* Suren Baghdasaryan <surenb@xxxxxxxxxx> [211007 17:32]: > On Thu, Oct 7, 2021 at 12:03 PM 'John Hubbard' via kernel-team > <kernel-team@xxxxxxxxxxx> wrote: > > > > On 10/7/21 11:50, Suren Baghdasaryan wrote: > > ... > > >>>>>>>>>> I believe Pavel meant something as simple as > > >>>>>>>>>> $ YOUR_FILE=$YOUR_IDS_DIR/my_string_name > > >>>>>>>>>> $ touch $YOUR_FILE > > >>>>>>>>>> $ stat -c %i $YOUR_FILE > > >>>>>>> > > >>>>>>> Ah, ok, now I understand the proposal. Thanks for the clarification! > > >>>>>>> So, this would use filesystem as a directory for inode->name mappings. > > >>>>>>> One rough edge for me is that the consumer would still need to parse > > >>>>>>> /proc/$pid/maps and convert [anon:inode] into [anon:name] instead of > > >>>>>>> just dumping the content for the user. Would it be acceptable if we > > >>>>>>> require the ID provided by prctl() to always be a valid inode and > > >>>>>>> show_map_vma() would do the inode-to-filename conversion when > > >>>>>>> generating maps/smaps files? I know that inode->dentry is not > > >>>>>>> one-to-one mapping but we can simply output the first dentry name. > > >>>>>>> WDYT? > > >>>>>> > > >>>>>> No. You do not want to dictate any particular way of the mapping. The > > >>>>>> above is just one way to do that without developing any actual mapping > > >>>>>> yourself. You just use a filesystem for that. Kernel doesn't and > > >>>>>> shouldn't understand the meaning of those numbers. It has no business in > > >>>>>> that. > > >>>>>> > > >>>>>> In a way this would be pushing policy into the kernel. > > >>>>> > > >>>>> I can see your point. Any other ideas on how to prevent tools from > > >>>>> doing this id-to-name conversion themselves? > > >>>> > > >>>> I really fail to understand why you really want to prevent them from that. > > >>>> Really, the whole thing is just a cookie that kernel maintains for memory > > >>>> mappings so that two parties can understand what the meaning of that > > >>>> mapping is from a higher level. They both have to agree on the naming > > >>>> but the kernel shouldn't dictate any specific convention because the > > >>>> kernel _doesn't_ _care_. These things are not really anything actionable > > >>>> for the kernel. It is just a metadata. > > >>> > > >>> The desire is for one of these two parties to be a human who can get > > >>> the data and use it as is without additional conversions. > > >>> /proc/$pid/maps could report FD numbers instead of pathnames, which > > >>> could be converted to pathnames in userspace. However we do not do > > >>> that because pathnames are more convenient for humans to identify a > > >>> specific resource. Same logic applies here IMHO. > > >> > > >> Yes, please. It really seems like the folks that are interested in this > > >> feature want strings. (I certainly do.) For those not interested in the > > >> feature, it sounds like a CONFIG to keep it away would be sufficient. > > >> Can we just move forward with that? > > > > > > Would love to if others are ok with this. > > > > > > > If this doesn't get accepted, then another way forward would to continue > > the ideas above to their logical conclusion, and create a new file system: > > vma-fs. Like debug-fs and other special file systems, similar policy and > > motivation. Also protected by a CONFIG option. > > TBH, I would prefer to have the current simple solution protected with > a CONFIG option. > > > > > Actually this seems at least as natural as the procfs approach, especially > > given the nature of these strings, which feel more like dir+file names, than > > simple strings. I think the current locking around VMAs makes this a very tricky, if not impossible, path. Watching a proc file which takes the mmap_lock() is painful enough. Considering how hard it has been to have this feature added, I cannot see locking changes being accepted as a more feasible approach nor can I see increased mmap_lock() contention from any feature being desired. I like the CONFIG option. The patches are in good shape and have a clever way around the (unlikely?) scalability issue that existed. Thanks, Liam
