[PATCH v2] erofs: complete a missing case for inplace I/O

[Gao Xiang]

From: Gao Xiang <hsiangkao at redhat.com>

Add a missing case which could cause unnecessary page allocation but
not directly use inplace I/O instead, which increases runtime extra
memory footprint.

The detail is, considering an online file-backed page, the right half
of the page is chosen to be cached (e.g. the end page of a readahead
request) and some of its data doesn't exist in managed cache, so the
pcluster will be definitely kept in the submission chain. (IOWs, it
cannot be decompressed without I/O, e.g., due to the bypass queue).

Currently, DELAYEDALLOC/TRYALLOC cases can be downgraded as NOINPLACE,
and stop online pages from inplace I/O. After this patch, unneeded page
allocations won't be observed in pickup_page_for_submission() then.

Signed-off-by: Gao Xiang <hsiangkao at redhat.com>
---
v1: https://lore.kernel.org/r/20201022145724.27284-4-hsiangkao@aol.com

This is a follow-up of previous patch, which can tolerate ro_fsstress
now. The principle is that DELAYEDALLOC/TRYALLOC with new managed pages
indicate that I/O is needed so no needed to downgrade to NOINPLACE
as mentioned in the commit message.

Post it independently rather than mixed in the upcoming bigpcluster
patchset thus make it easier for users to decide to backport or not.

 fs/erofs/zdata.c | 44 +++++++++++++++++++++++++++++---------------
 1 file changed, 29 insertions(+), 15 deletions(-)

diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
index 7ab8a4e3dfcb..cd9b76216925 100644
--- a/fs/erofs/zdata.c
+++ b/fs/erofs/zdata.c
@@ -104,6 +104,12 @@ enum z_erofs_collectmode {
 	 * |_______PRIMARY_FOLLOWED_______|________PRIMARY_HOOKED___________|
 	 */
 	COLLECT_PRIMARY_HOOKED,
+	/*
+	 * a weak form of COLLECT_PRIMARY_FOLLOWED, the difference is that it
+	 * could be dispatched into bypass queue later due to uptodated managed
+	 * pages. All related online pages cannot be reused for inplace I/O (or
+	 * pagevec) since it can be directly decoded without I/O submission.
+	 */
 	COLLECT_PRIMARY_FOLLOWED_NOINPLACE,
 	/*
 	 * The current collection has been linked with the owned chain, and
@@ -186,21 +192,25 @@ static void preload_compressed_pages(struct z_erofs_collector *clt,
 
 		if (page) {
 			t = tag_compressed_page_justfound(page);
-		} else if (type == DELAYEDALLOC) {
-			t = tagptr_init(compressed_page_t, PAGE_UNALLOCATED);
-		} else if (type == TRYALLOC) {
-			newpage = erofs_allocpage(pagepool, gfp);
-			if (!newpage)
-				goto dontalloc;
-
-			set_page_private(newpage, Z_EROFS_PREALLOCATED_PAGE);
-			t = tag_compressed_page_justfound(newpage);
-		} else {	/* DONTALLOC */
-dontalloc:
-			if (standalone)
-				clt->compressedpages = pages;
+		} else {
+			/* I/O is needed, no possible to decompress directly */
 			standalone = false;
-			continue;
+			switch (type) {
+			case DELAYEDALLOC:
+				t = tagptr_init(compressed_page_t,
+						PAGE_UNALLOCATED);
+				break;
+			case TRYALLOC:
+				newpage = erofs_allocpage(pagepool, gfp);
+				if (!newpage)
+					continue;
+				set_page_private(newpage,
+						 Z_EROFS_PREALLOCATED_PAGE);
+				t = tag_compressed_page_justfound(newpage);
+				break;
+			default:        /* DONTALLOC */
+				continue;
+			}
 		}
 
 		if (!cmpxchg_relaxed(pages, NULL, tagptr_cast_ptr(t)))
@@ -214,7 +224,11 @@ static void preload_compressed_pages(struct z_erofs_collector *clt,
 		}
 	}
 
-	if (standalone)		/* downgrade to PRIMARY_FOLLOWED_NOINPLACE */
+	/*
+	 * don't do inplace I/O if all compressed pages are available in
+	 * managed cache since it can be moved to the bypass queue instead.
+	 */
+	if (standalone)
 		clt->mode = COLLECT_PRIMARY_FOLLOWED_NOINPLACE;
 }
 
-- 
2.20.1