| Message ID | 1486555078-10383-1-git-send-email-t.vivek@samsung.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Wed 08-02-17 17:27:58, Vivek Trivedi wrote: > It has been observed that apps may block in sys_sync for long time if there > is parallel mount request for large size storage block device in a loaded > environment. > > For example, sys_sync is reported to be hunged when a large size disk > (e.g. 4TB/8TB disks) is connected. sys_mount may take time for reading large > amount of metedata - free space accounting by reading bitmap during mount. > The larger the volume, the larger the size of the bitmaps read during mount. > > During mount operation s_umount lock is taken by sys_mount. The lock is not > released till the mount is completed. The sync() process keeps on waiting till > s_umount is relased by sys_mount. > > Scenario > Process1 Process2 > sys_sync() sys_mount() > iterate_supers do_mount() > ... vfs_kern_mount() > mount_fs() (Filesystem_mount) > ... mount_bdev() > sget() > alloc_super() > down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING); => LOCK HELD by MOUNT > ... > down_read(&sb->s_umount); => WAITING FOR LOCK ... > . ... > . fill_super() -> time TAKING (as per filesystem) > . up_write(&sb->s_umount); => LOCK RELEASE by mount process > . > . STUCKED TILL MOUNT is completed > > Since, the superblock gets added to the list during the mount path alloc_super, > so the 'sb' is visible in the s_list. But the behaviour of waiting to sync() a > filesystem which is not active yet, seems ambigous here. > > To avoid this issue, may be we should consider about having to check only > the ACTIVE filesystem for doing operations from the superblock list. > 'sb' is valid and referencable as long as part of the s_list and MS_ACTIVE is > set after successful mount and cleared during the umount path from > generic_shutdown_super. > > --- > Fixed a typo and updated the condition in iterate_supers. Changelog of a patch belongs under the diffstat so that it does not get into the final commit log. > Signed-off-by: Vivek Trivedi <t.vivek@samsung.com> > Reviewed-by: Amit Sahrawat <a.sahrawat@samsung.com> > --- > fs/super.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/fs/super.c b/fs/super.c > index b8b6a08..01711a4 100644 > --- a/fs/super.c > +++ b/fs/super.c > @@ -587,6 +587,10 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg) > list_for_each_entry(sb, &super_blocks, s_list) { > if (hlist_unhashed(&sb->s_instances)) > continue; > + > + if (!(sb->s_flags & MS_ACTIVE)) > + continue; > + So I have two comments to this: 1) Using MS_BORN would be more appropriate here - and consistent with the fact that we do check it in iterate_supers(), just under s_umount. 2) iterate_supers_type() should be updated as well to keep them consistent. Otherwise the patch looks OK to me. Honza
diff --git a/fs/super.c b/fs/super.c index b8b6a08..01711a4 100644 --- a/fs/super.c +++ b/fs/super.c @@ -587,6 +587,10 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg) list_for_each_entry(sb, &super_blocks, s_list) { if (hlist_unhashed(&sb->s_instances)) continue; + + if (!(sb->s_flags & MS_ACTIVE)) + continue; + sb->s_count++; spin_unlock(&sb_lock);
It has been observed that apps may block in sys_sync for long time if there is parallel mount request for large size storage block device in a loaded environment. For example, sys_sync is reported to be hunged when a large size disk (e.g. 4TB/8TB disks) is connected. sys_mount may take time for reading large amount of metedata - free space accounting by reading bitmap during mount. The larger the volume, the larger the size of the bitmaps read during mount. During mount operation s_umount lock is taken by sys_mount. The lock is not released till the mount is completed. The sync() process keeps on waiting till s_umount is relased by sys_mount. Scenario Process1 Process2 sys_sync() sys_mount() iterate_supers do_mount() ... vfs_kern_mount() mount_fs() (Filesystem_mount) ... mount_bdev() sget() alloc_super() down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING); => LOCK HELD by MOUNT ... down_read(&sb->s_umount); => WAITING FOR LOCK ... . ... . fill_super() -> time TAKING (as per filesystem) . up_write(&sb->s_umount); => LOCK RELEASE by mount process . . STUCKED TILL MOUNT is completed Since, the superblock gets added to the list during the mount path alloc_super, so the 'sb' is visible in the s_list. But the behaviour of waiting to sync() a filesystem which is not active yet, seems ambigous here. To avoid this issue, may be we should consider about having to check only the ACTIVE filesystem for doing operations from the superblock list. 'sb' is valid and referencable as long as part of the s_list and MS_ACTIVE is set after successful mount and cleared during the umount path from generic_shutdown_super. --- Fixed a typo and updated the condition in iterate_supers. Signed-off-by: Vivek Trivedi <t.vivek@samsung.com> Reviewed-by: Amit Sahrawat <a.sahrawat@samsung.com> --- fs/super.c | 4 ++++ 1 file changed, 4 insertions(+)