syzbot reported a task hang issue due to a deadlock case where it is
waiting for the folio lock of a cached folio that will be used for
cache I/Os.

After looking into the crafted fuzzed image, I found it's formed with
several overlapped big pclusters as below:

 Ext:   logical offset   |  length :     physical offset    |  length
   0:        0..   16384 |   16384 :     151552..    167936 |   16384
   1:    16384..   32768 |   16384 :     155648..    172032 |   16384
   2:    32768..   49152 |   16384 :  537223168.. 537239552 |   16384
...

Here, extent 0/1 are physically overlapped although it's entirely
_impossible_ for normal filesystem images generated by mkfs.

First, managed folios containing compressed data will be marked as
up-to-date and then unlocked immediately (unlike in-place folios) when
compressed I/Os are complete.  If physical blocks are not submitted in
the incremental order, there should be separate BIOs to avoid dependency
issues.  However, the current code mis-arranges z_erofs_fill_bio_vec()
and BIO submission which causes unexpected BIO waits.

Second, managed folios will be connected to their own pclusters for
efficient inter-queries.  However, this is somewhat hard to implement
easily if overlapped big pclusters exist.  Again, these only appear in
fuzzed images so let's simply fall back to temporary short-lived pages
for correctness.

Additionally, it justifies that referenced managed folios cannot be
truncated for now and reverts part of commit 2080ca1ed3e4 ("erofs: tidy
up `struct z_erofs_bvec`") for simplicity although it shouldn't be any
difference.

Reported-by: syzbot+4fc98ed414ae63d1ada2@syzkaller.appspotmail.com
Reported-by: syzbot+de04e06b28cfecf2281c@syzkaller.appspotmail.com
Reported-by: syzbot+c8c8238b394be4a1087d@syzkaller.appspotmail.com
Tested-by: syzbot+4fc98ed414ae63d1ada2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/r/0000000000002fda01061e334873@google.com
Fixes: 8e6c8fa9f2e9 ("erofs: enable big pcluster feature")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
---
changes since v1:
 - Avoid re-entering z_erofs_fill_bio_vec() if bio_add_page() fails;

 fs/erofs/zdata.c | 71 ++++++++++++++++++++++++++----------------------
 1 file changed, 38 insertions(+), 33 deletions(-)

diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
index 2271cb74ae3a..dca11ab0ab75 100644
--- a/fs/erofs/zdata.c
+++ b/fs/erofs/zdata.c
@@ -1392,6 +1392,7 @@ static void z_erofs_fill_bio_vec(struct bio_vec *bvec,
 	struct z_erofs_bvec zbv;
 	struct address_space *mapping;
 	struct folio *folio;
+	struct page *page;
 	int bs = i_blocksize(f->inode);
 
 	/* Except for inplace folios, the entire folio can be used for I/Os */
@@ -1414,7 +1415,6 @@ static void z_erofs_fill_bio_vec(struct bio_vec *bvec,
 	 * file-backed folios will be used instead.
 	 */
 	if (folio->private == (void *)Z_EROFS_PREALLOCATED_PAGE) {
-		folio->private = 0;
 		tocache = true;
 		goto out_tocache;
 	}
@@ -1432,7 +1432,7 @@ static void z_erofs_fill_bio_vec(struct bio_vec *bvec,
 	}
 
 	folio_lock(folio);
-	if (folio->mapping == mc) {
+	if (likely(folio->mapping == mc)) {
 		/*
 		 * The cached folio is still in managed cache but without
 		 * a valid `->private` pcluster hint.  Let's reconnect them.
@@ -1442,41 +1442,44 @@ static void z_erofs_fill_bio_vec(struct bio_vec *bvec,
 			/* compressed_bvecs[] already takes a ref before */
 			folio_put(folio);
 		}
-
-		/* no need to submit if it is already up-to-date */
-		if (folio_test_uptodate(folio)) {
-			folio_unlock(folio);
-			bvec->bv_page = NULL;
+		if (likely(folio->private == pcl))  {
+			/* don't submit cache I/Os again if already uptodate */
+			if (folio_test_uptodate(folio)) {
+				folio_unlock(folio);
+				bvec->bv_page = NULL;
+			}
+			return;
 		}
-		return;
+		/*
+		 * Already linked with another pcluster, which only appears in
+		 * crafted images by fuzzers for now.  But handle this anyway.
+		 */
+		tocache = false;	/* use temporary short-lived pages */
+	} else {
+		DBG_BUGON(1); /* referenced managed folios can't be truncated */
+		tocache = true;
 	}
-
-	/*
-	 * It has been truncated, so it's unsafe to reuse this one. Let's
-	 * allocate a new page for compressed data.
-	 */
-	DBG_BUGON(folio->mapping);
-	tocache = true;
 	folio_unlock(folio);
 	folio_put(folio);
 out_allocfolio:
-	zbv.page = erofs_allocpage(&f->pagepool, gfp | __GFP_NOFAIL);
+	page = erofs_allocpage(&f->pagepool, gfp | __GFP_NOFAIL);
 	spin_lock(&pcl->obj.lockref.lock);
-	if (pcl->compressed_bvecs[nr].page) {
-		erofs_pagepool_add(&f->pagepool, zbv.page);
+	if (unlikely(pcl->compressed_bvecs[nr].page != zbv.page)) {
+		erofs_pagepool_add(&f->pagepool, page);
 		spin_unlock(&pcl->obj.lockref.lock);
 		cond_resched();
 		goto repeat;
 	}
-	bvec->bv_page = pcl->compressed_bvecs[nr].page = zbv.page;
-	folio = page_folio(zbv.page);
-	/* first mark it as a temporary shortlived folio (now 1 ref) */
-	folio->private = (void *)Z_EROFS_SHORTLIVED_PAGE;
+	bvec->bv_page = pcl->compressed_bvecs[nr].page = page;
+	folio = page_folio(page);
 	spin_unlock(&pcl->obj.lockref.lock);
 out_tocache:
 	if (!tocache || bs != PAGE_SIZE ||
-	    filemap_add_folio(mc, folio, pcl->obj.index + nr, gfp))
+	    filemap_add_folio(mc, folio, pcl->obj.index + nr, gfp)) {
+		/* turn into a temporary shortlived folio (1 ref) */
+		folio->private = (void *)Z_EROFS_SHORTLIVED_PAGE;
 		return;
+	}
 	folio_attach_private(folio, pcl);
 	/* drop a refcount added by allocpage (then 2 refs in total here) */
 	folio_put(folio);
@@ -1611,13 +1614,10 @@ static void z_erofs_submit_queue(struct z_erofs_decompress_frontend *f,
 		cur = mdev.m_pa;
 		end = cur + pcl->pclustersize;
 		do {
-			z_erofs_fill_bio_vec(&bvec, f, pcl, i++, mc);
-			if (!bvec.bv_page)
-				continue;
-
+			bvec.bv_page = NULL;
 			if (bio && (cur != last_pa ||
 				    bio->bi_bdev != mdev.m_bdev)) {
-io_retry:
+drain_io:
 				if (erofs_is_fileio_mode(EROFS_SB(sb)))
 					erofs_fileio_submit_bio(bio);
 				else if (erofs_is_fscache_mode(sb))
@@ -1632,6 +1632,15 @@ static void z_erofs_submit_queue(struct z_erofs_decompress_frontend *f,
 				bio = NULL;
 			}
 
+			if (!bvec.bv_page) {
+				z_erofs_fill_bio_vec(&bvec, f, pcl, i++, mc);
+				if (!bvec.bv_page)
+					continue;
+				if (cur + bvec.bv_len > end)
+					bvec.bv_len = end - cur;
+				DBG_BUGON(bvec.bv_len < sb->s_blocksize);
+			}
+
 			if (unlikely(PageWorkingset(bvec.bv_page)) &&
 			    !memstall) {
 				psi_memstall_enter(&pflags);
@@ -1654,13 +1663,9 @@ static void z_erofs_submit_queue(struct z_erofs_decompress_frontend *f,
 				++nr_bios;
 			}
 
-			if (cur + bvec.bv_len > end)
-				bvec.bv_len = end - cur;
-			DBG_BUGON(bvec.bv_len < sb->s_blocksize);
 			if (!bio_add_page(bio, bvec.bv_page, bvec.bv_len,
 					  bvec.bv_offset))
-				goto io_retry;
-
+				goto drain_io;
 			last_pa = cur + bvec.bv_len;
 			bypass = false;
 		} while ((cur += bvec.bv_len) < end);
-- 
2.43.5