Re: [PATCH v2] zswap: do not crash the kernel on decompression failure

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Feb 27, 2025 at 07:29:45AM +0000, Yosry Ahmed wrote:
> Maybe we can do something like this:
> 
> /* Returns true if the folio was in the zeromap or zswap */
> bool swap_read_folio_in_memory(struct folio *folio)
> {
> 	int ret;
> 
> 	ret = swap_read_folio_zeromap(folio);
> 	if (ret == -ENOENT)
> 		ret = zswap_load(folio);
> 
> 	if (ret == 0) {
> 		folio_mark_uptodate(folio);
> 		folio_unlock(folio);
> 		return true;
> 	} else if (ret != -ENOENT) {
> 		folio_unlock(folio);
> 		return true;
> 	} else {
> 		return false;
> 	}
> }

Eh, I think we're getting colder again.

This looks repetitive, zswap_load() is kind of awkward in that error
leg, and combining the two into one function is a bit of a stretch.

There is also something to be said about folio_mark_uptodate() and
folio_unlock() ususally being done by the backend implementation - in
what the page cache would call the "filler" method - to signal when
it's done reading, and what the outcome was.

E.g. for fs it's always in the specific ->read implementation:

static int simple_read_folio(struct file *file, struct folio *folio)
{
	folio_zero_range(folio, 0, folio_size(folio));
	flush_dcache_folio(folio);
	folio_mark_uptodate(folio);
	folio_unlock(folio);
	return 0;
}

and not in the generic manifold:

$ grep -c folio_mark_uptodate mm/filemap.c 
0

I'd actually rather push those down into zeromap and zswap as well to
follow that pattern more closely:

diff --git a/mm/page_io.c b/mm/page_io.c
index 9b983de351f9..1fb5ce1884bd 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -538,6 +538,7 @@ static bool swap_read_folio_zeromap(struct folio *folio)
 
 	folio_zero_range(folio, 0, folio_size(folio));
 	folio_mark_uptodate(folio);
+	folio_unlock(folio);
 	return true;
 }
 
@@ -635,13 +636,11 @@ void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
 	}
 	delayacct_swapin_start();
 
-	if (swap_read_folio_zeromap(folio)) {
-		folio_unlock(folio);
+	if (swap_read_folio_zeromap(folio))
 		goto finish;
-	} else if (zswap_load(folio)) {
-		folio_unlock(folio);
+
+	if (zswap_load(folio) != -ENOENT)
 		goto finish;
-	}
 
 	/* We have to read from slower devices. Increase zswap protection. */
 	zswap_folio_swapin(folio);
diff --git a/mm/zswap.c b/mm/zswap.c
index 6dbf31bd2218..76b2a964b0cd 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1620,7 +1620,7 @@ bool zswap_store(struct folio *folio)
 	return ret;
 }
 
-bool zswap_load(struct folio *folio)
+int zswap_load(struct folio *folio)
 {
 	swp_entry_t swp = folio->swap;
 	pgoff_t offset = swp_offset(swp);
@@ -1631,7 +1631,7 @@ bool zswap_load(struct folio *folio)
 	VM_WARN_ON_ONCE(!folio_test_locked(folio));
 
 	if (zswap_never_enabled())
-		return false;
+		return -ENOENT;
 
 	/*
 	 * Large folios should not be swapped in while zswap is being used, as
@@ -1641,8 +1641,25 @@ bool zswap_load(struct folio *folio)
 	 * Return true without marking the folio uptodate so that an IO error is
 	 * emitted (e.g. do_swap_page() will sigbus).
 	 */
-	if (WARN_ON_ONCE(folio_test_large(folio)))
-		return true;
+	if (WARN_ON_ONCE(folio_test_large(folio))) {
+		folio_unlock(folio);
+		return -EINVAL;
+	}
+
+	entry = xa_load(tree, offset);
+	if (!entry)
+		return -ENOENT;
+
+	if (!zswap_decompress(entry, folio)) {
+		folio_unlock(folio);
+		return -EIO;
+	}
+
+	folio_mark_uptodate(folio);
+
+	count_vm_event(ZSWPIN);
+	if (entry->objcg)
+		count_objcg_events(entry->objcg, ZSWPIN, 1);
 
 	/*
 	 * When reading into the swapcache, invalidate our entry. The
@@ -1656,27 +1673,14 @@ bool zswap_load(struct folio *folio)
 	 * files, which reads into a private page and may free it if
 	 * the fault fails. We remain the primary owner of the entry.)
 	 */
-	if (swapcache)
-		entry = xa_erase(tree, offset);
-	else
-		entry = xa_load(tree, offset);
-
-	if (!entry)
-		return false;
-
-	zswap_decompress(entry, folio);
-
-	count_vm_event(ZSWPIN);
-	if (entry->objcg)
-		count_objcg_events(entry->objcg, ZSWPIN, 1);
-
 	if (swapcache) {
-		zswap_entry_free(entry);
 		folio_mark_dirty(folio);
+		xa_erase(tree, offset);
+		zswap_entry_free(entry);
 	}
 
-	folio_mark_uptodate(folio);
-	return true;
+	folio_unlock(folio);
+	return 0;
 }
 
 void zswap_invalidate(swp_entry_t swp)