block: add partition resize function to blkpg ioctl

Add a new operation code (BLKPG_RESIZE_PARTITION) to the BLKPG ioctl that
allows altering the size of an existing partition, even if it is currently
in use.

This patch converts hd_struct->nr_sects into sequence counter because
One might extend a partition while IO is happening to it and update of
nr_sects can be non-atomic on 32bit machines with 64bit sector_t. This
can lead to issues like reading inconsistent size of a partition. Sequence
counter have been used so that readers don't have to take bdev mutex lock
as we call sector_in_part() very frequently.

Now all the access to hd_struct->nr_sects should happen using sequence
counter read/update helper functions part_nr_sects_read/part_nr_sects_write.
There is one exception though, set_capacity()/get_capacity(). I think
theoritically race should exist there too but this patch does not
modify set_capacity()/get_capacity() due to sheer number of call sites
and I am afraid that change might break something. I have left that as a
TODO item. We can handle it later if need be. This patch does not introduce
any new races as such w.r.t set_capacity()/get_capacity().

v2: Add CONFIG_LBDAF test to UP preempt case as suggested by Phillip.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Phillip Susi <psusi@ubuntu.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

5 files changed, 132 insertions, 9 deletions

diff --git a/block/genhd.c b/block/genhd.c
index 9cf5583c90ff..cac7366957c3 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -154,7 +154,7 @@ struct hd_struct *disk_part_iter_next(struct disk_part_iter *piter)
                 part = rcu_dereference(ptbl->part[piter->idx]);
                 if (!part)
                         continue;
-                if (!part->nr_sects &&
+                if (!part_nr_sects_read(part) &&
                     !(piter->flags & DISK_PITER_INCL_EMPTY) &&
                     !(piter->flags & DISK_PITER_INCL_EMPTY_PART0 &&
                       piter->idx == 0))
@@ -191,7 +191,7 @@ EXPORT_SYMBOL_GPL(disk_part_iter_exit);
 static inline int sector_in_part(struct hd_struct *part, sector_t sector)
 {
         return part->start_sect <= sector &&
-                sector < part->start_sect + part->nr_sects;
+                sector < part->start_sect + part_nr_sects_read(part);
 }
 
 /**
@@ -769,8 +769,8 @@ void __init printk_all_partitions(void)
 
                         printk("%s%s %10llu %s %s", is_part0 ? "" : "  ",
                                bdevt_str(part_devt(part), devt_buf),
-                               (unsigned long long)part->nr_sects >> 1,
-                               disk_name(disk, part->partno, name_buf),
+                               (unsigned long long)part_nr_sects_read(part) >> 1
+                               , disk_name(disk, part->partno, name_buf),
                                uuid_buf);
                         if (is_part0) {
                                 if (disk->driverfs_dev != NULL &&
@@ -862,7 +862,7 @@ static int show_partition(struct seq_file *seqf, void *v)
         while ((part = disk_part_iter_next(&piter)))
                 seq_printf(seqf, "%4d  %7d %10llu %s\n",
                            MAJOR(part_devt(part)), MINOR(part_devt(part)),
-                           (unsigned long long)part->nr_sects >> 1,
+                           (unsigned long long)part_nr_sects_read(part) >> 1,
                            disk_name(sgp, part->partno, buf));
         disk_part_iter_exit(&piter);
 
@@ -1268,6 +1268,16 @@ struct gendisk *alloc_disk_node(int minors, int node_id)
                 }
                 disk->part_tbl->part[0] = &disk->part0;
 
+                /*
+                 * set_capacity() and get_capacity() currently don't use
+                 * seqcounter to read/update the part0->nr_sects. Still init
+                 * the counter as we can read the sectors in IO submission
+                 * patch using seqence counters.
+                 *
+                 * TODO: Ideally set_capacity() and get_capacity() should be
+                 * converted to make use of bd_mutex and sequence counters.
+                 */
+                seqcount_init(&disk->part0.nr_sects_seq);
                 hd_ref_init(&disk->part0);
 
                 disk->minors = minors;
diff --git a/block/ioctl.c b/block/ioctl.c
index ba15b2dbfb98..4476e0e85d16 100644
--- a/block/ioctl.c
+++ b/block/ioctl.c
@@ -13,7 +13,7 @@ static int blkpg_ioctl(struct block_device *bdev, struct blkpg_ioctl_arg __user
 {
         struct block_device *bdevp;
         struct gendisk *disk;
-        struct hd_struct *part;
+        struct hd_struct *part, *lpart;
         struct blkpg_ioctl_arg a;
         struct blkpg_partition p;
         struct disk_part_iter piter;
@@ -36,8 +36,8 @@ static int blkpg_ioctl(struct block_device *bdev, struct blkpg_ioctl_arg __user
                 case BLKPG_ADD_PARTITION:
                         start = p.start >> 9;
                         length = p.length >> 9;
-                        /* check for fit in a hd_struct */ 
-                        if (sizeof(sector_t) == sizeof(long) && 
+                        /* check for fit in a hd_struct */
+                        if (sizeof(sector_t) == sizeof(long) &&
                             sizeof(long long) > sizeof(long)) {
                                 long pstart = start, plength = length;
                                 if (pstart != start || plength != length
@@ -92,6 +92,59 @@ static int blkpg_ioctl(struct block_device *bdev, struct blkpg_ioctl_arg __user
                         bdput(bdevp);
 
                         return 0;
+                case BLKPG_RESIZE_PARTITION:
+                        start = p.start >> 9;
+                        /* new length of partition in bytes */
+                        length = p.length >> 9;
+                        /* check for fit in a hd_struct */
+                        if (sizeof(sector_t) == sizeof(long) &&
+                            sizeof(long long) > sizeof(long)) {
+                                long pstart = start, plength = length;
+                                if (pstart != start || plength != length
+                                    || pstart < 0 || plength < 0)
+                                        return -EINVAL;
+                        }
+                        part = disk_get_part(disk, partno);
+                        if (!part)
+                                return -ENXIO;
+                        bdevp = bdget(part_devt(part));
+                        if (!bdevp) {
+                                disk_put_part(part);
+                                return -ENOMEM;
+                        }
+                        mutex_lock(&bdevp->bd_mutex);
+                        mutex_lock_nested(&bdev->bd_mutex, 1);
+                        if (start != part->start_sect) {
+                                mutex_unlock(&bdevp->bd_mutex);
+                                mutex_unlock(&bdev->bd_mutex);
+                                bdput(bdevp);
+                                disk_put_part(part);
+                                return -EINVAL;
+                        }
+                        /* overlap? */
+                        disk_part_iter_init(&piter, disk,
+                                            DISK_PITER_INCL_EMPTY);
+                        while ((lpart = disk_part_iter_next(&piter))) {
+                                if (lpart->partno != partno &&
+                                   !(start + length <= lpart->start_sect ||
+                                   start >= lpart->start_sect + lpart->nr_sects)
+                                   ) {
+                                        disk_part_iter_exit(&piter);
+                                        mutex_unlock(&bdevp->bd_mutex);
+                                        mutex_unlock(&bdev->bd_mutex);
+                                        bdput(bdevp);
+                                        disk_put_part(part);
+                                        return -EBUSY;
+                                }
+                        }
+                        disk_part_iter_exit(&piter);
+                        part_nr_sects_write(part, (sector_t)length);
+                        i_size_write(bdevp->bd_inode, p.length);
+                        mutex_unlock(&bdevp->bd_mutex);
+                        mutex_unlock(&bdev->bd_mutex);
+                        bdput(bdevp);
+                        disk_put_part(part);
+                        return 0;
                 default:
                         return -EINVAL;
         }
diff --git a/block/partition-generic.c b/block/partition-generic.c
index 6df5d6928a44..f1d14519cc04 100644
--- a/block/partition-generic.c
+++ b/block/partition-generic.c
@@ -84,7 +84,7 @@ ssize_t part_size_show(struct device *dev,
                        struct device_attribute *attr, char *buf)
 {
         struct hd_struct *p = dev_to_part(dev);
-        return sprintf(buf, "%llu\n",(unsigned long long)p->nr_sects);
+        return sprintf(buf, "%llu\n",(unsigned long long)part_nr_sects_read(p));
 }
 
 static ssize_t part_ro_show(struct device *dev,
@@ -294,6 +294,8 @@ struct hd_struct *add_partition(struct gendisk *disk, int partno,
                 err = -ENOMEM;
                 goto out_free;
         }
+
+        seqcount_init(&p->nr_sects_seq);
         pdev = part_to_dev(p);
 
         p->start_sect = start;
diff --git a/include/linux/blkpg.h b/include/linux/blkpg.h
index faf8a45af210..a8519446c111 100644
--- a/include/linux/blkpg.h
+++ b/include/linux/blkpg.h
@@ -40,6 +40,7 @@ struct blkpg_ioctl_arg {
 /* The subfunctions (for the op field) */
 #define BLKPG_ADD_PARTITION     1
 #define BLKPG_DEL_PARTITION     2
+#define BLKPG_RESIZE_PARTITION  3
 
 /* Sizes of name fields. Unused at present. */
 #define BLKPG_DEVNAMELTH        64
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index 017a7fb5a1fc..b88723b81b3d 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -98,7 +98,13 @@ struct partition_meta_info {
 
 struct hd_struct {
         sector_t start_sect;
+        /*
+         * nr_sects is protected by sequence counter. One might extend a
+         * partition while IO is happening to it and update of nr_sects
+         * can be non-atomic on 32bit machines with 64bit sector_t.
+         */
         sector_t nr_sects;
+        seqcount_t nr_sects_seq;
         sector_t alignment_offset;
         unsigned int discard_alignment;
         struct device __dev;
@@ -648,6 +654,57 @@ static inline void hd_struct_put(struct hd_struct *part)
                 __delete_partition(part);
 }
 
+/*
+ * Any access of part->nr_sects which is not protected by partition
+ * bd_mutex or gendisk bdev bd_mutex, should be done using this
+ * accessor function.
+ *
+ * Code written along the lines of i_size_read() and i_size_write().
+ * CONFIG_PREEMPT case optimizes the case of UP kernel with preemption
+ * on.
+ */
+static inline sector_t part_nr_sects_read(struct hd_struct *part)
+{
+#if BITS_PER_LONG==32 && defined(CONFIG_LBDAF) && defined(CONFIG_SMP)
+        sector_t nr_sects;
+        unsigned seq;
+        do {
+                seq = read_seqcount_begin(&part->nr_sects_seq);
+                nr_sects = part->nr_sects;
+        } while (read_seqcount_retry(&part->nr_sects_seq, seq));
+        return nr_sects;
+#elif BITS_PER_LONG==32 && defined(CONFIG_LBDAF) && defined(CONFIG_PREEMPT)
+        sector_t nr_sects;
+
+        preempt_disable();
+        nr_sects = part->nr_sects;
+        preempt_enable();
+        return nr_sects;
+#else
+        return part->nr_sects;
+#endif
+}
+
+/*
+ * Should be called with mutex lock held (typically bd_mutex) of partition
+ * to provide mutual exlusion among writers otherwise seqcount might be
+ * left in wrong state leaving the readers spinning infinitely.
+ */
+static inline void part_nr_sects_write(struct hd_struct *part, sector_t size)
+{
+#if BITS_PER_LONG==32 && defined(CONFIG_LBDAF) && defined(CONFIG_SMP)
+        write_seqcount_begin(&part->nr_sects_seq);
+        part->nr_sects = size;
+        write_seqcount_end(&part->nr_sects_seq);
+#elif BITS_PER_LONG==32 && defined(CONFIG_LBDAF) && defined(CONFIG_PREEMPT)
+        preempt_disable();
+        part->nr_sects = size;
+        preempt_enable();
+#else
+        part->nr_sects = size;
+#endif
+}
+
 #else /* CONFIG_BLOCK */
 
 static inline void printk_all_partitions(void) { }