[PATCH 9/9] blk-crypto: use mempool_alloc_bulk for encrypted bio page allocation

[Christoph Hellwig <hch@lst.de>]

Calling mempool_alloc in a lot is not safe unless the maximum allocation
size times the maximum number of threads using it is less than the
minimum pool size.  Use the new mempool_alloc_bulk helper to allocate
all missing elements in one pass to remove this deadlock risk.  This
also means that non-pool allocations now use alloc_pages_bulk which can
be significantly faster than a loop over individual page allocations.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 block/blk-crypto-fallback.c | 54 ++++++++++++++++++++++++++++++-------
 1 file changed, 45 insertions(+), 9 deletions(-)

diff --git a/block/blk-crypto-fallback.c b/block/blk-crypto-fallback.c
index 33aa7b26ed37..2f78027f0cce 100644
--- a/block/blk-crypto-fallback.c
+++ b/block/blk-crypto-fallback.c
@@ -22,7 +22,7 @@
 #include "blk-cgroup.h"
 #include "blk-crypto-internal.h"
 
-static unsigned int num_prealloc_bounce_pg = 32;
+static unsigned int num_prealloc_bounce_pg = BIO_MAX_VECS;
 module_param(num_prealloc_bounce_pg, uint, 0);
 MODULE_PARM_DESC(num_prealloc_bounce_pg,
 		 "Number of preallocated bounce pages for the blk-crypto crypto API fallback");
@@ -164,11 +164,21 @@ static bool blk_crypto_fallback_bio_valid(struct bio *bio)
 static void blk_crypto_fallback_encrypt_endio(struct bio *enc_bio)
 {
 	struct bio *src_bio = enc_bio->bi_private;
-	int i;
+	struct page **pages = (struct page **)enc_bio->bi_io_vec;
+	struct bio_vec *bv;
+	unsigned int i;
 
-	for (i = 0; i < enc_bio->bi_vcnt; i++)
-		mempool_free(enc_bio->bi_io_vec[i].bv_page,
-			     blk_crypto_bounce_page_pool);
+	/*
+	 * Use the same trick as the alloc side to avoid the need for an extra
+	 * pages array.
+	 */
+	bio_for_each_bvec_all(bv, enc_bio, i)
+		pages[i] = bv->bv_page;
+
+	i = mempool_free_bulk(blk_crypto_bounce_page_pool, (void **)pages,
+			enc_bio->bi_vcnt);
+	if (i < enc_bio->bi_vcnt)
+		release_pages(pages + i, enc_bio->bi_vcnt - i);
 
 	src_bio->bi_status = enc_bio->bi_status;
 
@@ -176,9 +186,12 @@ static void blk_crypto_fallback_encrypt_endio(struct bio *enc_bio)
 	bio_endio(src_bio);
 }
 
+#define PAGE_PTRS_PER_BVEC     (sizeof(struct bio_vec) / sizeof(struct page *))
+
 static struct bio *blk_crypto_alloc_enc_bio(struct bio *bio_src,
-		unsigned int nr_segs)
+		unsigned int nr_segs, struct page ***pages_ret)
 {
+	struct page **pages;
 	struct bio *bio;
 
 	bio = bio_alloc_bioset(bio_src->bi_bdev, nr_segs, bio_src->bi_opf,
@@ -192,6 +205,29 @@ static struct bio *blk_crypto_alloc_enc_bio(struct bio *bio_src,
 	bio->bi_write_stream	= bio_src->bi_write_stream;
 	bio->bi_iter.bi_sector	= bio_src->bi_iter.bi_sector;
 	bio_clone_blkg_association(bio, bio_src);
+
+	/*
+	 * Move page array up in the allocated memory for the bio vecs as far as
+	 * possible so that we can start filling biovecs from the beginning
+	 * without overwriting the temporary page array.
+	 */
+	static_assert(PAGE_PTRS_PER_BVEC > 1);
+	pages = (struct page **)bio->bi_io_vec;
+	pages += nr_segs * (PAGE_PTRS_PER_BVEC - 1);
+
+	/*
+	 * Try a bulk allocation first.  This could leave random pages in the
+	 * array unallocated, but we'll fix that up later in mempool_alloc_bulk.
+	 *
+	 * Note: alloc_pages_bulk needs the array to be zeroed, as it assumes
+	 * any non-zero slot already contains a valid allocation.
+	 */
+	memset(pages, 0, sizeof(struct page *) * nr_segs);
+	if (alloc_pages_bulk(GFP_NOFS, nr_segs, pages) < nr_segs) {
+		mempool_alloc_bulk(blk_crypto_bounce_page_pool, (void **)pages,
+				nr_segs, GFP_NOIO);
+	}
+	*pages_ret = pages;
 	return bio;
 }
 
@@ -234,6 +270,7 @@ static blk_status_t __blk_crypto_fallback_encrypt_bio(struct bio *src_bio,
 	struct scatterlist src, dst;
 	union blk_crypto_iv iv;
 	struct bio *enc_bio = NULL;
+	struct page **enc_pages;
 	unsigned int nr_segs;
 	unsigned int enc_idx = 0;
 	unsigned int j;
@@ -259,11 +296,10 @@ static blk_status_t __blk_crypto_fallback_encrypt_bio(struct bio *src_bio,
 
 		if (!enc_bio) {
 			enc_bio = blk_crypto_alloc_enc_bio(src_bio,
-					min(nr_segs, BIO_MAX_VECS));
+					min(nr_segs, BIO_MAX_VECS), &enc_pages);
 		}
 
-		enc_page = mempool_alloc(blk_crypto_bounce_page_pool,
-				GFP_NOIO);
+		enc_page = enc_pages[enc_idx];
 		__bio_add_page(enc_bio, enc_page, src_bv.bv_len,
 				src_bv.bv_offset);
 
-- 
2.47.3