L2ARC: Cleanup buffer re-compression

When compressed ARC is disabled, we may have to re-compress when
writing into L2ARC.  If doing so we can't fit it into the original
physical size, we should just fail immediately, since even if it
may still fit into allocation size, its checksum will never match.

While there, refactor the code similar to other compression places
without using abd_return_buf_copy().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes openzfs#16038

module/zfs/arc.c

@@ -8902,7 +8902,6 @@ l2arc_apply_transforms(spa_t *spa, arc_buf_hdr_t *hdr, uint64_t asize,
     abd_t **abd_out)
 {
 	int ret;
-	void *tmp = NULL;
 	abd_t *cabd = NULL, *eabd = NULL, *to_write = hdr->b_l1hdr.b_pabd;
 	enum zio_compress compress = HDR_GET_COMPRESS(hdr);
 	uint64_t psize = HDR_GET_PSIZE(hdr);
@@ -8923,12 +8922,11 @@ l2arc_apply_transforms(spa_t *spa, arc_buf_hdr_t *hdr, uint64_t asize,
 	 * and copy the data. This may be done to eliminate a dependency on a
 	 * shared buffer or to reallocate the buffer to match asize.
 	 */
-	if (HDR_HAS_RABD(hdr) && asize != psize) {
-		ASSERT3U(asize, >=, psize);
+	if (HDR_HAS_RABD(hdr)) {
+		ASSERT3U(asize, >, psize);
 		to_write = abd_alloc_for_io(asize, ismd);
 		abd_copy(to_write, hdr->b_crypt_hdr.b_rabd, psize);
-		if (psize != asize)
-			abd_zero_off(to_write, psize, asize - psize);
+		abd_zero_off(to_write, psize, asize - psize);
 		goto out;
 	}
 
@@ -8937,48 +8935,31 @@ l2arc_apply_transforms(spa_t *spa, arc_buf_hdr_t *hdr, uint64_t asize,
 		ASSERT3U(size, ==, psize);
 		to_write = abd_alloc_for_io(asize, ismd);
 		abd_copy(to_write, hdr->b_l1hdr.b_pabd, size);
-		if (size != asize)
+		if (asize > size)
 			abd_zero_off(to_write, size, asize - size);
 		goto out;
 	}
 
 	if (compress != ZIO_COMPRESS_OFF && !HDR_COMPRESSION_ENABLED(hdr)) {
-		/*
-		 * In some cases, we can wind up with size > asize, so
-		 * we need to opt for the larger allocation option here.
-		 *
-		 * (We also need abd_return_buf_copy in all cases because
-		 * it's an ASSERT() to modify the buffer before returning it
-		 * with arc_return_buf(), and all the compressors
-		 * write things before deciding to fail compression in nearly
-		 * every case.)
-		 */
-		uint64_t bufsize = MAX(size, asize);
-		cabd = abd_alloc_for_io(bufsize, ismd);
-		tmp = abd_borrow_buf(cabd, bufsize);
-
-		psize = zio_compress_data(compress, to_write, &tmp, size,
-		    hdr->b_complevel);
-
-		if (psize >= asize) {
-			psize = HDR_GET_PSIZE(hdr);
-			abd_return_buf_copy(cabd, tmp, bufsize);
-			HDR_SET_COMPRESS(hdr, ZIO_COMPRESS_OFF);
-			to_write = cabd;
-			abd_copy(to_write, hdr->b_l1hdr.b_pabd, psize);
-			if (psize != asize)
-				abd_zero_off(to_write, psize, asize - psize);
-			goto encrypt;
+		size_t bufsize = MAX(size, asize);
+		void *buf = zio_buf_alloc(bufsize);
+		uint64_t csize = zio_compress_data(compress, to_write, &buf,
+		    size, hdr->b_complevel);
+		if (csize > psize) {
+			/*
+			 * We can't re-compress the block into the original
+			 * psize.  Even if it fits into asize, it does not
+			 * matter, since checksum will never match on read.
+			 */
+			zio_buf_free(buf, bufsize);
+			return (SET_ERROR(EIO));
 		}
-		ASSERT3U(psize, <=, HDR_GET_PSIZE(hdr));
-		if (psize < asize)
-			memset((char *)tmp + psize, 0, bufsize - psize);
-		psize = HDR_GET_PSIZE(hdr);
-		abd_return_buf_copy(cabd, tmp, bufsize);
-		to_write = cabd;
+		if (asize > csize)
+			memset((char *)buf + csize, 0, asize - csize);
+		to_write = cabd = abd_get_from_buf(buf, bufsize);
+		abd_take_ownership_of_buf(cabd, B_TRUE);
 	}
 
-encrypt:
 	if (HDR_ENCRYPTED(hdr)) {
 		eabd = abd_alloc_for_io(asize, ismd);