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Commit c5bde238 authored by Jerome Marchand's avatar Jerome Marchand Committed by Greg Kroah-Hartman
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Staging: zram: Replace mutex lock by a R/W semaphore 



Currently, nothing protects zram table from concurrent access.
For instance, ZRAM_UNCOMPRESSED bit can be cleared by zram_free_page()
called from a concurrent write between the time ZRAM_UNCOMPRESSED has
been set and the time it is tested to unmap KM_USER0 in
zram_bvec_write(). This ultimately leads to kernel panic.

Also, a read request can occurs when the page has been freed by a
running write request and before it has been updated, leading to
zero filled block being incorrectly read and "Read before write"
error message.

This patch replace the current mutex by a rw_semaphore. It extends
the protection to zram table (currently, only compression buffers are
protected) and read requests (currently, only write requests are
protected).

Signed-off-by: default avatarJerome Marchand <jmarchan@redhat.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@suse.de>
parent 924bd88d

drivers/staging/zram/zram_drv.c

+13 −12
Original line number Diff line number Diff line
@@ -359,8 +359,6 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index, 
	    zram_test_flag(zram, index, ZRAM_ZERO))

		zram_free_page(zram, index);



	mutex_lock(&zram->lock);



	user_mem = kmap_atomic(page, KM_USER0);



	if (is_partial_io(bvec))
@@ -371,7 +369,6 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index, 


	if (page_zero_filled(uncmem)) {

		kunmap_atomic(user_mem, KM_USER0);

		mutex_unlock(&zram->lock);

		if (is_partial_io(bvec))

			kfree(uncmem);

		zram_stat_inc(&zram->stats.pages_zero);
@@ -388,7 +385,6 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index, 
			kfree(uncmem);



	if (unlikely(ret != LZO_E_OK)) {

		mutex_unlock(&zram->lock);

		pr_err("Compression failed! err=%d\n", ret);

		goto out;

	}
@@ -402,7 +398,6 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index, 
		clen = PAGE_SIZE;

		page_store = alloc_page(GFP_NOIO | __GFP_HIGHMEM);

		if (unlikely(!page_store)) {

			mutex_unlock(&zram->lock);

			pr_info("Error allocating memory for "

				"incompressible page: %u\n", index);

			ret = -ENOMEM;
@@ -420,7 +415,6 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index, 
	if (xv_malloc(zram->mem_pool, clen + sizeof(*zheader),

		      &zram->table[index].page, &store_offset,

		      GFP_NOIO | __GFP_HIGHMEM)) {

		mutex_unlock(&zram->lock);

		pr_info("Error allocating memory for compressed "

			"page: %u, size=%zu\n", index, clen);

		ret = -ENOMEM;
@@ -454,8 +448,6 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index, 
	if (clen <= PAGE_SIZE / 2)

		zram_stat_inc(&zram->stats.good_compress);



	mutex_unlock(&zram->lock);



	return 0;



out:
@@ -467,10 +459,19 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index, 
static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,

			int offset, struct bio *bio, int rw)

{

	if (rw == READ)

		return zram_bvec_read(zram, bvec, index, offset, bio);

	int ret;



	return zram_bvec_write(zram, bvec, index, offset);

	if (rw == READ) {

		down_read(&zram->lock);

		ret = zram_bvec_read(zram, bvec, index, offset, bio);

		up_read(&zram->lock);

	} else {

		down_write(&zram->lock);

		ret = zram_bvec_write(zram, bvec, index, offset);

		up_write(&zram->lock);

	}



	return ret;

}



static void update_position(u32 *index, int *offset, struct bio_vec *bvec)
@@ -701,7 +702,7 @@ static int create_device(struct zram *zram, int device_id) 
{

	int ret = 0;



	mutex_init(&zram->lock);

	init_rwsem(&zram->lock);

	mutex_init(&zram->init_lock);

	spin_lock_init(&zram->stat64_lock);

drivers/staging/zram/zram_drv.h

+2 −2
Original line number Diff line number Diff line
@@ -107,8 +107,8 @@ struct zram { 
	void *compress_buffer;

	struct table *table;

	spinlock_t stat64_lock;	/* protect 64-bit stats */

	struct mutex lock;	/* protect compression buffers against

				 * concurrent writes */

	struct rw_semaphore lock; /* protect compression buffers and table

				   * against concurrent read and writes */

	struct request_queue *queue;

	struct gendisk *disk;

	int init_done;