Messages in this thread | | | Date | Thu, 1 Jun 2023 16:43:32 +0800 | Subject | Re: [PATCH 6/8] xfs: introduce xfs_fs_destroy_super() | From | Qi Zheng <> |
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Hi Dave,
On 2023/6/1 07:48, Dave Chinner wrote: > On Wed, May 31, 2023 at 09:57:40AM +0000, Qi Zheng wrote: >> From: Kirill Tkhai <tkhai@ya.ru> >> >> xfs_fs_nr_cached_objects() touches sb->s_fs_info, >> and this patch makes it to be destructed later. >> >> After this patch xfs_fs_nr_cached_objects() is safe >> for splitting unregister_shrinker(): mp->m_perag_tree >> is stable till destroy_super_work(), while iteration >> over it is already RCU-protected by internal XFS >> business. >> >> Signed-off-by: Kirill Tkhai <tkhai@ya.ru> >> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com> >> --- >> fs/xfs/xfs_super.c | 25 ++++++++++++++++++++++--- >> 1 file changed, 22 insertions(+), 3 deletions(-) >> >> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c >> index 7e706255f165..694616524c76 100644 >> --- a/fs/xfs/xfs_super.c >> +++ b/fs/xfs/xfs_super.c >> @@ -743,11 +743,18 @@ xfs_fs_drop_inode( >> } >> >> static void >> -xfs_mount_free( >> +xfs_free_names( >> struct xfs_mount *mp) >> { >> kfree(mp->m_rtname); >> kfree(mp->m_logname); >> +} >> + >> +static void >> +xfs_mount_free( >> + struct xfs_mount *mp) >> +{ >> + xfs_free_names(mp); >> kmem_free(mp); >> } >> >> @@ -1136,8 +1143,19 @@ xfs_fs_put_super( >> xfs_destroy_mount_workqueues(mp); >> xfs_close_devices(mp); >> >> - sb->s_fs_info = NULL; >> - xfs_mount_free(mp); >> + xfs_free_names(mp); >> +} >> + >> +static void >> +xfs_fs_destroy_super( >> + struct super_block *sb) >> +{ >> + if (sb->s_fs_info) { >> + struct xfs_mount *mp = XFS_M(sb); >> + >> + kmem_free(mp); >> + sb->s_fs_info = NULL; >> + } >> } >> >> static long >> @@ -1165,6 +1183,7 @@ static const struct super_operations xfs_super_operations = { >> .dirty_inode = xfs_fs_dirty_inode, >> .drop_inode = xfs_fs_drop_inode, >> .put_super = xfs_fs_put_super, >> + .destroy_super = xfs_fs_destroy_super, >> .sync_fs = xfs_fs_sync_fs, >> .freeze_fs = xfs_fs_freeze, >> .unfreeze_fs = xfs_fs_unfreeze, > > I don't really like this ->destroy_super() callback, especially as > it's completely undocumented as to why it exists. This is purely a > work-around for handling extended filesystem superblock shrinker > functionality, yet there's nothing that tells the reader this. > > It also seems to imply that the superblock shrinker can continue to > run after the existing unregister_shrinker() call before ->kill_sb() > is called. This violates the assumption made in filesystems that the > superblock shrinkers have been stopped and will never run again > before ->kill_sb() is called. Hence ->kill_sb() implementations > assume there is nothing else accessing filesystem owned structures > and it can tear down internal structures safely. > > Realistically, the days of XFS using this superblock shrinker > extension are numbered. We've got a lot of the infrastructure we > need in place to get rid of the background inode reclaim > infrastructure that requires this shrinker extension, and it's on my > list of things that need to be addressed in the near future. > > In fact, now that I look at it, I think the shmem usage of this > superblock shrinker interface is broken - it returns SHRINK_STOP to > ->free_cached_objects(), but the only valid return value is the > number of objects freed (i.e. 0 is nothing freed). These special > superblock extension interfaces do not work like a normal > shrinker.... > > Hence I think the shmem usage should be replaced with an separate > internal shmem shrinker that is managed by the filesystem itself > (similar to how XFS has multiple internal shrinkers). > > At this point, then the only user of this interface is (again) XFS. > Given this, adding new VFS methods for a single filesystem > for functionality that is planned to be removed is probably not the > best approach to solving the problem.
Thanks for such a detailed analysis. Kirill Tkhai just proposeed a new method[1], I cc'd you on the email.
[1]. https://lore.kernel.org/lkml/bab60fe4-964c-43a6-ecce-4cbd4981d875@ya.ru/
Thanks, Qi
> > Cheers, > > Dave.
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