Re: [RFC v3 2/4] mm: move PG_slab flag to page_type

Vlastimil Babka
2023-01-12T16:27:30Z

On 12/18/22 11:18, Hyeonggon Yoo wrote:
> For now, only SLAB uses _mapcount field as a number of active objects in
> a slab, and other slab allocators do not. As 16 bits are enough for that,
> use remaining 16 bits of _mapcount as page_type even when SLAB is used.
> And then move PG_slab flag to page_type.
>
> As suggested by Matthew, store number of active objects in negative
> form and use helper when accessing or modifying it.
>
> page_type is always placed in upper half of _mapcount to avoid
> confusing normal _mapcount as page_type. As underflow (actually I mean,
> yeah, overflow) is not a concern anymore, use more lower bits.
>
> Add more folio helpers for PAGE_TYPE_OPS() not to break existing
> slab implementations. To preserve current behavior apply page policy
> in PAGE_TYPE_OPS() and use PF_NO_TAIL for slab pages and PF_ANY for others.
>
> Remove PG_slab check from PAGE_FLAGS_CHECK_AT_FREE. buddy will still
> check if _mapcount is properly set at free.
>
> Note that with this change, page_mapped() and folio_mapped() always return
> false for a slab page.
>
> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

Seems like quite some changes to page_type to accomodate SLAB, which is
hopefully going away soon(TM). Could we perhaps avoid that?

- Alternative 1: convert only SLUB for now, let SLAB keep using PG_slab
until it's gone.

- Alternative 2: SLAB's s_mem field seems wasteful to be a full blown
pointer, it could share the word with "active" if it was an offset from
page_addr(), see how it's normally set up:

void *addr = slab_address(slab);
slab->s_mem = addr + colour_off;

so basically we would store the colour_off and perform this calculation
on-demand.

One caveat is KFENCE's kfence_guarded_alloc() which seems to be creating
some fake slab? And setting s_mem to something that maybe isn't an offset up
to 16 bits from slab_address(). That has to be investigated. Worst case,
SLAB is going away soon(TM) so it's fine if KFENCE only works with SLUB?

> ---
> fs/proc/page.c | 13 ++----
> include/linux/mm_types.h | 11 +++--
> include/linux/page-flags.h | 77 ++++++++++++++++++++++++----------
> include/trace/events/mmflags.h | 1 -
> kernel/crash_core.c | 3 +-
> mm/slab.c | 44 ++++++++++++-------
> mm/slab.h | 3 +-
> 7 files changed, 98 insertions(+), 54 deletions(-)
>
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 6249c347809a..e7524f21cefe 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -67,7 +67,7 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
> */
> ppage = pfn_to_online_page(pfn);
>
> - if (!ppage || PageSlab(ppage) || page_has_type(ppage))
> + if (!ppage || page_has_type(ppage))
> pcount = 0;
> else
> pcount = page_mapcount(ppage);
> @@ -124,11 +124,8 @@ u64 stable_page_flags(struct page *page)
>
> /*
> * pseudo flags for the well known (anonymous) memory mapped pages
> - *
> - * Note that page->_mapcount is overloaded in SLOB/SLUB/SLQB, so the
> - * simple test in page_mapped() is not enough.
> */
> - if (!PageSlab(page) && page_mapped(page))
> + if (page_mapped(page))
> u |= 1 << KPF_MMAP;
> if (PageAnon(page))
> u |= 1 << KPF_ANON;
> @@ -178,16 +175,14 @@ u64 stable_page_flags(struct page *page)
> u |= 1 << KPF_OFFLINE;
> if (PageTable(page))
> u |= 1 << KPF_PGTABLE;
> + if (PageSlab(page))
> + u |= 1 << KPF_SLAB;
>
> if (page_is_idle(page))
> u |= 1 << KPF_IDLE;
>
> u |= kpf_copy_bit(k, KPF_LOCKED, PG_locked);
>
> - u |= kpf_copy_bit(k, KPF_SLAB, PG_slab);
> - if (PageTail(page) && PageSlab(compound_head(page)))
> - u |= 1 << KPF_SLAB;
> -
> u |= kpf_copy_bit(k, KPF_ERROR, PG_error);
> u |= kpf_copy_bit(k, KPF_DIRTY, PG_dirty);
> u |= kpf_copy_bit(k, KPF_UPTODATE, PG_uptodate);
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 3b8475007734..6b04ae65241d 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -207,10 +207,13 @@ struct page {
> atomic_t _mapcount;
>
> /*
> - * If the page is neither PageSlab nor mappable to userspace,
> - * the value stored here may help determine what this page
> - * is used for. See page-flags.h for a list of page types
> - * which are currently stored here.
> + * If the page is not mappable to userspace, the value
> + * stored here may help determine what this page is used for.
> + * See page-flags.h for a list of page types which are currently
> + * stored here.
> + *
> + * Note that only upper half is used for page types and lower
> + * half is reserved for SLAB.
> */
> unsigned int page_type;
> };
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 69e93a0c1277..07063d60efe3 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -107,7 +107,6 @@ enum pageflags {
> PG_workingset,
> PG_waiters, /* Page has waiters, check its waitqueue. Must be bit #7 and in the same byte as "PG_locked" */
> PG_error,
> - PG_slab,
> PG_owner_priv_1, /* Owner use. If pagecache, fs may use*/
> PG_arch_1,
> PG_reserved,
> @@ -482,7 +481,6 @@ PAGEFLAG(Active, active, PF_HEAD) __CLEARPAGEFLAG(Active, active, PF_HEAD)
> TESTCLEARFLAG(Active, active, PF_HEAD)
> PAGEFLAG(Workingset, workingset, PF_HEAD)
> TESTCLEARFLAG(Workingset, workingset, PF_HEAD)
> -__PAGEFLAG(Slab, slab, PF_NO_TAIL)
> __PAGEFLAG(SlobFree, slob_free, PF_NO_TAIL)
> PAGEFLAG(Checked, checked, PF_NO_COMPOUND) /* Used by some filesystems */
>
> @@ -906,42 +904,72 @@ static inline bool is_page_hwpoison(struct page *page)
> }
>
> /*
> - * For pages that are never mapped to userspace (and aren't PageSlab),
> - * page_type may be used. Because it is initialised to -1, we invert the
> - * sense of the bit, so __SetPageFoo *clears* the bit used for PageFoo, and
> - * __ClearPageFoo *sets* the bit used for PageFoo. We reserve a few high and
> - * low bits so that an underflow or overflow of page_mapcount() won't be
> - * mistaken for a page type value.
> + * For pages that are never mapped to userspace, page_type may be used.
> + * Because it is initialised to -1, we invert the sense of the bit,
> + * so __SetPageFoo *clears* the bit used for PageFoo, and __ClearPageFoo
> + * *sets* the bit used for PageFoo. We reserve a few high and low bits
> + * so that an underflow or overflow of page_mapcount() won't be mistaken
> + * for a page type value.
> */
>
> #define PAGE_TYPE_BASE 0xf0000000
> -/* Reserve 0x0000007f to catch underflows of page_mapcount */
> -#define PAGE_MAPCOUNT_RESERVE -128
> -#define PG_buddy 0x00000080
> -#define PG_offline 0x00000100
> -#define PG_table 0x00000200
> -#define PG_guard 0x00000400
> +#define PG_buddy 0x00010000
> +#define PG_offline 0x00020000
> +#define PG_table 0x00040000
> +#define PG_guard 0x00080000
> +#define PG_slab 0x00100000
>
> #define PageType(page, flag) \
> ((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
>
> -static inline int page_has_type(struct page *page)
> +#define PAGE_TYPE_MASK 0xffff0000
> +
> +static inline bool page_type_has_type(unsigned int page_type)
> {
> - return (int)page->page_type < PAGE_MAPCOUNT_RESERVE;
> + return ((int)page_type < (int)PAGE_TYPE_MASK);
> }
>
> -#define PAGE_TYPE_OPS(uname, lname) \
> +static inline bool page_has_type(struct page *page)
> +{
> + return page_type_has_type(page->page_type);
> +}
> +
> +
> +#define PAGE_TYPE_OPS(uname, lname, policy) \
> static __always_inline int Page##uname(struct page *page) \
> { \
> + page = policy(page, 0); \
> + return PageType(page, PG_##lname); \
> +} \
> +static __always_inline int folio_test_##lname(struct folio *folio) \
> +{ \
> + struct page *page = &folio->page; \
> + \
> return PageType(page, PG_##lname); \
> } \
> static __always_inline void __SetPage##uname(struct page *page) \
> { \
> + page = policy(page, 1); \
> + VM_BUG_ON_PAGE(!PageType(page, 0), page); \
> + page->page_type &= ~PG_##lname; \
> +} \
> +static __always_inline void __folio_set_##lname(struct folio *folio) \
> +{ \
> + struct page *page = &folio->page; \
> + \
> VM_BUG_ON_PAGE(!PageType(page, 0), page); \
> page->page_type &= ~PG_##lname; \
> } \
> static __always_inline void __ClearPage##uname(struct page *page) \
> { \
> + page = policy(page, 1); \
> + VM_BUG_ON_PAGE(!Page##uname(page), page); \
> + page->page_type |= PG_##lname; \
> +} \
> +static __always_inline void __folio_clear_##lname(struct folio *folio) \
> +{ \
> + struct page *page = &folio->page; \
> + \
> VM_BUG_ON_PAGE(!Page##uname(page), page); \
> page->page_type |= PG_##lname; \
> }
> @@ -950,7 +978,7 @@ static __always_inline void __ClearPage##uname(struct page *page) \
> * PageBuddy() indicates that the page is free and in the buddy system
> * (see mm/page_alloc.c).
> */
> -PAGE_TYPE_OPS(Buddy, buddy)
> +PAGE_TYPE_OPS(Buddy, buddy, PF_ANY)
>
> /*
> * PageOffline() indicates that the page is logically offline although the
> @@ -974,7 +1002,10 @@ PAGE_TYPE_OPS(Buddy, buddy)
> * pages should check PageOffline() and synchronize with such drivers using
> * page_offline_freeze()/page_offline_thaw().
> */
> -PAGE_TYPE_OPS(Offline, offline)
> +PAGE_TYPE_OPS(Offline, offline, PF_ANY)
> +
> +/* PageSlab() indicates that the page is used by slab subsystem. */
> +PAGE_TYPE_OPS(Slab, slab, PF_NO_TAIL)
>
> extern void page_offline_freeze(void);
> extern void page_offline_thaw(void);
> @@ -984,12 +1015,12 @@ extern void page_offline_end(void);
> /*
> * Marks pages in use as page tables.
> */
> -PAGE_TYPE_OPS(Table, table)
> +PAGE_TYPE_OPS(Table, table, PF_ANY)
>
> /*
> * Marks guardpages used with debug_pagealloc.
> */
> -PAGE_TYPE_OPS(Guard, guard)
> +PAGE_TYPE_OPS(Guard, guard, PF_ANY)
>
> extern bool is_free_buddy_page(struct page *page);
>
> @@ -1037,8 +1068,8 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
> (1UL << PG_lru | 1UL << PG_locked | \
> 1UL << PG_private | 1UL << PG_private_2 | \
> 1UL << PG_writeback | 1UL << PG_reserved | \
> - 1UL << PG_slab | 1UL << PG_active | \
> - 1UL << PG_unevictable | __PG_MLOCKED | LRU_GEN_MASK)
> + 1UL << PG_active | 1UL << PG_unevictable | \
> + __PG_MLOCKED | LRU_GEN_MASK)
>
> /*
> * Flags checked when a page is prepped for return by the page allocator.
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index 412b5a46374c..8301912f8c25 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -113,7 +113,6 @@
> {1UL << PG_lru, "lru" }, \
> {1UL << PG_active, "active" }, \
> {1UL << PG_workingset, "workingset" }, \
> - {1UL << PG_slab, "slab" }, \
> {1UL << PG_owner_priv_1, "owner_priv_1" }, \
> {1UL << PG_arch_1, "arch_1" }, \
> {1UL << PG_reserved, "reserved" }, \
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index 87ef6096823f..1ea8198c26e1 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -482,13 +482,14 @@ static int __init crash_save_vmcoreinfo_init(void)
> VMCOREINFO_NUMBER(PG_private);
> VMCOREINFO_NUMBER(PG_swapcache);
> VMCOREINFO_NUMBER(PG_swapbacked);
> - VMCOREINFO_NUMBER(PG_slab);
> #ifdef CONFIG_MEMORY_FAILURE
> VMCOREINFO_NUMBER(PG_hwpoison);
> #endif
> VMCOREINFO_NUMBER(PG_head_mask);
> #define PAGE_BUDDY_MAPCOUNT_VALUE (~PG_buddy)
> VMCOREINFO_NUMBER(PAGE_BUDDY_MAPCOUNT_VALUE);
> +#define PAGE_SLAB_MAPCOUNT_VALUE (~PG_slab)
> + VMCOREINFO_NUMBER(PAGE_SLAB_MAPCOUNT_VALUE);
> #ifdef CONFIG_HUGETLB_PAGE
> VMCOREINFO_NUMBER(HUGETLB_PAGE_DTOR);
> #define PAGE_OFFLINE_MAPCOUNT_VALUE (~PG_offline)
> diff --git a/mm/slab.c b/mm/slab.c
> index 7a269db050ee..eee46f71c4b8 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -2269,6 +2269,21 @@ void __kmem_cache_release(struct kmem_cache *cachep)
> }
> }
>
> +static inline unsigned int slab_get_active(struct slab *slab)
> +{
> + return ~(slab->page_type | PG_slab);
> +}
> +
> +static inline void slab_inc_active(struct slab *slab)
> +{
> + slab->page_type--;
> +}
> +
> +static inline void slab_dec_active(struct slab *slab)
> +{
> + slab->page_type++;
> +}
> +
> /*
> * Get the memory for a slab management obj.
> *
> @@ -2291,7 +2306,6 @@ static void *alloc_slabmgmt(struct kmem_cache *cachep,
> void *addr = slab_address(slab);
>
> slab->s_mem = addr + colour_off;
> - slab->active = 0;
>
> if (OBJFREELIST_SLAB(cachep))
> freelist = NULL;
> @@ -2510,8 +2524,8 @@ static void *slab_get_obj(struct kmem_cache *cachep, struct slab *slab)
> {
> void *objp;
>
> - objp = index_to_obj(cachep, slab, get_free_obj(slab, slab->active));
> - slab->active++;
> + objp = index_to_obj(cachep, slab, get_free_obj(slab, slab_get_active(slab)));
> + slab_inc_active(slab);
>
> return objp;
> }
> @@ -2524,7 +2538,7 @@ static void slab_put_obj(struct kmem_cache *cachep,
> unsigned int i;
>
> /* Verify double free bug */
> - for (i = slab->active; i < cachep->num; i++) {
> + for (i = slab_get_active(slab); i < cachep->num; i++) {
> if (get_free_obj(slab, i) == objnr) {
> pr_err("slab: double free detected in cache '%s', objp %px\n",
> cachep->name, objp);
> @@ -2532,11 +2546,11 @@ static void slab_put_obj(struct kmem_cache *cachep,
> }
> }
> #endif
> - slab->active--;
> + slab_dec_active(slab);
> if (!slab->freelist)
> slab->freelist = objp + obj_offset(cachep);
>
> - set_free_obj(slab, slab->active, objnr);
> + set_free_obj(slab, slab_get_active(slab), objnr);
> }
>
> /*
> @@ -2635,14 +2649,14 @@ static void cache_grow_end(struct kmem_cache *cachep, struct slab *slab)
>
> raw_spin_lock(&n->list_lock);
> n->total_slabs++;
> - if (!slab->active) {
> + if (!slab_get_active(slab)) {
> list_add_tail(&slab->slab_list, &n->slabs_free);
> n->free_slabs++;
> } else
> fixup_slab_list(cachep, n, slab, &list);
>
> STATS_INC_GROWN(cachep);
> - n->free_objects += cachep->num - slab->active;
> + n->free_objects += cachep->num - slab_get_active(slab);
> raw_spin_unlock(&n->list_lock);
>
> fixup_objfreelist_debug(cachep, &list);
> @@ -2744,7 +2758,7 @@ static inline void fixup_slab_list(struct kmem_cache *cachep,
> {
> /* move slabp to correct slabp list: */
> list_del(&slab->slab_list);
> - if (slab->active == cachep->num) {
> + if (slab_get_active(slab) == cachep->num) {
> list_add(&slab->slab_list, &n->slabs_full);
> if (OBJFREELIST_SLAB(cachep)) {
> #if DEBUG
> @@ -2783,7 +2797,7 @@ static noinline struct slab *get_valid_first_slab(struct kmem_cache_node *n,
>
> /* Move pfmemalloc slab to the end of list to speed up next search */
> list_del(&slab->slab_list);
> - if (!slab->active) {
> + if (!slab_get_active(slab)) {
> list_add_tail(&slab->slab_list, &n->slabs_free);
> n->free_slabs++;
> } else
> @@ -2865,9 +2879,9 @@ static __always_inline int alloc_block(struct kmem_cache *cachep,
> * There must be at least one object available for
> * allocation.
> */
> - BUG_ON(slab->active >= cachep->num);
> + BUG_ON(slab_get_active(slab) >= cachep->num);
>
> - while (slab->active < cachep->num && batchcount--) {
> + while (slab_get_active(slab) < cachep->num && batchcount--) {
> STATS_INC_ALLOCED(cachep);
> STATS_INC_ACTIVE(cachep);
> STATS_SET_HIGH(cachep);
> @@ -3162,7 +3176,7 @@ static void *____cache_alloc_node(struct kmem_cache *cachep, gfp_t flags,
> STATS_INC_ACTIVE(cachep);
> STATS_SET_HIGH(cachep);
>
> - BUG_ON(slab->active == cachep->num);
> + BUG_ON(slab_get_active(slab) == cachep->num);
>
> obj = slab_get_obj(cachep, slab);
> n->free_objects--;
> @@ -3297,7 +3311,7 @@ static void free_block(struct kmem_cache *cachep, void **objpp,
> STATS_DEC_ACTIVE(cachep);
>
> /* fixup slab chains */
> - if (slab->active == 0) {
> + if (slab_get_active(slab) == 0) {
> list_add(&slab->slab_list, &n->slabs_free);
> n->free_slabs++;
> } else {
> @@ -3352,7 +3366,7 @@ static void cache_flusharray(struct kmem_cache *cachep, struct array_cache *ac)
> struct slab *slab;
>
> list_for_each_entry(slab, &n->slabs_free, slab_list) {
> - BUG_ON(slab->active);
> + BUG_ON(slab_get_active(slab));
>
> i++;
> }
> diff --git a/mm/slab.h b/mm/slab.h
> index 7cc432969945..c6ffe6799436 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -20,7 +20,8 @@ struct slab {
> };
> struct rcu_head rcu_head;
> };
> - unsigned int active;
> + /* lower half of page_type is used as active objects counter */
> + unsigned int page_type;
>
> #elif defined(CONFIG_SLUB)
>