erofs: complete a missing case for inplace I/O

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.

Link: https://lore.kernel.org/r/20210321183227.5182-1-hsiangkao@aol.com
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>

diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
index 7ab8a4e3dfcb824b75e4777546238bcf73046952..cd9b7621692524d9c0ae9395cc514749f3da0d80 100644 (file)
--- 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 @@ dontalloc:
                }
        }
 
-       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;
 }