linux-kernel - [PATCH 1/5] zram: handle multiple pages attached bio's bvec

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Date:   Mon, 3 Apr 2017 14:17:29 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        <kernel-team@....com>, Minchan Kim <minchan@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, Hannes Reinecke <hare@...e.com>,
        Johannes Thumshirn <jthumshirn@...e.de>
Subject: [PATCH 1/5] zram: handle multiple pages attached bio's bvec

Johannes Thumshirn reported system goes the panic when using NVMe over
Fabrics loopback target with zram.

The reason is zram expects each bvec in bio contains a single page
but nvme can attach a huge bulk of pages attached to the bio's bvec
so that zram's index arithmetic could be wrong so that out-of-bound
access makes panic.

It can be solved by limiting max_sectors with SECTORS_PER_PAGE like
[1] but it makes zram slow because bio should split with each pages
so this patch makes zram aware of multiple pages in a bvec so it
could solve without any regression.

[1] 0bc315381fe9, zram: set physical queue limits to avoid array out of
    bounds accesses

Cc: Jens Axboe <axboe@...nel.dk>
Cc: Hannes Reinecke <hare@...e.com>
Reported-by: Johannes Thumshirn <jthumshirn@...e.de>
Tested-by: Johannes Thumshirn <jthumshirn@...e.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@...e.de>
Signed-off-by: Johannes Thumshirn <jthumshirn@...e.de>
Signed-off-by: Minchan Kim <minchan@...nel.org>
---
 drivers/block/zram/zram_drv.c | 39 ++++++++++-----------------------------
 1 file changed, 10 insertions(+), 29 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 01944419b1f3..28c2836f8c96 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -137,8 +137,7 @@ static inline bool valid_io_request(struct zram *zram,
 
 static void update_position(u32 *index, int *offset, struct bio_vec *bvec)
 {
-	if (*offset + bvec->bv_len >= PAGE_SIZE)
-		(*index)++;
+	*index  += (*offset + bvec->bv_len) / PAGE_SIZE;
 	*offset = (*offset + bvec->bv_len) % PAGE_SIZE;
 }
 
@@ -838,34 +837,20 @@ static void __zram_make_request(struct zram *zram, struct bio *bio)
 	}
 
 	bio_for_each_segment(bvec, bio, iter) {
-		int max_transfer_size = PAGE_SIZE - offset;
-
-		if (bvec.bv_len > max_transfer_size) {
-			/*
-			 * zram_bvec_rw() can only make operation on a single
-			 * zram page. Split the bio vector.
-			 */
-			struct bio_vec bv;
-
-			bv.bv_page = bvec.bv_page;
-			bv.bv_len = max_transfer_size;
-			bv.bv_offset = bvec.bv_offset;
+		struct bio_vec bv = bvec;
+		unsigned int remained = bvec.bv_len;
 
+		do {
+			bv.bv_len = min_t(unsigned int, PAGE_SIZE, remained);
 			if (zram_bvec_rw(zram, &bv, index, offset,
-					 op_is_write(bio_op(bio))) < 0)
+					op_is_write(bio_op(bio))) < 0)
 				goto out;
 
-			bv.bv_len = bvec.bv_len - max_transfer_size;
-			bv.bv_offset += max_transfer_size;
-			if (zram_bvec_rw(zram, &bv, index + 1, 0,
-					 op_is_write(bio_op(bio))) < 0)
-				goto out;
-		} else
-			if (zram_bvec_rw(zram, &bvec, index, offset,
-					 op_is_write(bio_op(bio))) < 0)
-				goto out;
+			bv.bv_offset += bv.bv_len;
+			remained -= bv.bv_len;
 
-		update_position(&index, &offset, &bvec);
+			update_position(&index, &offset, &bv);
+		} while (remained);
 	}
 
 	bio_endio(bio);
@@ -882,8 +867,6 @@ static blk_qc_t zram_make_request(struct request_queue *queue, struct bio *bio)
 {
 	struct zram *zram = queue->queuedata;
 
-	blk_queue_split(queue, &bio, queue->bio_split);
-
 	if (!valid_io_request(zram, bio->bi_iter.bi_sector,
 					bio->bi_iter.bi_size)) {
 		atomic64_inc(&zram->stats.invalid_io);
@@ -1191,8 +1174,6 @@ static int zram_add(void)
 	blk_queue_io_min(zram->disk->queue, PAGE_SIZE);
 	blk_queue_io_opt(zram->disk->queue, PAGE_SIZE);
 	zram->disk->queue->limits.discard_granularity = PAGE_SIZE;
-	zram->disk->queue->limits.max_sectors = SECTORS_PER_PAGE;
-	zram->disk->queue->limits.chunk_sectors = 0;
 	blk_queue_max_discard_sectors(zram->disk->queue, UINT_MAX);
 	/*
 	 * zram_bio_discard() will clear all logical blocks if logical block
-- 
2.7.4