On Wed, Oct 6, 2021 at 7:29 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Wed, 6 Oct 2021 08:20:20 -0700 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > 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. > > If it gets this fancy then the 80 char limit is likely to become a > significant limitation and the choice should be explained & justified. > > Why not 97? 1034? Why not just strndup_user() and be done with it? The original patch from 8 years ago used 256 as the limit but Rasmus argued that the string content should be human-readable, so 80 chars seems to be a reasonable limit (see: https://lore.kernel.org/all/d8619a98-2380-ca96-001e-60fe9c6204a6@xxxxxxxxxxxxxxxxxx), which makes sense to me. We should be able to handle the 80 char limit by trimming it before calling prctl(). > > > > 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. > > Has the performance hit been quantified? I'll try to get the data that was collected or at least an estimate. I imagine collecting such data would require considerable userspace redesign. > I've seen this many times down the ages. Something which *could* be > done in userspace is instead done in the kernel because coordinating > userspace is Just So Damn Hard. I guess the central problem is that > userspace isn't centrally coordinated. I wish we were better at this. It's not just hard, it's also inefficient. And for our usecase performance is important. > > > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx. >
