On Thu, May 27, 2021 at 03:59:51PM -0700, Darrick J. Wong wrote:

On Thu, May 27, 2021 at 08:47:15AM +1000, Dave Chinner wrote:

quoted

From: Dave Chinner <redacted>

Because it's more efficient than allocating pages one at a time in a
loop.

Signed-off-by: Dave Chinner <redacted>
---
 fs/xfs/xfs_buf.c | 62 +++++++++++++++++++-----------------------------
 1 file changed, 24 insertions(+), 38 deletions(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index b1610115d401..8ca4add138c5 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c

@@ -386,10 +386,7 @@ xfs_buf_alloc_pages(
 	xfs_buf_flags_t	flags)
 {
 	gfp_t		gfp_mask = xb_to_gfp(flags);
-	size_t		size;
-	size_t		offset;
-	size_t		nbytes;
-	int		i;
+	long		filled = 0;
 	int		error;
 
 	/* Assure zeroed buffer for non-read cases. */

@@ -400,50 +397,39 @@ xfs_buf_alloc_pages(
 	if (unlikely(error))
 		return error;
 
-	offset = bp->b_offset;
 	bp->b_flags |= _XBF_PAGES;
 
-	for (i = 0; i < bp->b_page_count; i++) {
-		struct page	*page;
-		uint		retries = 0;
-retry:
-		page = alloc_page(gfp_mask);
-		if (unlikely(page == NULL)) {
-			if (flags & XBF_READ_AHEAD) {
-				bp->b_page_count = i;
-				error = -ENOMEM;
-				goto out_free_pages;
-			}
+	/*
+	 * Bulk filling of pages can take multiple calls. Not filling the entire
+	 * array is not an allocation failure, so don't back off if we get at
+	 * least one extra page.
+	 */
+	for (;;) {
+		long	last = filled;
 
-			/*
-			 * This could deadlock.
-			 *
-			 * But until all the XFS lowlevel code is revamped to
-			 * handle buffer allocation failures we can't do much.
-			 */
-			if (!(++retries % 100))
-				xfs_err(NULL,
-		"%s(%u) possible memory allocation deadlock in %s (mode:0x%x)",
-					current->comm, current->pid,
-					__func__, gfp_mask);
-
-			XFS_STATS_INC(bp->b_mount, xb_page_retries);
-			congestion_wait(BLK_RW_ASYNC, HZ/50);
-			goto retry;
+		filled = alloc_pages_bulk_array(gfp_mask, bp->b_page_count,
+						bp->b_pages);
+		if (filled == bp->b_page_count) {
+			XFS_STATS_INC(bp->b_mount, xb_page_found);
+			break;
 		}
 
-		XFS_STATS_INC(bp->b_mount, xb_page_found);
+		if (filled != last)
+			continue;
 
-		nbytes = min_t(size_t, size, PAGE_SIZE - offset);
-		size -= nbytes;
-		bp->b_pages[i] = page;
-		offset = 0;
+		if (flags & XBF_READ_AHEAD) {
+			error = -ENOMEM;
+			goto out_free_pages;
+		}
+
+		XFS_STATS_INC(bp->b_mount, xb_page_retries);
+		congestion_wait(BLK_RW_ASYNC, HZ/50);

Nit: spaces around operators ("HZ / 50").

With that fixed,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

I have a question about _xfs_buf_get_pages:

Never mind, you fixed all this in the next patch, which my grep didn't
find.  Question withdrawn.

--D

STATIC int
_xfs_buf_get_pages(
	struct xfs_buf		*bp,
	int			page_count)
{
	/* Make sure that we have a page list */
	if (bp->b_pages == NULL) {
		bp->b_page_count = page_count;
		if (page_count <= XB_PAGES) {
			bp->b_pages = bp->b_page_array;
		} else {
			bp->b_pages = kmem_alloc(sizeof(struct page *) *
						 page_count, KM_NOFS);
			if (bp->b_pages == NULL)
				return -ENOMEM;
		}
		memset(bp->b_pages, 0, sizeof(struct page *) * page_count);
	}
	return 0;
}

xfs_bufs are kmem_cache_zalloc'd, which means that b_page_array should
be zeroed, right?

And we could use kmem_zalloc for the pagecount > XB_PAGES case, which
would make the memset necessary, wouldn't it?

OFC that only holds if a buffer that fails the memory allocation is
immediately fed to _xfs_buf_free_pages to null out b_pages, which I
think is true...?

--D

quoted

 	}
 	return 0;
 
 out_free_pages:
-	for (i = 0; i < bp->b_page_count; i++)
-		__free_page(bp->b_pages[i]);
+	while (--filled >= 0)
+		__free_page(bp->b_pages[filled]);
 	bp->b_flags &= ~_XBF_PAGES;
 	return error;
 }
-- 
2.31.1