On Wed, Oct 6, 2021 at 8:08 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 06.10.21 17:01, Suren Baghdasaryan wrote: > > On Wed, Oct 6, 2021 at 2:27 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > >> > >> On 06.10.21 10:27, Michal Hocko wrote: > >>> On Tue 05-10-21 23:57:36, John Hubbard wrote: > >>> [...] > >>>> 1) Yes, just leave the strings in the kernel, that's simple and > >>>> it works, and the alternatives don't really help your case nearly > >>>> enough. > >>> > >>> I do not have a strong opinion. Strings are easier to use but they > >>> are more involved and the necessity of kref approach just underlines > >>> that. There are going to be new allocations and that always can lead > >>> to surprising side effects. These are small (80B at maximum) so the > >>> overall footpring shouldn't all that large by default but it can grow > >>> quite large with a very high max_map_count. There are workloads which > >>> really require the default to be set high (e.g. heavy mremap users). So > >>> if anything all those should be __GFP_ACCOUNT and memcg accounted. > >>> > >>> I do agree that numbers are just much more simpler from accounting, > >>> performance and implementation POV. > >> > >> +1 > >> > >> I can understand that having a string can be quite beneficial e.g., when > >> dumping mmaps. If only user space knows the id <-> string mapping, that > >> can be quite tricky. > >> > >> However, I also do wonder if there would be a way to standardize/reserve > >> ids, such that a given id always corresponds to a specific user. If we > >> use an uint64_t for an id, there would be plenty room to reserve ids ... > >> > >> I'd really prefer if we can avoid using strings and instead using ids. > > > > I wish it was that simple and for some names like [anon:.bss] or > > [anon:dalvik-zygote space] reserving a unique id would work, however > > some names like [anon:dalvik-/system/framework/boot-core-icu4j.art] > > are generated dynamically at runtime and include package name. > > Valuable information Yeah, I should have described it clearer the first time around. > > > Packages are constantly evolving, new ones are developed, names can > > change, etc. So assigning a unique id for these names is not really > > feasible. > > So, you'd actually want to generate/reserve an id for a given string at > runtime, assign that id to the VMA, and have a way to match id <-> > string somehow? If we go with ids then yes, that is what we would have to do. > That reservation service could be inside the kernel or even (better?) in > user space. The service could for example de-duplicates strings. Yes but it would require an IPC call to that service potentially on every mmap() when we want to name a mapped vma. This would be prohibitive. Even on consumption side, instead of just dumping /proc/$pid/maps we would have to parse the file and convert all [anon:id] into [anon:name] with each conversion requiring an IPC call (assuming no id->name pair caching on the client side). > My question would be, if we really have to expose these strings to the > kernel, or if an id is sufficient. Sure, it would move complexity to > user space, but keeping complexity out of the kernel is usually a good idea. My worry here is not the additional complexity on the userspace side but the performance hit we would have to endure due to these conversions. > -- > Thanks, > > David / dhildenb >
