nfs: add support for multiple nfs reqs per page

Add "page groups" - a circular list of nfs requests (struct nfs_page)
that all reference the same page. This gives nfs read and write paths
the ability to account for sub-page regions independently.  This
somewhat follows the design of struct buffer_head's sub-page
accounting.

Only "head" requests are ever added/removed from the inode list in
the buffered write path. "head" and "sub" requests are treated the
same through the read path and the rest of the write/commit path.
Requests are given an extra reference across the life of the list.

Page groups are never rejoined after being split. If the read/write
request fails and the client falls back to another path (ie revert
to MDS in PNFS case), the already split requests are pushed through
the recoalescing code again, which may split them further and then
coalesce them into properly sized requests on the wire. Fragmentation
shouldn't be a problem with the current design, because we flush all
requests in page group when a non-contiguous request is added, so
the only time resplitting should occur is on a resend of a read or
write.

This patch lays the groundwork for sub-page splitting, but does not
actually do any splitting. For now all page groups have one request
as pg_test functions don't yet split pages. There are several related
patches that are needed support multiple requests per page group.

Signed-off-by: Weston Andros Adamson <dros@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/direct.c

@@ -380,7 +380,7 @@ static ssize_t nfs_direct_read_schedule_segment(struct nfs_pageio_descriptor *de
 			struct nfs_page *req;
 			unsigned int req_len = min_t(size_t, bytes, PAGE_SIZE - pgbase);
 			/* XXX do we need to do the eof zeroing found in async_filler? */
-			req = nfs_create_request(dreq->ctx, pagevec[i],
+			req = nfs_create_request(dreq->ctx, pagevec[i], NULL,
 						 pgbase, req_len);
 			if (IS_ERR(req)) {
 				result = PTR_ERR(req);
@@ -749,7 +749,7 @@ static ssize_t nfs_direct_write_schedule_segment(struct nfs_pageio_descriptor *d
 			struct nfs_page *req;
 			unsigned int req_len = min_t(size_t, bytes, PAGE_SIZE - pgbase);
 
-			req = nfs_create_request(dreq->ctx, pagevec[i],
+			req = nfs_create_request(dreq->ctx, pagevec[i], NULL,
 						 pgbase, req_len);
 			if (IS_ERR(req)) {
 				result = PTR_ERR(req);
@@ -827,13 +827,16 @@ static void nfs_direct_write_completion(struct nfs_pgio_header *hdr)
 	spin_unlock(&dreq->lock);
 
 	while (!list_empty(&hdr->pages)) {
+		bool do_destroy = true;
+
 		req = nfs_list_entry(hdr->pages.next);
 		nfs_list_remove_request(req);
 		switch (bit) {
 		case NFS_IOHDR_NEED_RESCHED:
 		case NFS_IOHDR_NEED_COMMIT:
 			kref_get(&req->wb_kref);
 			nfs_mark_request_commit(req, hdr->lseg, &cinfo);
+			do_destroy = false;
 		}
 		nfs_unlock_and_release_request(req);
 	}

fs/nfs/pagelist.c

@@ -29,6 +29,8 @@
 static struct kmem_cache *nfs_page_cachep;
 static const struct rpc_call_ops nfs_pgio_common_ops;
 
+static void nfs_free_request(struct nfs_page *);
+
 static bool nfs_pgarray_set(struct nfs_page_array *p, unsigned int pagecount)
 {
 	p->npages = pagecount;
@@ -136,10 +138,151 @@ nfs_iocounter_wait(struct nfs_io_counter *c)
 	return __nfs_iocounter_wait(c);
 }
 
+/*
+ * nfs_page_group_lock - lock the head of the page group
+ * @req - request in group that is to be locked
+ *
+ * this lock must be held if modifying the page group list
+ */
+void
+nfs_page_group_lock(struct nfs_page *req)
+{
+	struct nfs_page *head = req->wb_head;
+	int err = -EAGAIN;
+
+	WARN_ON_ONCE(head != head->wb_head);
+
+	while (err)
+		err = wait_on_bit_lock(&head->wb_flags, PG_HEADLOCK,
+			nfs_wait_bit_killable, TASK_KILLABLE);
+}
+
+/*
+ * nfs_page_group_unlock - unlock the head of the page group
+ * @req - request in group that is to be unlocked
+ */
+void
+nfs_page_group_unlock(struct nfs_page *req)
+{
+	struct nfs_page *head = req->wb_head;
+
+	WARN_ON_ONCE(head != head->wb_head);
+
+	smp_mb__before_clear_bit();
+	clear_bit(PG_HEADLOCK, &head->wb_flags);
+	smp_mb__after_clear_bit();
+	wake_up_bit(&head->wb_flags, PG_HEADLOCK);
+}
+
+/*
+ * nfs_page_group_sync_on_bit_locked
+ *
+ * must be called with page group lock held
+ */
+static bool
+nfs_page_group_sync_on_bit_locked(struct nfs_page *req, unsigned int bit)
+{
+	struct nfs_page *head = req->wb_head;
+	struct nfs_page *tmp;
+
+	WARN_ON_ONCE(!test_bit(PG_HEADLOCK, &head->wb_flags));
+	WARN_ON_ONCE(test_and_set_bit(bit, &req->wb_flags));
+
+	tmp = req->wb_this_page;
+	while (tmp != req) {
+		if (!test_bit(bit, &tmp->wb_flags))
+			return false;
+		tmp = tmp->wb_this_page;
+	}
+
+	/* true! reset all bits */
+	tmp = req;
+	do {
+		clear_bit(bit, &tmp->wb_flags);
+		tmp = tmp->wb_this_page;
+	} while (tmp != req);
+
+	return true;
+}
+
+/*
+ * nfs_page_group_sync_on_bit - set bit on current request, but only
+ *   return true if the bit is set for all requests in page group
+ * @req - request in page group
+ * @bit - PG_* bit that is used to sync page group
+ */
+bool nfs_page_group_sync_on_bit(struct nfs_page *req, unsigned int bit)
+{
+	bool ret;
+
+	nfs_page_group_lock(req);
+	ret = nfs_page_group_sync_on_bit_locked(req, bit);
+	nfs_page_group_unlock(req);
+
+	return ret;
+}
+
+/*
+ * nfs_page_group_init - Initialize the page group linkage for @req
+ * @req - a new nfs request
+ * @prev - the previous request in page group, or NULL if @req is the first
+ *         or only request in the group (the head).
+ */
+static inline void
+nfs_page_group_init(struct nfs_page *req, struct nfs_page *prev)
+{
+	WARN_ON_ONCE(prev == req);
+
+	if (!prev) {
+		req->wb_head = req;
+		req->wb_this_page = req;
+	} else {
+		WARN_ON_ONCE(prev->wb_this_page != prev->wb_head);
+		WARN_ON_ONCE(!test_bit(PG_HEADLOCK, &prev->wb_head->wb_flags));
+		req->wb_head = prev->wb_head;
+		req->wb_this_page = prev->wb_this_page;
+		prev->wb_this_page = req;
+
+		/* grab extra ref if head request has extra ref from
+		 * the write/commit path to handle handoff between write
+		 * and commit lists */
+		if (test_bit(PG_INODE_REF, &prev->wb_head->wb_flags))
+			kref_get(&req->wb_kref);
+	}
+}
+
+/*
+ * nfs_page_group_destroy - sync the destruction of page groups
+ * @req - request that no longer needs the page group
+ *
+ * releases the page group reference from each member once all
+ * members have called this function.
+ */
+static void
+nfs_page_group_destroy(struct kref *kref)
+{
+	struct nfs_page *req = container_of(kref, struct nfs_page, wb_kref);
+	struct nfs_page *tmp, *next;
+
+	if (!nfs_page_group_sync_on_bit(req, PG_TEARDOWN))
+		return;
+
+	tmp = req;
+	do {
+		next = tmp->wb_this_page;
+		/* unlink and free */
+		tmp->wb_this_page = tmp;
+		tmp->wb_head = tmp;
+		nfs_free_request(tmp);
+		tmp = next;
+	} while (tmp != req);
+}
+
 /**
  * nfs_create_request - Create an NFS read/write request.
  * @ctx: open context to use
  * @page: page to write
+ * @last: last nfs request created for this page group or NULL if head
  * @offset: starting offset within the page for the write
  * @count: number of bytes to read/write
  *
@@ -149,7 +292,8 @@ nfs_iocounter_wait(struct nfs_io_counter *c)
  */
 struct nfs_page *
 nfs_create_request(struct nfs_open_context *ctx, struct page *page,
-		   unsigned int offset, unsigned int count)
+		   struct nfs_page *last, unsigned int offset,
+		   unsigned int count)
 {
 	struct nfs_page		*req;
 	struct nfs_lock_context *l_ctx;
@@ -181,6 +325,7 @@ nfs_create_request(struct nfs_open_context *ctx, struct page *page,
 	req->wb_bytes   = count;
 	req->wb_context = get_nfs_open_context(ctx);
 	kref_init(&req->wb_kref);
+	nfs_page_group_init(req, last);
 	return req;
 }
 
@@ -238,16 +383,18 @@ static void nfs_clear_request(struct nfs_page *req)
 	}
 }
 
-
 /**
  * nfs_release_request - Release the count on an NFS read/write request
  * @req: request to release
  *
  * Note: Should never be called with the spinlock held!
  */
-static void nfs_free_request(struct kref *kref)
+static void nfs_free_request(struct nfs_page *req)
 {
-	struct nfs_page *req = container_of(kref, struct nfs_page, wb_kref);
+	WARN_ON_ONCE(req->wb_this_page != req);
+
+	/* extra debug: make sure no sync bits are still set */
+	WARN_ON_ONCE(test_bit(PG_TEARDOWN, &req->wb_flags));
 
 	/* Release struct file and open context */
 	nfs_clear_request(req);
@@ -256,7 +403,7 @@ static void nfs_free_request(struct kref *kref)
 
 void nfs_release_request(struct nfs_page *req)
 {
-	kref_put(&req->wb_kref, nfs_free_request);
+	kref_put(&req->wb_kref, nfs_page_group_destroy);
 }
 
 static int nfs_wait_bit_uninterruptible(void *word)
@@ -832,21 +979,66 @@ static void nfs_pageio_doio(struct nfs_pageio_descriptor *desc)
  * @desc: destination io descriptor
  * @req: request
  *
+ * This may split a request into subrequests which are all part of the
+ * same page group.
+ *
  * Returns true if the request 'req' was successfully coalesced into the
  * existing list of pages 'desc'.
  */
 static int __nfs_pageio_add_request(struct nfs_pageio_descriptor *desc,
 			   struct nfs_page *req)
 {
-	while (!nfs_pageio_do_add_request(desc, req)) {
-		desc->pg_moreio = 1;
-		nfs_pageio_doio(desc);
-		if (desc->pg_error < 0)
-			return 0;
-		desc->pg_moreio = 0;
-		if (desc->pg_recoalesce)
-			return 0;
-	}
+	struct nfs_page *subreq;
+	unsigned int bytes_left = 0;
+	unsigned int offset, pgbase;
+
+	nfs_page_group_lock(req);
+
+	subreq = req;
+	bytes_left = subreq->wb_bytes;
+	offset = subreq->wb_offset;
+	pgbase = subreq->wb_pgbase;
+
+	do {
+		if (!nfs_pageio_do_add_request(desc, subreq)) {
+			/* make sure pg_test call(s) did nothing */
+			WARN_ON_ONCE(subreq->wb_bytes != bytes_left);
+			WARN_ON_ONCE(subreq->wb_offset != offset);
+			WARN_ON_ONCE(subreq->wb_pgbase != pgbase);
+
+			nfs_page_group_unlock(req);
+			desc->pg_moreio = 1;
+			nfs_pageio_doio(desc);
+			if (desc->pg_error < 0)
+				return 0;
+			desc->pg_moreio = 0;
+			if (desc->pg_recoalesce)
+				return 0;
+			/* retry add_request for this subreq */
+			nfs_page_group_lock(req);
+			continue;
+		}
+
+		/* check for buggy pg_test call(s) */
+		WARN_ON_ONCE(subreq->wb_bytes + subreq->wb_pgbase > PAGE_SIZE);
+		WARN_ON_ONCE(subreq->wb_bytes > bytes_left);
+		WARN_ON_ONCE(subreq->wb_bytes == 0);
+
+		bytes_left -= subreq->wb_bytes;
+		offset += subreq->wb_bytes;
+		pgbase += subreq->wb_bytes;
+
+		if (bytes_left) {
+			subreq = nfs_create_request(req->wb_context,
+					req->wb_page,
+					subreq, pgbase, bytes_left);
+			nfs_lock_request(subreq);
+			subreq->wb_offset  = offset;
+			subreq->wb_index = req->wb_index;
+		}
+	} while (bytes_left > 0);
+
+	nfs_page_group_unlock(req);
 	return 1;
 }
 

fs/nfs/read.c

@@ -85,7 +85,7 @@ int nfs_readpage_async(struct nfs_open_context *ctx, struct inode *inode,
 	len = nfs_page_length(page);
 	if (len == 0)
 		return nfs_return_empty_page(page);
-	new = nfs_create_request(ctx, page, 0, len);
+	new = nfs_create_request(ctx, page, NULL, 0, len);
 	if (IS_ERR(new)) {
 		unlock_page(page);
 		return PTR_ERR(new);
@@ -311,7 +311,7 @@ readpage_async_filler(void *data, struct page *page)
 	if (len == 0)
 		return nfs_return_empty_page(page);
 
-	new = nfs_create_request(desc->ctx, page, 0, len);
+	new = nfs_create_request(desc->ctx, page, NULL, 0, len);
 	if (IS_ERR(new))
 		goto out_error;