abd: add page iterator

The regular ABD iterators yield data buffers, so they have to map and
unmap pages into kernel memory. vdev_disk only needs the page pointers,
so has to then reverse the math to get them, and the map/unmap is just
unnecessary overhead.

This adds adb_iterate_page_func, which yields unmapped struct page
instead, and then updates vdev_disk to use it.

Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.

include/sys/abd.h

@@ -79,6 +79,9 @@ typedef struct abd {
 
 typedef int abd_iter_func_t(void *buf, size_t len, void *priv);
 typedef int abd_iter_func2_t(void *bufa, void *bufb, size_t len, void *priv);
+#if defined(__linux__) && defined(_KERNEL)
+typedef int abd_iter_page_func_t(struct page *, size_t, size_t, void *);
+#endif
 
 extern int zfs_abd_scatter_enabled;
 
@@ -125,6 +128,10 @@ void abd_release_ownership_of_buf(abd_t *);
 int abd_iterate_func(abd_t *, size_t, size_t, abd_iter_func_t *, void *);
 int abd_iterate_func2(abd_t *, abd_t *, size_t, size_t, size_t,
     abd_iter_func2_t *, void *);
+#if defined(__linux__) && defined(_KERNEL)
+int abd_iterate_page_func(abd_t *, size_t, size_t, abd_iter_page_func_t *,
+    void *);
+#endif
 void abd_copy_off(abd_t *, abd_t *, size_t, size_t, size_t);
 void abd_copy_from_buf_off(abd_t *, const void *, size_t, size_t);
 void abd_copy_to_buf_off(void *, abd_t *, size_t, size_t);

include/sys/abd_impl.h

@@ -38,7 +38,9 @@ typedef enum abd_stats_op {
 	ABDSTAT_DECR  /* Decrease abdstat values */
 } abd_stats_op_t;
 
-struct scatterlist; /* forward declaration */
+/* forward declarations */
+struct scatterlist;
+struct page;
 
 struct abd_iter {
 	/* public interface */
@@ -51,6 +53,10 @@ struct abd_iter {
 	size_t		iter_offset;	/* offset in current sg/abd_buf, */
 					/* abd_offset included */
 	struct scatterlist *iter_sg;	/* current sg */
+
+	struct page	*iter_page;		/* current page */
+	size_t		iter_page_dsize;	/* size of data in page */
+	size_t		iter_page_doff;		/* offset of data in page */
 };
 
 extern abd_t *abd_zero_scatter;
@@ -79,6 +85,7 @@ boolean_t abd_iter_at_end(struct abd_iter *);
 void abd_iter_advance(struct abd_iter *, size_t);
 void abd_iter_map(struct abd_iter *);
 void abd_iter_unmap(struct abd_iter *);
+void abd_iter_page(struct abd_iter *);
 
 /*
  * Helper macros

module/os/linux/zfs/abd_os.c

@@ -21,6 +21,7 @@
 /*
  * Copyright (c) 2014 by Chunwei Chen. All rights reserved.
  * Copyright (c) 2019 by Delphix. All rights reserved.
+ * Copyright (c) 2023, Klara Inc.
  */
 
 /*
@@ -59,6 +60,7 @@
 #include <sys/zfs_znode.h>
 #ifdef _KERNEL
 #include <linux/kmap_compat.h>
+#include <linux/mm_compat.h>
 #include <linux/scatterlist.h>
 #else
 #define	MAX_ORDER	1
@@ -926,6 +928,9 @@ abd_iter_init(struct abd_iter *aiter, abd_t *abd)
 		aiter->iter_offset = ABD_SCATTER(abd).abd_offset;
 		aiter->iter_sg = ABD_SCATTER(abd).abd_sgl;
 	}
+	aiter->iter_page = NULL;
+	aiter->iter_page_doff = 0;
+	aiter->iter_page_dsize = 0;
 }
 
 /*
@@ -935,6 +940,7 @@ abd_iter_init(struct abd_iter *aiter, abd_t *abd)
 boolean_t
 abd_iter_at_end(struct abd_iter *aiter)
 {
+	ASSERT3U(aiter->iter_pos, <=, aiter->iter_abd->abd_size);
 	return (aiter->iter_pos == aiter->iter_abd->abd_size);
 }
 
@@ -1028,10 +1034,90 @@ abd_cache_reap_now(void)
 {
 }
 
-#if defined(_KERNEL)
+#ifdef _KERNEL
+void
+abd_iter_page(struct abd_iter *aiter)
+{
+	if (abd_iter_at_end(aiter)) {
+		aiter->iter_page = NULL;
+		aiter->iter_page_doff = 0;
+		aiter->iter_page_dsize = 0;
+		return;
+	}
+
+	struct page *page;
+	size_t doff, dsize;
+
+	if (abd_is_linear(aiter->iter_abd)) {
+		ASSERT3U(aiter->iter_pos, ==, aiter->iter_offset);
+
+		/* memory address at iter_pos */
+		void *paddr = ABD_LINEAR_BUF(aiter->iter_abd) + aiter->iter_pos;
+
+		/* struct page for address */
+		page = is_vmalloc_addr(paddr) ?
+		    vmalloc_to_page(paddr) : virt_to_page(paddr);
+
+		/* offset of address within the page */
+		doff = offset_in_page(paddr);
+
+		/* total data remaining in abd from this position */
+		dsize = aiter->iter_abd->abd_size - aiter->iter_offset;
+	} else {
+		ASSERT(!abd_is_gang(aiter->iter_abd));
+
+		/* current scatter page */
+		page = sg_page(aiter->iter_sg);
+
+		/* position within page */
+		doff = aiter->iter_offset;
+
+		/* remaining data in scatterlist */
+		dsize = MIN(aiter->iter_sg->length - aiter->iter_offset,
+		    aiter->iter_abd->abd_size - aiter->iter_pos);
+	}
+	ASSERT(page);
+
+	if (PageTail(page)) {
+
+		/*
+		 * This page is part of a "compound page", which is a group of
+		 * pages that can be referenced from a single struct page *.
+		 * Its organised as a "head" page, followed by a series of
+		 * "tail" pages.
+		 *
+		 * In OpenZFS, compound pages are allocated using the
+		 * __GFP_COMP flag, which we get from scatter ABDs and SPL
+		 * vmalloc slabs (ie >16K allocations). So a great many of the
+		 * IO buffers we get are going to be of this type.
+		 *
+		 * The tail pages are just regular PAGE_SIZE pages, and can be
+		 * safely used as-is. However, the head page has length
+		 * covering itself and all the tail pages. If this ABD chunk
+		 * spans multiple pages, then we can use the head page and a
+		 * >PAGE_SIZE length, which is far more efficient.
+		 *
+		 * To do this, we need to adjust the offset to be counted from
+		 * the head page. struct page for compound pages are stored
+		 * contiguously, so we can just adjust by a simple offset.
+		 */
+		struct page *head = compound_head(page);
+		doff += ((page - head) * PAGESIZE);
+		page = head;
+	}
+
+	/* final page and position within it */
+	aiter->iter_page = page;
+	aiter->iter_page_doff = doff;
+
+	/* amount of data in the chunk, up to the end of the page */
+	aiter->iter_page_dsize = MIN(dsize, page_size(page) - doff);
+}
+
+
 /*
- * Note: ABD BIO functions only needed to support vdev_classic. See comments in
- * vdev_disk.c.
+ * Note: The below ABD BIO functions only needed to support vdev_classic. See
+ * comments in vdev_disk.c.
  */
 
 /*
@@ -1188,4 +1274,5 @@ MODULE_PARM_DESC(zfs_abd_scatter_min_size,
 module_param(zfs_abd_scatter_max_order, uint, 0644);
 MODULE_PARM_DESC(zfs_abd_scatter_max_order,
 	"Maximum order allocation used for a scatter ABD.");
-#endif
+
+#endif /* _KERNEL */

module/os/linux/zfs/vdev_disk.c

@@ -38,7 +38,6 @@
 #include <linux/blkpg.h>
 #include <linux/msdos_fs.h>
 #include <linux/vfs_compat.h>
-#include <linux/mm_compat.h>
 #ifdef HAVE_LINUX_BLK_CGROUP_HEADER
 #include <linux/blk-cgroup.h>
 #endif
@@ -847,45 +846,6 @@ BIO_END_IO_PROTO(vdev_disk_io_rw_completion, bio, error)
 	vbio_put(vbio);
 }
 
-static inline void
-_buf_to_page_and_offset(const void *buf, struct page **pagep, uint_t *offp)
-{
-	struct page *page = is_vmalloc_addr(buf) ?
-	    vmalloc_to_page(buf) : virt_to_page(buf);
-
-	if (!PageCompound(page)) {
-		/* Single boring page, nothing more to see */
-		*pagep = page;
-		*offp = offset_in_page(buf);
-		return;
-	}
-
-	/*
-	 * This page is part of a "compound page", which is a group of pages
-	 * that can be referenced from a single struct page *. Its organised as
-	 * a "head" page, followed by a series of "tail" pages.
-	 *
-	 * In OpenZFS, compound pages are allocated using the __GFP_COMP flag,
-	 * which we get from scatter ABDs and SPL vmalloc slabs (ie >16K
-	 * allocations). So a great many of the IO buffers we get are going to
-	 * be of this type.
-	 *
-	 * The tail pages are just regular PAGE_SIZE pages, and we can just
-	 * load them into the BIO the same as we would for non-compound pages
-	 * above, and it all works just fine. However, the head page has length
-	 * covering itself and all the tail pages. If our buffer spans multiple
-	 * pages, then we can load the head page and a >PAGE_SIZE length into
-	 * the BIO, which is far more efficient.
-	 *
-	 * To do this, we need to calculate the offset of the buffer from the
-	 * head page (offset_in_page() is the offset within its PAGE_SIZE'd
-	 * page ie just a simple ~(PAGE_SIZE-1) mask).
-	 */
-
-	*pagep = compound_head(page);
-	*offp = (uint_t)((uintptr_t)(buf) & (page_size(*pagep)-1));
-}
-
 /*
  * Iterator callback to count ABD pages and check their size & alignment.
  *
@@ -902,13 +862,10 @@ typedef struct {
 } vdev_disk_check_pages_t;
 
 static int
-vdev_disk_check_pages_cb(void *buf, size_t len, void *priv)
+vdev_disk_check_pages_cb(struct page *page, size_t off, size_t len, void *priv)
 {
 	vdev_disk_check_pages_t *s = priv;
 
-	struct page *page;
-	uint_t off;
-
 	/*
 	 * If we didn't finish on a block size boundary last time, then there
 	 * would be a gap if we tried to use this ABD as-is, so abort.
@@ -922,24 +879,11 @@ vdev_disk_check_pages_cb(void *buf, size_t len, void *priv)
 	 */
 	s->end = len & s->bmask;
 
-	while (len > 0) {
-		_buf_to_page_and_offset(buf, &page, &off);
-
-		/*
-		 * All blocks after the first must start on a block size
-		 * boundary.
-		 */
-		if (s->npages != 0 && (off & s->bmask) != 0)
-			return (1);
-
-		uint_t take = MIN(len, page_size(page)-off);
-
-		buf += take;
-		len -= take;
-
-		s->npages++;
-	}
+	/* All blocks after the first must start on a block size boundary. */
+	if (s->npages != 0 && (off & s->bmask) != 0)
+		return (1);
 
+	s->npages++;
 	return (0);
 }
 
@@ -956,36 +900,18 @@ vdev_disk_check_pages(abd_t *abd, uint64_t size, uint_t lbs)
 	    .end = 0
 	};
 
-	if (abd_iterate_func(abd, 0, size, vdev_disk_check_pages_cb, &s))
+	if (abd_iterate_page_func(abd, 0, size, vdev_disk_check_pages_cb, &s))
 		return (0);
 
 	return (s.npages);
 }
 
 /* Iterator callback to submit ABD pages to the vbio. */
 static int
-vdev_disk_fill_vbio_cb(void *buf, size_t len, void *priv)
+vdev_disk_fill_vbio_cb(struct page *page, size_t off, size_t len, void *priv)
 {
 	vbio_t *vbio = priv;
-	int err;
-
-	struct page *page;
-	uint_t off;
-
-	while (len > 0) {
-		_buf_to_page_and_offset(buf, &page, &off);
-
-		uint_t take = MIN(len, page_size(page)-off);
-
-		err = vbio_add_page(vbio, page, take, off);
-		if (err != 0)
-			return (err);
-
-		buf += take;
-		len -= take;
-	}
-
-	return (0);
+	return (vbio_add_page(vbio, page, len, off));
 }
 
 static int
@@ -1063,7 +989,7 @@ vdev_disk_io_rw(zio_t *zio)
 		vbio->vbio_abd = abd;
 
 	/* Fill it with pages */
-	error = abd_iterate_func(abd, 0, zio->io_size,
+	error = abd_iterate_page_func(abd, 0, zio->io_size,
 	    vdev_disk_fill_vbio_cb, vbio);
 	if (error != 0) {
 		vbio_free(vbio);

module/zfs/abd.c

@@ -828,6 +828,44 @@ abd_iterate_func(abd_t *abd, size_t off, size_t size,
 	return (ret);
 }
 
+#if defined(__linux__) && defined(_KERNEL)
+int
+abd_iterate_page_func(abd_t *abd, size_t off, size_t size,
+    abd_iter_page_func_t *func, void *private)
+{
+	struct abd_iter aiter;
+	int ret = 0;
+
+	if (size == 0)
+		return (0);
+
+	abd_verify(abd);
+	ASSERT3U(off + size, <=, abd->abd_size);
+
+	abd_t *c_abd = abd_init_abd_iter(abd, &aiter, off);
+
+	while (size > 0) {
+		IMPLY(abd_is_gang(abd), c_abd != NULL);
+
+		abd_iter_page(&aiter);
+
+		size_t len = MIN(aiter.iter_page_dsize, size);
+		ASSERT3U(len, >, 0);
+
+		ret = func(aiter.iter_page, aiter.iter_page_doff,
+		    len, private);
+
+		if (ret != 0)
+			break;
+
+		size -= len;
+		c_abd = abd_advance_abd_iter(abd, c_abd, &aiter, len);
+	}
+
+	return (ret);
+}
+#endif
+
 struct buf_arg {
 	void *arg_buf;
 };