Currently there is one 'struct page_frag' for every 'struct
sock' and 'struct task_struct', we are about to replace the
'struct page_frag' with 'struct page_frag_cache' for them.
Before begin the replacing, we need to ensure the size of
'struct page_frag_cache' is not bigger than the size of
'struct page_frag', as there may be tens of thousands of
'struct sock' and 'struct task_struct' instances in the
system.
By or'ing the page order & pfmemalloc with lower bits of
'va' instead of using 'u16' or 'u32' for page size and 'u8'
for pfmemalloc, we are able to avoid 3 or 5 bytes space waste.
And page address & pfmemalloc & order is unchanged for the
same page in the same 'page_frag_cache' instance, it makes
sense to fit them together.
Also, it is better to replace 'offset' with 'remaining', which
is the remaining size for the cache in a 'page_frag_cache'
instance, we are able to do a single 'fragsz > remaining'
checking for the case of cache not being enough, which should be
the fast path if we ensure size is zoro when 'va' == NULL by
memset'ing 'struct page_frag_cache' in page_frag_cache_init()
and page_frag_cache_drain().
After this patch, the size of 'struct page_frag_cache' should be
the same as the size of 'struct page_frag'.
CC: Alexander Duyck <alexander.duyck@xxxxxxxxx>
Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
---
include/linux/page_frag_cache.h | 76 +++++++++++++++++++++++-----
mm/page_frag_cache.c | 90 ++++++++++++++++++++-------------
2 files changed, 118 insertions(+), 48 deletions(-)
diff --git a/include/linux/page_frag_cache.h b/include/linux/page_frag_cache.h
index 6ac3a25089d1..b33904d4494f 100644
--- a/include/linux/page_frag_cache.h
+++ b/include/linux/page_frag_cache.h
@@ -8,29 +8,81 @@
#define PAGE_FRAG_CACHE_MAX_SIZE __ALIGN_MASK(32768, ~PAGE_MASK)
#define PAGE_FRAG_CACHE_MAX_ORDER get_order(PAGE_FRAG_CACHE_MAX_SIZE)
-struct page_frag_cache {
- void *va;
+/*
+ * struct encoded_va - a nonexistent type marking this pointer
+ *
+ * An 'encoded_va' pointer is a pointer to a aligned virtual address, which is
+ * at least aligned to PAGE_SIZE, that means there are at least 12 lower bits
+ * space available for other purposes.
+ *
+ * Currently we use the lower 8 bits and bit 9 for the order and PFMEMALLOC
+ * flag of the page this 'va' is corresponding to.
+ *
+ * Use the supplied helper functions to endcode/decode the pointer and bits.
+ */
+struct encoded_va;
+
+#define PAGE_FRAG_CACHE_ORDER_MASK GENMASK(7, 0)
+#define PAGE_FRAG_CACHE_PFMEMALLOC_BIT BIT(8)
+#define PAGE_FRAG_CACHE_PFMEMALLOC_SHIFT 8
+
+static inline struct encoded_va *encode_aligned_va(void *va,
+ unsigned int order,
+ bool pfmemalloc)
+{
#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
- __u16 offset;
- __u16 size;
+ return (struct encoded_va *)((unsigned long)va | order |
+ pfmemalloc << PAGE_FRAG_CACHE_PFMEMALLOC_SHIFT);
#else
- __u32 offset;
+ return (struct encoded_va *)((unsigned long)va |
+ pfmemalloc << PAGE_FRAG_CACHE_PFMEMALLOC_SHIFT);
+#endif
+}
+
+static inline unsigned long encoded_page_order(struct encoded_va *encoded_va)
+{
+#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
+ return PAGE_FRAG_CACHE_ORDER_MASK & (unsigned long)encoded_va;
+#else
+ return 0;
+#endif
+}
+
+static inline bool encoded_page_pfmemalloc(struct encoded_va *encoded_va)
+{
+ return PAGE_FRAG_CACHE_PFMEMALLOC_BIT & (unsigned long)encoded_va;
+}
+
+static inline void *encoded_page_address(struct encoded_va *encoded_va)
+{
+ return (void *)((unsigned long)encoded_va & PAGE_MASK);
+}
+
+struct page_frag_cache {
+ struct encoded_va *encoded_va;
+
+#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) && (BITS_PER_LONG <= 32)
+ u16 pagecnt_bias;
+ u16 remaining;
+#else
+ u32 pagecnt_bias;
+ u32 remaining;
#endif
- /* we maintain a pagecount bias, so that we dont dirty cache line
- * containing page->_refcount every time we allocate a fragment.
- */
- unsigned int pagecnt_bias;
- bool pfmemalloc;
};
static inline void page_frag_cache_init(struct page_frag_cache *nc)
{
- nc->va = NULL;
+ memset(nc, 0, sizeof(*nc));
}
static inline bool page_frag_cache_is_pfmemalloc(struct page_frag_cache *nc)
{
- return !!nc->pfmemalloc;
+ return encoded_page_pfmemalloc(nc->encoded_va);
+}
+
+static inline unsigned int page_frag_cache_page_size(struct encoded_va *encoded_va)
+{
+ return PAGE_SIZE << encoded_page_order(encoded_va);
}
void page_frag_cache_drain(struct page_frag_cache *nc);
diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c
index e5fd4acae003..756ef68cce21 100644
--- a/mm/page_frag_cache.c
+++ b/mm/page_frag_cache.c
@@ -18,11 +18,20 @@
#include <linux/page_frag_cache.h>
#include "internal.h"
+static void *page_frag_cache_current_va(struct page_frag_cache *nc)
+{
+ struct encoded_va *encoded_va = nc->encoded_va;
+
+ return (void *)(((unsigned long)encoded_va & PAGE_MASK) |
+ (page_frag_cache_page_size(encoded_va) - nc->remaining));
+}
+
static struct page *__page_frag_cache_refill(struct page_frag_cache *nc,
gfp_t gfp_mask)
{
struct page *page = NULL;
gfp_t gfp = gfp_mask;
+ unsigned int order;
#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
/* Ensure free_unref_page() can be used to free the page fragment */
@@ -32,23 +41,41 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *nc,
__GFP_NOWARN | __GFP_NORETRY | __GFP_NOMEMALLOC;
page = alloc_pages_node(NUMA_NO_NODE, gfp_mask,
PAGE_FRAG_CACHE_MAX_ORDER);
- nc->size = page ? PAGE_FRAG_CACHE_MAX_SIZE : PAGE_SIZE;
#endif
- if (unlikely(!page))
+ if (unlikely(!page)) {
page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);
+ if (unlikely(!page)) {
+ memset(nc, 0, sizeof(*nc));
+ return NULL;
+ }
+
+ order = 0;
+ nc->remaining = PAGE_SIZE;
+ } else {
+ order = PAGE_FRAG_CACHE_MAX_ORDER;
+ nc->remaining = PAGE_FRAG_CACHE_MAX_SIZE;
+ }
- nc->va = page ? page_address(page) : NULL;
+ /* Even if we own the page, we do not use atomic_set().
+ * This would break get_page_unless_zero() users.
+ */
+ page_ref_add(page, PAGE_FRAG_CACHE_MAX_SIZE);
+ /* reset page count bias of new frag */
+ nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
+ nc->encoded_va = encode_aligned_va(page_address(page), order,
+ page_is_pfmemalloc(page));
return page;
}
void page_frag_cache_drain(struct page_frag_cache *nc)
{
- if (!nc->va)
+ if (!nc->encoded_va)
return;
- __page_frag_cache_drain(virt_to_head_page(nc->va), nc->pagecnt_bias);
- nc->va = NULL;
+ __page_frag_cache_drain(virt_to_head_page(nc->encoded_va),
+ nc->pagecnt_bias);
+ memset(nc, 0, sizeof(*nc));
}
EXPORT_SYMBOL(page_frag_cache_drain);
@@ -65,51 +92,41 @@ void *__page_frag_alloc_va_align(struct page_frag_cache *nc,
unsigned int fragsz, gfp_t gfp_mask,
unsigned int align_mask)
{
- unsigned int size = PAGE_SIZE;
+ struct encoded_va *encoded_va = nc->encoded_va;
struct page *page;
- int offset;
+ int remaining;
+ void *va;
- if (unlikely(!nc->va)) {
+ if (unlikely(!encoded_va)) {
refill:
- page = __page_frag_cache_refill(nc, gfp_mask);
- if (!page)
+ if (unlikely(!__page_frag_cache_refill(nc, gfp_mask)))
return NULL;
- /* Even if we own the page, we do not use atomic_set().
- * This would break get_page_unless_zero() users.
- */
- page_ref_add(page, PAGE_FRAG_CACHE_MAX_SIZE);
-
- /* reset page count bias and offset to start of new frag */
- nc->pfmemalloc = page_is_pfmemalloc(page);
- nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
- nc->offset = 0;
+ encoded_va = nc->encoded_va;
}
-#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
- /* if size can vary use size else just use PAGE_SIZE */
- size = nc->size;
-#endif
-
- offset = __ALIGN_KERNEL_MASK(nc->offset, ~align_mask);
- if (unlikely(offset + fragsz > size)) {
- page = virt_to_page(nc->va);
-
+ remaining = nc->remaining & align_mask;
+ remaining -= fragsz;
+ if (unlikely(remaining < 0)) {
+ page = virt_to_page(encoded_va);
if (!page_ref_sub_and_test(page, nc->pagecnt_bias))
goto refill;
- if (unlikely(nc->pfmemalloc)) {
- free_unref_page(page, compound_order(page));
+ if (unlikely(encoded_page_pfmemalloc(encoded_va))) {
+ VM_BUG_ON(compound_order(page) !=
+ encoded_page_order(encoded_va));
+ free_unref_page(page, encoded_page_order(encoded_va));
goto refill;
}
/* OK, page count is 0, we can safely set it */
set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
- /* reset page count bias and offset to start of new frag */
+ /* reset page count bias and remaining of new frag */
nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
- offset = 0;
- if (unlikely(fragsz > PAGE_SIZE)) {
+ nc->remaining = remaining = page_frag_cache_page_size(encoded_va);
+ remaining -= fragsz;
+ if (unlikely(remaining < 0)) {
/*
* The caller is trying to allocate a fragment
* with fragsz > PAGE_SIZE but the cache isn't big
@@ -123,10 +140,11 @@ void *__page_frag_alloc_va_align(struct page_frag_cache *nc,
}
}
+ va = page_frag_cache_current_va(nc);
nc->pagecnt_bias--;
- nc->offset = offset + fragsz;
+ nc->remaining = remaining;
- return nc->va + offset;
+ return va;
}
EXPORT_SYMBOL(__page_frag_alloc_va_align);
--
2.33.0
