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? > >>> 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 */ > >>> }; > > >> > >> This was already done for s_fsnotify_marks, so you can follow the recipe > >> of 07a3b8d0bf72 ("fsnotify: lazy attach fsnotify_sb_info state to sb") > >> and create an fsnotify_inode_info container. > >> > > > > On second thought, fsnotify_sb_info container is allocated and attached > > in the context of userspace adding a mark. > > > > If you will need allocate and attach fsnotify_inode_info in the content of > > fast path fanotify hook in order to add the inode to the map, I don't > > think that is going to fly?? > > Do you mean we may not be able to allocate memory in the fast path > hook? AFAICT, the fast path is still in the process context, so I > think this is not a problem? > > Thanks, > Song >
