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Message-ID: <601b8d3d-4de2-7dda-d839-5960841529f8@redhat.com>
Date: Wed, 27 Mar 2019 13:51:08 +0000
From: Andrew Price <anprice@...hat.com>
To: David Howells <dhowells@...hat.com>
Cc: miklos@...redi.hu, viro@...iv.linux.org.uk,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/4] vfs: Create fs_context-aware mount_bdev()
replacement
On 27/03/2019 11:23, David Howells wrote:
> Andrew Price <anprice@...hat.com> wrote:
>
>>> + up_write(&s->s_umount);
>>> + blkdev_put(bdev, fc->bdev_mode);
>>> + down_write(&s->s_umount);
>>
>> fc->bdev should be NULLed here (or, on the way out of sget_fc() might be more
>> appropriate) otherwise we get a double-blkdev_put() leading to NULL pointer
>> derefs later. This happens when I mount a device twice and then unmount them,
>> or mount it 3 times.
>
> How about the attached changes? Note that they conflict a bit with the mtd
> changes from where I took the destructor piece.
>
> David
It fixes my bug and I see nothing unexpected from an xfstests -g quick
run with my gfs2 patch on top. Looks good.
Thanks,
Andy
> ---
> commit 161602f963ae5a05bdb834461f7a4896286fbde0
> Author: David Howells <dhowells@...hat.com>
> Date: Wed Mar 27 11:12:57 2019 +0000
>
> bdev handling fix
>
> diff --git a/fs/fs_context.c b/fs/fs_context.c
> index 9e22fe6aafe7..fc8fda4b9fe9 100644
> --- a/fs/fs_context.c
> +++ b/fs/fs_context.c
> @@ -425,10 +425,8 @@ void put_fs_context(struct fs_context *fc)
>
> if (fc->need_free && fc->ops && fc->ops->free)
> fc->ops->free(fc);
> -#ifdef CONFIG_BLOCK
> - if (fc->bdev)
> - blkdev_put(fc->bdev, fc->bdev_mode);
> -#endif
> + if (fc->dev_destructor)
> + fc->dev_destructor(fc);
>
> security_free_mnt_opts(&fc->security);
> put_net(fc->net_ns);
> diff --git a/fs/super.c b/fs/super.c
> index 85851adb0f19..e9e678d2c37c 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1211,8 +1211,17 @@ int vfs_get_super(struct fs_context *fc,
> EXPORT_SYMBOL(vfs_get_super);
>
> #ifdef CONFIG_BLOCK
> +static void fc_bdev_destructor(struct fs_context *fc)
> +{
> + if (fc->bdev) {
> + blkdev_put(fc->bdev, fc->bdev_mode);
> + fc->bdev = NULL;
> + }
> +}
> +
> static int set_bdev_super_fc(struct super_block *s, struct fs_context *fc)
> {
> + s->s_mode = fc->bdev_mode;
> s->s_bdev = fc->bdev;
> s->s_dev = s->s_bdev->bd_dev;
> s->s_bdi = bdi_get(s->s_bdev->bd_bdi);
> @@ -1252,6 +1261,9 @@ int vfs_get_block_super(struct fs_context *fc,
> return PTR_ERR(bdev);
> }
>
> + fc->dev_destructor = fc_bdev_destructor;
> + fc->bdev = bdev;
> +
> /* Once the superblock is inserted into the list by sget_fc(), s_umount
> * will protect the lockfs code from trying to start a snapshot while
> * we are mounting
> @@ -1260,18 +1272,14 @@ int vfs_get_block_super(struct fs_context *fc,
> if (bdev->bd_fsfreeze_count > 0) {
> mutex_unlock(&bdev->bd_fsfreeze_mutex);
> warnf(fc, "%pg: Can't mount, blockdev is frozen", bdev);
> - error = -EBUSY;
> - goto error_bdev;
> + return -EBUSY;
> }
>
> - fc->bdev = bdev;
> fc->sb_flags |= SB_NOSEC;
> s = sget_fc(fc, test_bdev_super_fc, set_bdev_super_fc);
> mutex_unlock(&bdev->bd_fsfreeze_mutex);
> - if (IS_ERR(s)) {
> - error = PTR_ERR(s);
> - goto error_bdev;
> - }
> + if (IS_ERR(s))
> + return PTR_ERR(s);
>
> if (s->s_root) {
> /* Don't summarily change the RO/RW state. */
> @@ -1281,16 +1289,10 @@ int vfs_get_block_super(struct fs_context *fc,
> goto error_sb;
> }
>
> - /* s_umount nests inside bd_mutex during __invalidate_device().
> - * blkdev_put() acquires bd_mutex and can't be called under
> - * s_umount. Drop s_umount temporarily. This is safe as we're
> - * holding an active reference.
> + /* Leave fc->bdev to fc_bdev_destructor() to clean up to avoid
> + * locking conflicts.
> */
> - up_write(&s->s_umount);
> - blkdev_put(bdev, fc->bdev_mode);
> - down_write(&s->s_umount);
> } else {
> - s->s_mode = fc->bdev_mode;
> snprintf(s->s_id, sizeof(s->s_id), "%pg", bdev);
> sb_set_blocksize(s, block_size(bdev));
> error = fill_super(s, fc);
> @@ -1307,11 +1309,7 @@ int vfs_get_block_super(struct fs_context *fc,
>
> error_sb:
> deactivate_locked_super(s);
> -error_bdev:
> - if (fc->bdev) {
> - blkdev_put(fc->bdev, fc->bdev_mode);
> - fc->bdev = NULL;
> - }
> + /* Leave fc->bdev to fc_bdev_destructor() to clean up */
> return error;
> }
> EXPORT_SYMBOL(vfs_get_block_super);
> @@ -1521,8 +1519,13 @@ int vfs_get_tree(struct fs_context *fc)
> * on the superblock.
> */
> error = fc->ops->get_tree(fc);
> - if (error < 0)
> + if (error < 0) {
> + if (fc->dev_destructor) {
> + fc->dev_destructor(fc);
> + fc->dev_destructor = NULL;
> + }
> return error;
> + }
>
> if (!fc->root) {
> pr_err("Filesystem %s get_tree() didn't set fc->root\n",
> diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
> index cb49b92f02af..9af788a3ef8e 100644
> --- a/include/linux/fs_context.h
> +++ b/include/linux/fs_context.h
> @@ -76,7 +76,9 @@ struct fs_context {
> const struct fs_context_operations *ops;
> struct file_system_type *fs_type;
> void *fs_private; /* The filesystem's context */
> - struct block_device *bdev; /* The backing blockdev (if applicable) */
> + union {
> + struct block_device *bdev; /* The backing blockdev (if applicable) */
> + };
> struct dentry *root; /* The root and superblock */
> struct user_namespace *user_ns; /* The user namespace for this mount */
> struct net *net_ns; /* The network namespace for this mount */
> @@ -93,6 +95,7 @@ struct fs_context {
> enum fs_context_purpose purpose:8;
> bool need_free:1; /* Need to call ops->free() */
> bool global:1; /* Goes into &init_user_ns */
> + void (*dev_destructor)(struct fs_context *fc); /* For block or mtd */
> };
>
> struct fs_context_operations {
>
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