On 2024/10/20 18:02, Bagas Sanjaya wrote:

Thanks, will try my best to not miss any 'alloc' typo for doc patch
next version:(

On Fri, Oct 18, 2024 at 06:53:50PM +0800, Yunsheng Lin wrote:

quoted

diff --git a/Documentation/mm/page_frags.rst b/Documentation/mm/page_frags.rst
index 503ca6cdb804..7fd9398aca4e 100644
--- a/Documentation/mm/page_frags.rst
+++ b/Documentation/mm/page_frags.rst

@@ -1,3 +1,5 @@
+.. SPDX-License-Identifier: GPL-2.0
+
 ==============
 Page fragments
 ==============

@@ -40,4 +42,176 @@ page via a single call.  The advantage to doing this is that it allows for
 cleaning up the multiple references that were added to a page in order to
 avoid calling get_page per allocation.
 
-Alexander Duyck, Nov 29, 2016.
+
+Architecture overview
+=====================
+
+.. code-block:: none
+
+                      +----------------------+
+                      | page_frag API caller |
+                      +----------------------+
+                                  |
+                                  |
+                                  v
+    +------------------------------------------------------------------+
+    |                   request page fragment                          |
+    +------------------------------------------------------------------+
+             |                                 |                     |
+             |                                 |                     |
+             |                          Cache not enough             |
+             |                                 |                     |
+             |                         +-----------------+           |
+             |                         | reuse old cache |--Usable-->|
+             |                         +-----------------+           |
+             |                                 |                     |
+             |                             Not usable                |
+             |                                 |                     |
+             |                                 v                     |
+        Cache empty                   +-----------------+            |
+             |                        | drain old cache |            |
+             |                        +-----------------+            |
+             |                                 |                     |
+             v_________________________________v                     |
+                              |                                      |
+                              |                                      |
+             _________________v_______________                       |
+            |                                 |              Cache is enough
+            |                                 |                      |
+ PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE         |                      |
+            |                                 |                      |
+            |               PAGE_SIZE >= PAGE_FRAG_CACHE_MAX_SIZE    |
+            v                                 |                      |
+    +----------------------------------+      |                      |
+    | refill cache with order > 0 page |      |                      |
+    +----------------------------------+      |                      |
+      |                    |                  |                      |
+      |                    |                  |                      |
+      |              Refill failed            |                      |
+      |                    |                  |                      |
+      |                    v                  v                      |
+      |      +------------------------------------+                  |
+      |      |   refill cache with order 0 page   |                  |
+      |      +----------------------------------=-+                  |
+      |                       |                                      |
+ Refill succeed               |                                      |
+      |                 Refill succeed                               |
+      |                       |                                      |
+      v                       v                                      v
+    +------------------------------------------------------------------+
+    |             allocate fragment from cache                         |
+    +------------------------------------------------------------------+
+
+API interface
+=============
+As the design and implementation of page_frag API implies, the allocation side
+does not allow concurrent calling. Instead it is assumed that the caller must
+ensure there is not concurrent alloc calling to the same page_frag_cache
+instance by using its own lock or rely on some lockless guarantee like NAPI
+softirq.
+
+Depending on different aligning requirement, the page_frag API caller may call
+page_frag_*_align*() to ensure the returned virtual address or offset of the
+page is aligned according to the 'align/alignment' parameter. Note the size of
+the allocated fragment is not aligned, the caller needs to provide an aligned
+fragsz if there is an alignment requirement for the size of the fragment.
+
+Depending on different use cases, callers expecting to deal with va, page or
+both va and page for them may call page_frag_alloc, page_frag_refill, or
+page_frag_alloc_refill API accordingly.
+
+There is also a use case that needs minimum memory in order for forward progress,
+but more performant if more memory is available. Using page_frag_*_prepare() and
+page_frag_commit*() related API, the caller requests the minimum memory it needs
+and the prepare API will return the maximum size of the fragment returned. The
+caller needs to either call the commit API to report how much memory it actually
+uses, or not do so if deciding to not use any memory.
+
+.. 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_refill_align page_frag_refill_align
+		 page_frag_refill __page_frag_refill_prepare_align
+		 page_frag_refill_prepare_align page_frag_refill_prepare
+		 __page_frag_alloc_refill_prepare_align
+		 page_frag_alloc_refill_prepare_align
+		 page_frag_alloc_refill_prepare page_frag_alloc_refill_probe
+		 page_frag_refill_probe page_frag_commit
+		 page_frag_commit_noref page_frag_alloc_abort
+
+.. kernel-doc:: mm/page_frag_cache.c
+   :identifiers: page_frag_cache_drain page_frag_free
+		 __page_frag_alloc_refill_probe_align
+
+Coding examples
+===============
+
+Initialization and draining API
+-------------------------------
+
+.. code-block:: c
+
+   page_frag_cache_init(nc);
+   ...
+   page_frag_cache_drain(nc);
+
+
+Allocation & freeing API
+------------------------
+
+.. code-block:: c
+
+    void *va;
+
+    va = page_frag_alloc_align(nc, size, gfp, align);
+    if (!va)
+        goto do_error;
+
+    err = do_something(va, size);
+    if (err) {
+        page_frag_abort(nc, size);
+        goto do_error;
+    }
+
+    ...
+
+    page_frag_free(va);
+
+
+Preparation & committing API
+----------------------------
+
+.. code-block:: c
+
+    struct page_frag page_frag, *pfrag;
+    bool merge = true;
+    void *va;
+
+    pfrag = &page_frag;
+    va = page_frag_alloc_refill_prepare(nc, 32U, pfrag, GFP_KERNEL);
+    if (!va)
+        goto wait_for_space;
+
+    copy = min_t(unsigned int, copy, pfrag->size);
+    if (!skb_can_coalesce(skb, i, pfrag->page, pfrag->offset)) {
+        if (i >= max_skb_frags)
+            goto new_segment;
+
+        merge = false;
+    }
+
+    copy = mem_schedule(copy);
+    if (!copy)
+        goto wait_for_space;
+
+    err = copy_from_iter_full_nocache(va, copy, iter);
+    if (err)
+        goto do_error;
+
+    if (merge) {
+        skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], copy);
+        page_frag_commit_noref(nc, pfrag, copy);
+    } else {
+        skb_fill_page_desc(skb, i, pfrag->page, pfrag->offset, copy);
+        page_frag_commit(nc, pfrag, copy);
+    }

Looks good.

quoted

+/**
+ * page_frag_cache_is_pfmemalloc() - Check for pfmemalloc.
+ * @nc: page_frag cache from which to check
+ *
+ * Used to check if the current page in page_frag cache is allocated from the

"Check if ..."

quoted

+ * pfmemalloc reserves. It has the same calling context expectation as the
+ * allocation API.
+ *
+ * Return:
+ * true if the current page in page_frag cache is allocated from the pfmemalloc
+ * reserves, otherwise return false.
+ */
<snipped>...
+/**
+ * page_frag_alloc() - Allocate a page fragment.
+ * @nc: page_frag cache from which to allocate
+ * @fragsz: the requested fragment size
+ * @gfp_mask: the allocation gfp to use when cache need to be refilled
+ *
+ * Alloc a page fragment from page_frag cache.

"Allocate a page fragment ..."

quoted

+ *
+ * Return:
+ * virtual address of the page fragment, otherwise return NULL.
+ */
 static inline void *page_frag_alloc(struct page_frag_cache *nc,
<snipped>...
+/**
+ * __page_frag_refill_prepare_align() - Prepare refilling a page_frag with
+ * aligning requirement.
+ * @nc: page_frag cache from which to refill
+ * @fragsz: the requested fragment size
+ * @pfrag: the page_frag to be refilled.
+ * @gfp_mask: the allocation gfp to use when cache need to be refilled
+ * @align_mask: the requested aligning requirement for the fragment
+ *
+ * Prepare refill a page_frag from page_frag cache with aligning requirement.

"Prepare refilling ..."

quoted

+ *
+ * Return:
+ * True if prepare refilling succeeds, otherwise return false.
+ */
<snipped>...
+/**
+ * __page_frag_alloc_refill_probe_align() - Probe allocing a fragment and
+ * refilling a page_frag with aligning requirement.
+ * @nc: page_frag cache from which to allocate and refill
+ * @fragsz: the requested fragment size
+ * @pfrag: the page_frag to be refilled.
+ * @align_mask: the requested aligning requirement for the fragment.
+ *
+ * Probe allocing a fragment and refilling a page_frag from page_frag cache with

"Probe allocating..."

quoted

+ * aligning requirement.
+ *
+ * Return:
+ * virtual address of the page fragment, otherwise return NULL.
+ */

Thanks.