[PATCH] mm/zswap: validate dlen and normalize comp_ret on compression success

[Suseendran Bhaskaran <suseendranb@google.com>]

In zswap_compress(), dlen is only meaningful when compression succeeds
(!comp_ret), as compressors do not consistently set req->dlen on error
(e.g. zstd, 842, and qat set dlen = 0; lz4, lz4hc, lzo, lzo-rle,
deflate, and iaa leave dlen at PAGE_SIZE; hisi_zip sets dlen to the
hardware-reported produced bytes).

When !comp_ret and dlen == PAGE_SIZE with zswap writeback disabled,
zswap_compress() sets comp_ret to -EINVAL instead of -ENOSPC,
miscounting the page under reject_compress_fail instead of
reject_compress_poor.

When !comp_ret, set comp_ret = -ENOSPC if dlen == PAGE_SIZE, or
comp_ret = -EINVAL if WARN_ON_ONCE(!dlen || dlen > PAGE_SIZE). This
allows simplifying the failure check to 'if (comp_ret)' and dropping
the ternary assignment in the writeback-disabled path.

Fixes: dca4437a5861 ("mm/zswap: store <PAGE_SIZE compression failed page as-is")
Signed-off-by: Suseendran Bhaskaran <suseendranb@google.com>
---
Previously sent as [PATCH 2/2] of:
https://lore.kernel.org/all/20261021205626.3286270-3-suseendranb@google.com/

Changes since v1:
- Split from patch 1/2 (which is going through independently).
- Keep existing writeback-enabled behavior for comp_ret != 0.
- Normalize comp_ret under !comp_ret and check 'if (comp_ret)'.

 mm/zswap.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 37f34e406c8e..1e727abcf9fb 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -873,6 +873,12 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry,
 	 */
 	comp_ret = crypto_wait_req(crypto_acomp_compress(acomp_ctx->req), &acomp_ctx->wait);
 	dlen = acomp_ctx->req->dlen;
+	if (!comp_ret) {
+		if (dlen == PAGE_SIZE)
+			comp_ret = -ENOSPC;
+		else if (WARN_ON_ONCE(!dlen || dlen > PAGE_SIZE))
+			comp_ret = -EINVAL;
+	}
 
 	/*
 	 * If a page cannot be compressed into a size smaller than PAGE_SIZE,
@@ -881,12 +887,11 @@ 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) {
 		rcu_read_lock();
 		if (!mem_cgroup_zswap_writeback_enabled(
 					folio_memcg(page_folio(page)))) {
 			rcu_read_unlock();
-			comp_ret = comp_ret ? comp_ret : -EINVAL;
 			goto unlock;
 		}
 		rcu_read_unlock();
-- 
2.56.0.257.gf7497c7d27-goog