct folio {
};
struct page page;
};
- unsigned long _flags_1;
- unsigned long __head;
- unsigned char _folio_dtor;
- unsigned char _folio_order;
- atomic_t _total_mapcount;
- atomic_t _pincount;
+ union {
+ struct {
+ unsigned long _flags_1;
+ unsigned long _head_1;
+ unsigned char _folio_dtor;
+ unsigned char _folio_order;
+ atomic_t _total_mapcount;
+ atomic_t _pincount;
#ifdef CONFIG_64BIT
- unsigned int _folio_nr_pages;
+ unsigned int _folio_nr_pages;
#endif
- unsigned long _private_1;
+ };
+ struct page page_1;
+ };
+ union {
+ struct {
+ unsigned long _flags_2;
+ unsigned long _head_2;
+ void *_hugetlb_subpool;
+ void *_hugetlb_cgroup;
+ void *_hugetlb_cgroup_rsvd;
+ void *_hugetlb_hwpoison;
+ };
+ struct page page_2;
+ };
};

#define FOLIO_MATCH(pg, fl) \
@@ -335,16 +360,25 @@ FOLIO_MATCH(memcg_data, memcg_data);
static_assert(offsetof(struct folio, fl) == \
offsetof(struct page, pg) + sizeof(struct page))
FOLIO_MATCH(flags, _flags_1);
-FOLIO_MATCH(compound_head, __head);
+FOLIO_MATCH(compound_head, _head_1);
FOLIO_MATCH(compound_dtor, _folio_dtor);
FOLIO_MATCH(compound_order, _folio_order);
FOLIO_MATCH(compound_mapcount, _total_mapcount);
FOLIO_MATCH(compound_pincount, _pincount);
#ifdef CONFIG_64BIT
FOLIO_MATCH(compound_nr, _folio_nr_pages);
-FOLIO_MATCH(_private_1, _private_1);
#endif
#undef FOLIO_MATCH
+#define FOLIO_MATCH(pg, fl) \
+ static_assert(offsetof(struct folio, fl) == \
+ offsetof(struct page, pg) + 2 * sizeof(struct page))
+FOLIO_MATCH(flags, _flags_2);
+FOLIO_MATCH(compound_head, _head_2);
+FOLIO_MATCH(hugetlb_subpool, _hugetlb_subpool);
+FOLIO_MATCH(hugetlb_cgroup, _hugetlb_cgroup);
+FOLIO_MATCH(hugetlb_cgroup_rsvd, _hugetlb_cgroup_rsvd);
+FOLIO_MATCH(hugetlb_hwpoison, _hugetlb_hwpoison);
+#undef FOLIO_MATCH

static inline atomic_t *folio_mapcount_ptr(struct folio *folio)
{
@@ -388,16 +422,6 @@ static inline void *folio_get_private(struct folio *folio)
return folio->private;
}

-static inline void folio_set_private_1(struct folio *folio, unsigned long private)
-{
- folio->_private_1 = private;
-}
-
-static inline unsigned long folio_get_private_1(struct folio *folio)
-{
- return folio->_private_1;
-}
-
struct page_frag_cache {
void * va;
#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
diff --git a/mm/Kconfig b/mm/Kconfig
index 57e1d8c5b505..bc7e7dacfcd5 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -775,7 +775,7 @@ endchoice

config THP_SWAP
def_bool y
- depends on TRANSPARENT_HUGEPAGE && ARCH_WANTS_THP_SWAP && SWAP
+ depends on TRANSPARENT_HUGEPAGE && ARCH_WANTS_THP_SWAP && SWAP && 64BIT
help
Swap transparent huge pages in one piece, without splitting.
XXX: For now, swap cluster backing transparent huge page
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 779a426d2cab..63d8501001c6 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1687,8 +1687,7 @@ EXPORT_SYMBOL_GPL(mf_dax_kill_procs);
#ifdef CONFIG_HUGETLB_PAGE
/*
* Struct raw_hwp_page represents information about "raw error page",
- * constructing singly linked list originated from ->private field of
- * SUBPAGE_INDEX_HWPOISON-th tail page.
+ * constructing singly linked list from ->_hugetlb_hwpoison field of folio.
*/
struct raw_hwp_page {
struct llist_node node;
@@ -1697,7 +1696,7 @@ struct raw_hwp_page {

static inline struct llist_head *raw_hwp_list_head(struct page *hpage)
{
- return (struct llist_head *)&page_private(hpage + SUBPAGE_INDEX_HWPOISON);
+ return (struct llist_head *)&page_folio(hpage)->_hugetlb_hwpoison;
}

static unsigned long __free_raw_hwp_pages(struct page *hpage, bool move_flag)
--
2.35.3

Next message: Hugh Dickins: "[PATCH 2/3] mm,thp,rmap: simplify compound page mapcount handling"
Previous message: Xu Yilun: "Re: [PATCH v4 3/4] fpga: dfl: add basic support DFHv1"
In reply to: Hugh Dickins: "[PATCH 0/3] mm,huge,rmap: unify and speed up compound mapcounts"
Next in thread: Sidhartha Kumar: "Re: [PATCH 1/3] mm,hugetlb: use folio fields in second tail page"
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