Messages in this thread | | | Subject | Re: [RFC v6 PATCH 2/2] mm: mmap: zap pages with read mmap_sem in munmap | From | Yang Shi <> | Date | Wed, 8 Aug 2018 10:19:54 -0700 |
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On 8/8/18 2:22 AM, Vlastimil Babka wrote: > On 08/08/2018 03:51 AM, Yang Shi wrote: >> On 8/6/18 10:45 PM, Michal Hocko wrote: >>> On Mon 06-08-18 15:19:06, Yang Shi wrote: >>>> On 8/6/18 1:52 PM, Michal Hocko wrote: >>>>> On Mon 06-08-18 13:48:35, Yang Shi wrote: >>>>>> On 8/6/18 1:41 PM, Michal Hocko wrote: >>>>>>> On Mon 06-08-18 09:46:30, Yang Shi wrote: >>>>>>>> On 8/6/18 2:40 AM, Michal Hocko wrote: >>>>>>>>> On Fri 03-08-18 14:01:58, Yang Shi wrote: >>>>>>>>>> On 8/3/18 2:07 AM, Michal Hocko wrote: >>>>>>>>>>> On Fri 27-07-18 02:10:14, Yang Shi wrote: >>>>>>> [...] >>>>>>>>>>>> If the vma has VM_LOCKED | VM_HUGETLB | VM_PFNMAP or uprobe, they are >>>>>>>>>>>> considered as special mappings. They will be dealt with before zapping >>>>>>>>>>>> pages with write mmap_sem held. Basically, just update vm_flags. >>>>>>>>>>> Well, I think it would be safer to simply fallback to the current >>>>>>>>>>> implementation with these mappings and deal with them on top. This would >>>>>>>>>>> make potential issues easier to bisect and partial reverts as well. >>>>>>>>>> Do you mean just call do_munmap()? It sounds ok. Although we may waste some >>>>>>>>>> cycles to repeat what has done, it sounds not too bad since those special >>>>>>>>>> mappings should be not very common. >>>>>>>>> VM_HUGETLB is quite spread. Especially for DB workloads. >>>>>>>> Wait a minute. In this way, it sounds we go back to my old implementation >>>>>>>> with special handling for those mappings with write mmap_sem held, right? >>>>>>> Yes, I would really start simple and add further enhacements on top. >>>>>> If updating vm_flags with read lock is safe in this case, we don't have to >>>>>> do this. The only reason for this special handling is about vm_flags update. >>>>> Yes, maybe you are right that this is safe. I would still argue to have >>>>> it in a separate patch for easier review, bisectability etc... >>>> Sorry, I'm a little bit confused. Do you mean I should have the patch >>>> *without* handling the special case (just like to assume it is safe to >>>> update vm_flags with read lock), then have the other patch on top of it, >>>> which simply calls do_munmap() to deal with the special cases? >>> Just skip those special cases in the initial implementation and handle >>> each special case in its own patch on top. >> Thanks. VM_LOCKED area will not be handled specially since it is easy to >> handle it, just follow what do_munmap does. The special cases will just >> handle VM_HUGETLB, VM_PFNMAP and uprobe mappings. > So I think you could maybe structure code like this: instead of > introducing do_munmap_zap_rlock() and all those "bool skip_vm_flags" > additions, add a boolean parameter in do_munmap() to use the new > behavior, with only the first user SYSCALL_DEFINE2(munmap) setting it to > true. If true, do_munmap() will do the > - down_write_killable() itself instead of assuming it's already locked > - munmap_lookup_vma() > - check if any of the vma's in the range is "special", if yes, change > the boolean param to "false", and continue like previously, e.g. no mmap > sem downgrade etc.
Thanks for the suggestion. Actually, I did the similar thing in v1 patches, which added a bool parameter in vm_munmap() to tell if releasing mmap_sem is acceptable for some code paths. But, it got pushed back by tglx since vm_munmap() is called by x86 specific code too (and some other architectures). He suggested to define a new function to do the optimization. So, I followed this approach in the later versions.
Yang
> > That would be a basis for further optimizing the special vma cases in > subsequent patches (maybe it's really ok to touch the vma flags with > mmap sem for read as vma's are detached), and to eventually convert more > do_munmap() callers to the new mode. > > HTH, > Vlastimil > > >
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