On Wed, Nov 20, 2024 at 12:19:51PM +0100, Amir Goldstein wrote: > On Wed, Nov 20, 2024 at 10:28 AM Christian Brauner <brauner@xxxxxxxxxx> wrote: > > > > On Tue, Nov 19, 2024 at 09:53:20PM +0000, Song Liu wrote: > > > Hi Jeff and Amir, > > > > > > Thanks for your inputs! > > > > > > > On Nov 19, 2024, at 7:30 AM, Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > > > > > > > On Tue, Nov 19, 2024 at 4:25 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > > >> > > > >> On Tue, Nov 19, 2024 at 3:21 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > >>> > > > > > > [...] > > > > > > >>> Longer term, I think it may be beneficial to come up with a way to attach > > > >>>>> private info to the inode in a way that doesn't cost us one pointer per > > > >>>>> funcionality that may possibly attach info to the inode. We already have > > > >>>>> i_crypt_info, i_verity_info, i_flctx, i_security, etc. It's always a tough > > > >>>>> call where the space overhead for everybody is worth the runtime & > > > >>>>> complexity overhead for users using the functionality... > > > >>>> > > > >>>> It does seem to be the right long term solution, and I am willing to > > > >>>> work on it. However, I would really appreciate some positive feedback > > > >>>> on the idea, so that I have better confidence my weeks of work has a > > > >>>> better chance to worth it. > > > >>>> > > > >>>> Thanks, > > > >>>> Song > > > >>>> > > > >>>> [1] https://github.com/systemd/systemd/blob/main/src/core/bpf/restrict_fs/restrict-fs.bpf.c > > > >>> > > > >>> fsnotify is somewhat similar to file locking in that few inodes on the > > > >>> machine actually utilize these fields. > > > >>> > > > >>> For file locking, we allocate and populate the inode->i_flctx field on > > > >>> an as-needed basis. The kernel then hangs on to that struct until the > > > >>> inode is freed. > > > > > > If we have some universal on-demand per-inode memory allocator, > > > I guess we can move i_flctx to it? > > > > > > >>> We could do something similar here. We have this now: > > > >>> > > > >>> #ifdef CONFIG_FSNOTIFY > > > >>> __u32 i_fsnotify_mask; /* all events this inode cares about */ > > > >>> /* 32-bit hole reserved for expanding i_fsnotify_mask */ > > > >>> struct fsnotify_mark_connector __rcu *i_fsnotify_marks; > > > >>> #endif > > > > > > And maybe some fsnotify fields too? > > > > > > With a couple users, I think it justifies to have some universal > > > on-demond allocator. > > > > > > >>> What if you were to turn these fields into a pointer to a new struct: > > > >>> > > > >>> struct fsnotify_inode_context { > > > >>> struct fsnotify_mark_connector __rcu *i_fsnotify_marks; > > > >>> struct bpf_local_storage __rcu *i_bpf_storage; > > > >>> __u32 i_fsnotify_mask; /* all events this inode cares about */ > > > >>> }; > > > >>> > > > >> > > > >> The extra indirection is going to hurt for i_fsnotify_mask > > > >> it is being accessed frequently in fsnotify hooks, so I wouldn't move it > > > >> into a container, but it could be moved to the hole after i_state. > > > > > > >>> Then whenever you have to populate any of these fields, you just > > > >>> allocate one of these structs and set the inode up to point to it. > > > >>> They're tiny too, so don't bother freeing it until the inode is > > > >>> deallocated. > > > >>> > > > >>> It'd mean rejiggering a fair bit of fsnotify code, but it would give > > > >>> the fsnotify code an easier way to expand per-inode info in the future. > > > >>> It would also slightly shrink struct inode too. > > > > > > I am hoping to make i_bpf_storage available to tracing programs. > > > Therefore, I would rather not limit it to fsnotify context. We can > > > still use the universal on-demand allocator. > > > > Can't we just do something like: > > > > diff --git a/include/linux/fs.h b/include/linux/fs.h > > index 7e29433c5ecc..cc05a5485365 100644 > > --- a/include/linux/fs.h > > +++ b/include/linux/fs.h > > @@ -627,6 +627,12 @@ is_uncached_acl(struct posix_acl *acl) > > #define IOP_DEFAULT_READLINK 0x0010 > > #define IOP_MGTIME 0x0020 > > > > +struct inode_addons { > > + struct fsnotify_mark_connector __rcu *i_fsnotify_marks; > > + struct bpf_local_storage __rcu *i_bpf_storage; > > + __u32 i_fsnotify_mask; /* all events this inode cares about */ > > +}; > > + > > /* > > * Keep mostly read-only and often accessed (especially for > > * the RCU path lookup and 'stat' data) fields at the beginning > > @@ -731,12 +737,7 @@ struct inode { > > unsigned i_dir_seq; > > }; > > > > - > > -#ifdef CONFIG_FSNOTIFY > > - __u32 i_fsnotify_mask; /* all events this inode cares about */ > > - /* 32-bit hole reserved for expanding i_fsnotify_mask */ > > - struct fsnotify_mark_connector __rcu *i_fsnotify_marks; > > -#endif > > + struct inode_addons *i_addons; > > > > #ifdef CONFIG_FS_ENCRYPTION > > struct fscrypt_inode_info *i_crypt_info; > > > > Then when either fsnotify or bpf needs that storage they can do a > > cmpxchg() based allocation for struct inode_addons just like I did with > > f_owner: > > > > int file_f_owner_allocate(struct file *file) > > { > > struct fown_struct *f_owner; > > > > f_owner = file_f_owner(file); > > if (f_owner) > > return 0; > > > > f_owner = kzalloc(sizeof(struct fown_struct), GFP_KERNEL); > > if (!f_owner) > > return -ENOMEM; > > > > rwlock_init(&f_owner->lock); > > f_owner->file = file; > > /* If someone else raced us, drop our allocation. */ > > if (unlikely(cmpxchg(&file->f_owner, NULL, f_owner))) > > kfree(f_owner); > > return 0; > > } > > > > The internal allocations for specific fields are up to the subsystem > > ofc. Does that make sense? > > > > Maybe, but as I wrote, i_fsnotify_mask should not be moved out > of inode struct, because it is accessed in fast paths of fsnotify vfs > hooks, where we do not want to have to deref another context, > but i_fsnotify_mask can be moved to the hole after i_state. > > And why stop at i_fsnotify/i_bfp? > If you go to "addons" why not also move i_security/i_crypt/i_verify? > Need to have some common rationale behind those decisions. The rationale is that we need a mechanism to stop bloating our structures with ever more stuff somehow. What happens to older members of struct inode is a cleanup matter and then it needs to be seen what can be moved into a substruct and not mind the additional pointer chase. It's just a generalization of your proposal in a way because I don't understand why you would move the bpf stuff into fsnotify specific parts.
