[PATCH 2/2] mm/zswap: classify compression dst overflow as poor compression

[Suseendran Bhaskaran <suseendranb@google.com>]

Commit dca4437a5861 ("mm/zswap: store <PAGE_SIZE compression failed page
as-is") and commit 0b1bf60c324a ("mm/zswap: reduce the size of the
compression buffer to a single page") updated zswap_compress() to pass a
PAGE_SIZE destination buffer to crypto_acomp_compress() (so
incompressible pages typically overflow the PAGE_SIZE buffer due to
compressor metadata overhead) and check:

    if (comp_ret || !dlen || dlen >= PAGE_SIZE)

Because crypto_acomp_compress() previously returned -EINVAL on
destination buffer overflow, this check conflated destination buffer
overflow (incompressible page) with unexpected compressor failures and
did not set comp_ret = -ENOSPC when rejecting incompressible pages,
resulting in two user-facing issues:

1. When zswap writeback is disabled (e.g. memory.zswap.writeback = 0),
   zswap_compress() sets 'comp_ret = comp_ret ? comp_ret : -EINVAL' and
   jumps to unlock. Whether the compressor fails on destination overflow
   with -EINVAL or succeeds with dlen == PAGE_SIZE, comp_ret becomes
   -EINVAL. Consequently, incompressible pages are misaccounted in
   debugfs under reject_compress_fail instead of reject_compress_poor
   (leaving reject_compress_poor stuck at 0 and triggering false
   compressor failure alerts on normal incompressible workloads).

2. When zswap writeback is enabled, any non-zero comp_ret (such as
   -EINVAL from an invalid compressor context, -ENOMEM, or a hardware
   accelerator error) is treated identically to an incompressible page:
   comp_ret is cleared to 0 and the raw page is stored uncompressed in
   zsmalloc. This silently masks genuine compressor failures
   (reject_compress_fail is never incremented) and miscounts pages that
   encountered a compressor error under stored_incompressible_pages.

Update zswap_compress() to treat only comp_ret == -ENOSPC or
(!comp_ret && dlen >= PAGE_SIZE) as an incompressible page (storing it
uncompressed in zswap when writeback is enabled to preserve LRU order,
or rejecting it with comp_ret = -ENOSPC when writeback is disabled so
zswap_reject_compress_poor is incremented), while jumping to unlock on
unexpected compressor errors (comp_ret || !dlen) so
zswap_reject_compress_fail is properly incremented.

Fixes: dca4437a5861 ("mm/zswap: store <PAGE_SIZE compression failed page as-is")
Signed-off-by: Suseendran Bhaskaran <suseendranb@google.com>
---
 mm/zswap.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 37f34e406c8e..5f2a941fcaf1 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -881,12 +881,12 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry,
 	 * only adds metadata overhead.  swap_writeout() will put the page back
 	 * to the active LRU list in the case.
 	 */
-	if (comp_ret || !dlen || dlen >= PAGE_SIZE) {
+	if (comp_ret == -ENOSPC || (!comp_ret && dlen >= PAGE_SIZE)) {
 		rcu_read_lock();
 		if (!mem_cgroup_zswap_writeback_enabled(
 					folio_memcg(page_folio(page)))) {
 			rcu_read_unlock();
-			comp_ret = comp_ret ? comp_ret : -EINVAL;
+			comp_ret = -ENOSPC;
 			goto unlock;
 		}
 		rcu_read_unlock();
@@ -894,6 +894,9 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry,
 		dlen = PAGE_SIZE;
 		dst = kmap_local_page(page);
 		mapped = true;
+	} else if (comp_ret || !dlen) {
+		comp_ret = comp_ret ? comp_ret : -EINVAL;
+		goto unlock;
 	}
 
 	gfp = GFP_NOWAIT | __GFP_NORETRY | __GFP_HIGHMEM | __GFP_MOVABLE;
-- 
2.56.0.218.g1ec904b173-goog