On Tue, Nov 19, 2024 at 3:21 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > On Fri, 2024-11-15 at 17:35 +0000, Song Liu wrote: > > Hi Jan, > > > > > On Nov 15, 2024, at 3:19 AM, Jan Kara <jack@xxxxxxx> wrote: > > > > [...] > > > > > > AFAICT, we need to modify how lsm blob are managed with > > > > CONFIG_BPF_SYSCALL=y && CONFIG_BPF_LSM=n case. The solution, even > > > > if it gets accepted, doesn't really save any memory. Instead of > > > > growing struct inode by 8 bytes, the solution will allocate 8 > > > > more bytes to inode->i_security. So the total memory consumption > > > > is the same, but the memory is more fragmented. > > > > > > I guess you've found a better solution for this based on James' suggestion. > > > > > > > Therefore, I think we should really step back and consider adding > > > > the i_bpf_storage to struct inode. While this does increase the > > > > size of struct inode by 8 bytes, it may end up with less overall > > > > memory consumption for the system. This is why. > > > > > > > > When the user cannot use inode local storage, the alternative is > > > > to use hash maps (use inode pointer as key). AFAICT, all hash maps > > > > comes with non-trivial overhead, in memory consumption, in access > > > > latency, and in extra code to manage the memory. OTOH, inode local > > > > storage doesn't have these issue, and is usually much more efficient: > > > > - memory is only allocated for inodes with actual data, > > > > - O(1) latency, > > > > - per inode data is freed automatically when the inode is evicted. > > > > Please refer to [1] where Amir mentioned all the work needed to > > > > properly manage a hash map, and I explained why we don't need to > > > > worry about these with inode local storage. > > > > > > Well, but here you are speaking of a situation where bpf inode storage > > > space gets actually used for most inodes. Then I agree i_bpf_storage is the > > > most economic solution. But I'd also expect that for vast majority of > > > systems the bpf inode storage isn't used at all and if it does get used, it > > > is used only for a small fraction of inodes. So we are weighting 8 bytes > > > per inode for all those users that don't need it against more significant > > > memory savings for users that actually do need per inode bpf storage. A > > > factor in this is that a lot of people are running some distribution kernel > > > which generally enables most config options that are at least somewhat > > > useful. So hiding the cost behind CONFIG_FOO doesn't really help such > > > people. > > > > Agreed that an extra pointer will be used if there is no actual users > > of it. However, in longer term, "most users do not use bpf inode > > storage" may not be true. As kernel engineers, we may not always notice > > when user space is using some BPF features. For example, systemd has > > a BPF LSM program "restrict_filesystems" [1]. It is enabled if the > > user have lsm=bpf in kernel args. I personally noticed it as a > > surprise when we enabled lsm=bpf. > > > > > I'm personally not *so* hung up about a pointer in struct inode but I can > > > see why Christian is and I agree adding a pointer there isn't a win for > > > everybody. > > > > I can also understand Christian's motivation. However, I am a bit > > frustrated because similar approach (adding a pointer to the struct) > > worked fine for other popular data structures: task_struct, sock, > > cgroup. > > > > There are (usually) a lot more inodes on a host than all of those other > structs combined. Worse, struct inode is often embedded in other > structs, and adding fields can cause alignment problems there. > > > > > 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. 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 > > 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. 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. Thanks, Amir.
