[PATCH RFC 04/10] mm: page_frag: introduce page_frag_alloc_abort() related API

[Yunsheng Lin <linyunsheng@huawei.com>]

For some case as tun_build_skb() without the needing of
using complicated prepare & commit API, add the abort API to
abort the operation of page_frag_alloc_*() related API for
error handling knowing that no one else is taking extra
reference to the just allocated fragment, and add abort_ref
API to only abort the reference counting of the allocated
fragment if it is already referenced by someone else.

CC: Alexander Duyck <alexander.duyck@gmail.com>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Linux-MM <linux-mm@kvack.org>
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
---
 Documentation/mm/page_frags.rst |  7 +++++--
 include/linux/page_frag_cache.h | 20 ++++++++++++++++++++
 mm/page_frag_cache.c            | 21 +++++++++++++++++++++
 3 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/Documentation/mm/page_frags.rst b/Documentation/mm/page_frags.rst
index 34e654c2956e..339e641beb53 100644
--- a/Documentation/mm/page_frags.rst
+++ b/Documentation/mm/page_frags.rst
@@ -114,9 +114,10 @@ fragsz if there is an alignment requirement for the size of the fragment.
 .. kernel-doc:: include/linux/page_frag_cache.h
    :identifiers: page_frag_cache_init page_frag_cache_is_pfmemalloc
 		 __page_frag_alloc_align page_frag_alloc_align page_frag_alloc
+		 page_frag_alloc_abort
 
 .. kernel-doc:: mm/page_frag_cache.c
-   :identifiers: page_frag_cache_drain page_frag_free
+   :identifiers: page_frag_cache_drain page_frag_free page_frag_alloc_abort_ref
 
 Coding examples
 ===============
@@ -143,8 +144,10 @@ Allocation & freeing API
         goto do_error;
 
     err = do_something(va, size);
-    if (err)
+    if (err) {
+        page_frag_alloc_abort(nc, va, size);
         goto do_error;
+    }
 
     ...
 
diff --git a/include/linux/page_frag_cache.h b/include/linux/page_frag_cache.h
index a2b1127e8ac8..c3347c97522c 100644
--- a/include/linux/page_frag_cache.h
+++ b/include/linux/page_frag_cache.h
@@ -141,5 +141,25 @@ static inline void *page_frag_alloc(struct page_frag_cache *nc,
 }
 
 void page_frag_free(void *addr);
+void page_frag_alloc_abort_ref(struct page_frag_cache *nc, void *va,
+			       unsigned int fragsz);
+
+/**
+ * page_frag_alloc_abort - Abort the page fragment allocation.
+ * @nc: page_frag cache to which the page fragment is aborted back
+ * @va: virtual address of page fragment to be aborted
+ * @fragsz: size of the page fragment to be aborted
+ *
+ * It is expected to be called from the same context as the allocation API.
+ * Mostly used for error handling cases to abort the fragment allocation knowing
+ * that no one else is taking extra reference to the just aborted fragment, so
+ * that the aborted fragment can be reused.
+ */
+static inline void page_frag_alloc_abort(struct page_frag_cache *nc, void *va,
+					 unsigned int fragsz)
+{
+	page_frag_alloc_abort_ref(nc, va, fragsz);
+	nc->offset -= fragsz;
+}
 
 #endif
diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c
index d014130fb893..4d5626da42ed 100644
--- a/mm/page_frag_cache.c
+++ b/mm/page_frag_cache.c
@@ -201,3 +201,24 @@ void page_frag_free(void *addr)
 		free_unref_page(page, compound_order(page));
 }
 EXPORT_SYMBOL(page_frag_free);
+
+/**
+ * page_frag_alloc_abort_ref - Abort the reference of allocated fragment.
+ * @nc: page_frag cache to which the page fragment is aborted back
+ * @va: virtual address of page fragment to be aborted
+ * @fragsz: size of the page fragment to be aborted
+ *
+ * It is expected to be called from the same context as the allocation API.
+ * Mostly used for error handling cases to abort the reference of allocated
+ * fragment if the fragment has been referenced for other usages, to aovid the
+ * atomic operation of page_frag_free() API.
+ */
+void page_frag_alloc_abort_ref(struct page_frag_cache *nc, void *va,
+			       unsigned int fragsz)
+{
+	VM_BUG_ON(va + fragsz !=
+		  encoded_page_decode_virt(nc->encoded_page) + nc->offset);
+
+	nc->pagecnt_bias++;
+}
+EXPORT_SYMBOL(page_frag_alloc_abort_ref);
-- 
2.33.0