cow: stop using mmap

We don't have an equivalent to mmap in the qemu block API, so read and
write the bitmap directly.  At least in the dumb implementation added
in this patch this is a lot less efficient, but it means cow can also
work on windows, and over nbd or curl.  And it fixes qemu-iotests testcase
012 which did not work properly due to issues with read-only mmap access.

In addition we can also get rid of the now unused get_mmap_addr function.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/cow.c

@@ -21,11 +21,9 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  * THE SOFTWARE.
  */
-#ifndef _WIN32
 #include "qemu-common.h"
 #include "block_int.h"
 #include "module.h"
-#include <sys/mman.h>
 
 /**************************************************************/
 /* COW block driver using file system holes */
@@ -45,9 +43,6 @@ struct cow_header_v2 {
 
 typedef struct BDRVCowState {
     int fd;
-    uint8_t *cow_bitmap; /* if non NULL, COW mappings are used first */
-    uint8_t *cow_bitmap_addr; /* mmap address of cow_bitmap */
-    int cow_bitmap_size;
     int64_t cow_sectors_offset;
 } BDRVCowState;
 
@@ -68,6 +63,7 @@ static int cow_open(BlockDriverState *bs, const char *filename, int flags)
     BDRVCowState *s = bs->opaque;
     int fd;
     struct cow_header_v2 cow_header;
+    int bitmap_size;
     int64_t size;
 
     fd = open(filename, O_RDWR | O_BINARY | O_LARGEFILE);
@@ -94,61 +90,92 @@ static int cow_open(BlockDriverState *bs, const char *filename, int flags)
     pstrcpy(bs->backing_file, sizeof(bs->backing_file),
             cow_header.backing_file);
 
-    /* mmap the bitmap */
-    s->cow_bitmap_size = ((bs->total_sectors + 7) >> 3) + sizeof(cow_header);
-    s->cow_bitmap_addr = (void *)mmap(get_mmap_addr(s->cow_bitmap_size),
-                                      s->cow_bitmap_size,
-                                      PROT_READ | PROT_WRITE,
-                                      MAP_SHARED, s->fd, 0);
-    if (s->cow_bitmap_addr == MAP_FAILED)
-        goto fail;
-    s->cow_bitmap = s->cow_bitmap_addr + sizeof(cow_header);
-    s->cow_sectors_offset = (s->cow_bitmap_size + 511) & ~511;
+    bitmap_size = ((bs->total_sectors + 7) >> 3) + sizeof(cow_header);
+    s->cow_sectors_offset = (bitmap_size + 511) & ~511;
     return 0;
  fail:
     close(fd);
     return -1;
 }
 
-static inline void cow_set_bit(uint8_t *bitmap, int64_t bitnum)
+/*
+ * XXX(hch): right now these functions are extremly ineffcient.
+ * We should just read the whole bitmap we'll need in one go instead.
+ */
+static inline int cow_set_bit(BlockDriverState *bs, int64_t bitnum)
 {
-    bitmap[bitnum / 8] |= (1 << (bitnum%8));
+    BDRVCowState *s = bs->opaque;
+    uint64_t offset = sizeof(struct cow_header_v2) + bitnum / 8;
+    uint8_t bitmap;
+
+    if (pread(s->fd, &bitmap, sizeof(bitmap), offset) !=
+	    sizeof(bitmap)) {
+       return -errno;
+    }
+
+    bitmap |= (1 << (bitnum % 8));
+
+    if (pwrite(s->fd, &bitmap, sizeof(bitmap), offset) !=
+	    sizeof(bitmap)) {
+       return -errno;
+    }
+    return 0;
 }
 
-static inline int is_bit_set(const uint8_t *bitmap, int64_t bitnum)
+static inline int is_bit_set(BlockDriverState *bs, int64_t bitnum)
 {
-    return !!(bitmap[bitnum / 8] & (1 << (bitnum%8)));
-}
+    BDRVCowState *s = bs->opaque;
+    uint64_t offset = sizeof(struct cow_header_v2) + bitnum / 8;
+    uint8_t bitmap;
 
+    if (pread(s->fd, &bitmap, sizeof(bitmap), offset) !=
+	    sizeof(bitmap)) {
+       return -errno;
+    }
+
+    return !!(bitmap & (1 << (bitnum % 8)));
+}
 
 /* Return true if first block has been changed (ie. current version is
  * in COW file).  Set the number of continuous blocks for which that
  * is true. */
-static inline int is_changed(uint8_t *bitmap,
-                             int64_t sector_num, int nb_sectors,
-                             int *num_same)
+static int cow_is_allocated(BlockDriverState *bs, int64_t sector_num,
+        int nb_sectors, int *num_same)
 {
     int changed;
 
-    if (!bitmap || nb_sectors == 0) {
+    if (nb_sectors == 0) {
 	*num_same = nb_sectors;
 	return 0;
     }
 
-    changed = is_bit_set(bitmap, sector_num);
+    changed = is_bit_set(bs, sector_num);
+    if (changed < 0) {
+        return 0; /* XXX: how to return I/O errors? */
+    }
+
     for (*num_same = 1; *num_same < nb_sectors; (*num_same)++) {
-	if (is_bit_set(bitmap, sector_num + *num_same) != changed)
+	if (is_bit_set(bs, sector_num + *num_same) != changed)
 	    break;
     }
 
     return changed;
 }
 
-static int cow_is_allocated(BlockDriverState *bs, int64_t sector_num,
-                            int nb_sectors, int *pnum)
+static int cow_update_bitmap(BlockDriverState *bs, int64_t sector_num,
+        int nb_sectors)
 {
-    BDRVCowState *s = bs->opaque;
-    return is_changed(s->cow_bitmap, sector_num, nb_sectors, pnum);
+    int error = 0;
+    int i;
+
+    for (i = 0; i < nb_sectors; i++) {
+        error = cow_set_bit(bs, sector_num + i);
+        if (error) {
+            break;
+        }
+    }
+
+    return error;
 }
 
 static int cow_read(BlockDriverState *bs, int64_t sector_num,
@@ -158,7 +185,7 @@ static int cow_read(BlockDriverState *bs, int64_t sector_num,
     int ret, n;
 
     while (nb_sectors > 0) {
-        if (is_changed(s->cow_bitmap, sector_num, nb_sectors, &n)) {
+        if (cow_is_allocated(bs, sector_num, nb_sectors, &n)) {
             ret = pread(s->fd, buf, n * 512,
                         s->cow_sectors_offset + sector_num * 512);
             if (ret != n * 512)
@@ -184,21 +211,19 @@ static int cow_write(BlockDriverState *bs, int64_t sector_num,
                      const uint8_t *buf, int nb_sectors)
 {
     BDRVCowState *s = bs->opaque;
-    int ret, i;
+    int ret;
 
     ret = pwrite(s->fd, buf, nb_sectors * 512,
                  s->cow_sectors_offset + sector_num * 512);
     if (ret != nb_sectors * 512)
         return -1;
-    for (i = 0; i < nb_sectors; i++)
-        cow_set_bit(s->cow_bitmap, sector_num + i);
-    return 0;
+
+    return cow_update_bitmap(bs, sector_num, nb_sectors);
 }
 
 static void cow_close(BlockDriverState *bs)
 {
     BDRVCowState *s = bs->opaque;
-    munmap((void *)s->cow_bitmap_addr, s->cow_bitmap_size);
     close(s->fd);
 }
 
@@ -308,4 +333,3 @@ static void bdrv_cow_init(void)
 }
 
 block_init(bdrv_cow_init);
-#endif

qemu-common.h

@@ -164,9 +164,6 @@ void qemu_free(void *ptr);
 char *qemu_strdup(const char *str);
 char *qemu_strndup(const char *str, size_t size);
 
-void *get_mmap_addr(unsigned long size);
-
-
 void qemu_mutex_lock_iothread(void);
 void qemu_mutex_unlock_iothread(void);
 

qemu-malloc.c

@@ -32,11 +32,6 @@ static void *oom_check(void *ptr)
     return ptr;
 }
 
-void *get_mmap_addr(unsigned long size)
-{
-    return NULL;
-}
-
 void qemu_free(void *ptr)
 {
     free(ptr);