lib/scatterlist: Fix chaining support in sgl_alloc_order()

This patch avoids that workloads with large block sizes (megabytes)
can trigger the following call stack with the ib_srpt driver (that
driver is the only driver that chains scatterlists allocated by
sgl_alloc_order()):

BUG: Bad page state in process kworker/0:1H  pfn:2423a78
page:fffffb03d08e9e00 count:-3 mapcount:0 mapping:          (null) index:0x0
flags: 0x57ffffc0000000()
raw: 0057ffffc0000000 0000000000000000 0000000000000000 fffffffdffffffff
raw: dead000000000100 dead000000000200 0000000000000000 0000000000000000
page dumped because: nonzero _count
CPU: 0 PID: 733 Comm: kworker/0:1H Tainted: G          I      4.15.0-rc7.bart+ #1
Hardware name: HP ProLiant DL380 G7, BIOS P67 08/16/2015
Workqueue: ib-comp-wq ib_cq_poll_work [ib_core]
Call Trace:
 dump_stack+0x5c/0x83
 bad_page+0xf5/0x10f
 get_page_from_freelist+0xa46/0x11b0
 __alloc_pages_nodemask+0x103/0x290
 sgl_alloc_order+0x101/0x180
 target_alloc_sgl+0x2c/0x40 [target_core_mod]
 srpt_alloc_rw_ctxs+0x173/0x2d0 [ib_srpt]
 srpt_handle_new_iu+0x61e/0x7f0 [ib_srpt]
 __ib_process_cq+0x55/0xa0 [ib_core]
 ib_cq_poll_work+0x1b/0x60 [ib_core]
 process_one_work+0x141/0x340
 worker_thread+0x47/0x3e0
 kthread+0xf5/0x130
 ret_from_fork+0x1f/0x30

Fixes: e80a0af4759a ("lib/scatterlist: Introduce sgl_alloc() and sgl_free()")
Reported-by: Laurence Oberman <loberman@redhat.com>
Tested-by: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Nicholas A. Bellinger <nab@linux-iscsi.org>
Cc: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/target/target_core_transport.c

@@ -2300,7 +2300,7 @@ queue_full:
 
 void target_free_sgl(struct scatterlist *sgl, int nents)
 {
-	sgl_free(sgl);
+	sgl_free_n_order(sgl, nents, 0);
 }
 EXPORT_SYMBOL(target_free_sgl);
 

include/linux/scatterlist.h

@@ -282,6 +282,7 @@ struct scatterlist *sgl_alloc_order(unsigned long long length,
 				    gfp_t gfp, unsigned int *nent_p);
 struct scatterlist *sgl_alloc(unsigned long long length, gfp_t gfp,
 			      unsigned int *nent_p);
+void sgl_free_n_order(struct scatterlist *sgl, int nents, int order);
 void sgl_free_order(struct scatterlist *sgl, int order);
 void sgl_free(struct scatterlist *sgl);
 #endif /* CONFIG_SGL_ALLOC */

lib/scatterlist.c

@@ -512,7 +512,7 @@ struct scatterlist *sgl_alloc_order(unsigned long long length,
 	if (!sgl)
 		return NULL;
 
-	sg_init_table(sgl, nent);
+	sg_init_table(sgl, nalloc);
 	sg = sgl;
 	while (length) {
 		elem_len = min_t(u64, length, PAGE_SIZE << order);
@@ -526,7 +526,7 @@ struct scatterlist *sgl_alloc_order(unsigned long long length,
 		length -= elem_len;
 		sg = sg_next(sg);
 	}
-	WARN_ON_ONCE(sg);
+	WARN_ONCE(length, "length = %lld\n", length);
 	if (nent_p)
 		*nent_p = nent;
 	return sgl;
@@ -549,22 +549,44 @@ struct scatterlist *sgl_alloc(unsigned long long length, gfp_t gfp,
 EXPORT_SYMBOL(sgl_alloc);
 
 /**
- * sgl_free_order - free a scatterlist and its pages
+ * sgl_free_n_order - free a scatterlist and its pages
  * @sgl: Scatterlist with one or more elements
+ * @nents: Maximum number of elements to free
  * @order: Second argument for __free_pages()
+ *
+ * Notes:
+ * - If several scatterlists have been chained and each chain element is
+ *   freed separately then it's essential to set nents correctly to avoid that a
+ *   page would get freed twice.
+ * - All pages in a chained scatterlist can be freed at once by setting @nents
+ *   to a high number.
  */
-void sgl_free_order(struct scatterlist *sgl, int order)
+void sgl_free_n_order(struct scatterlist *sgl, int nents, int order)
 {
 	struct scatterlist *sg;
 	struct page *page;
+	int i;
 
-	for (sg = sgl; sg; sg = sg_next(sg)) {
+	for_each_sg(sgl, sg, nents, i) {
+		if (!sg)
+			break;
 		page = sg_page(sg);
 		if (page)
 			__free_pages(page, order);
 	}
 	kfree(sgl);
 }
+EXPORT_SYMBOL(sgl_free_n_order);
+
+/**
+ * sgl_free_order - free a scatterlist and its pages
+ * @sgl: Scatterlist with one or more elements
+ * @order: Second argument for __free_pages()
+ */
+void sgl_free_order(struct scatterlist *sgl, int order)
+{
+	sgl_free_n_order(sgl, INT_MAX, order);
+}
 EXPORT_SYMBOL(sgl_free_order);
 
 /**