Messages in this thread | | | Date | Wed, 29 Dec 2021 19:01:18 +0800 | Subject | Re: [PATCH v2 3/3] x86: Support huge vmalloc mappings | From | Kefeng Wang <> |
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On 2021/12/29 0:14, Dave Hansen wrote: > On 12/28/21 2:26 AM, Kefeng Wang wrote: >>>> There are some disadvantages about this feature[2], one of the main >>>> concerns is the possible memory fragmentation/waste in some scenarios, >>>> also archs must ensure that any arch specific vmalloc allocations that >>>> require PAGE_SIZE mappings(eg, module alloc with STRICT_MODULE_RWX) >>>> use the VM_NO_HUGE_VMAP flag to inhibit larger mappings. >>> That just says that x86 *needs* PAGE_SIZE allocations. But, what >>> happens if VM_NO_HUGE_VMAP is not passed (like it was in v1)? Will the >>> subsequent permission changes just fragment the 2M mapping? >> Yes, without VM_NO_HUGE_VMAP, it could fragment the 2M mapping. >> >> When module alloc with STRICT_MODULE_RWX on x86, it calls >> __change_page_attr() >> >> from set_memory_ro/rw/nx which will split large page, so there is no >> need to make >> >> module alloc with HUGE_VMALLOC. > This all sounds very fragile to me. Every time a new architecture would > get added for huge vmalloc() support, the developer needs to know to go > find that architecture's module_alloc() and add this flag. They next > guy is going to forget, just like you did. > > Considering that this is not a hot path, a weak function would be a nice > choice: > > /* vmalloc() flags used for all module allocations. */ > unsigned long __weak arch_module_vm_flags() > { > /* > * Modules use a single, large vmalloc(). Different > * permissions are applied later and will fragment > * huge mappings. Avoid using huge pages for modules. > */ > return VM_NO_HUGE_VMAP;
For x86, it only fragment, but for arm64, due to apply_to_page_range() in
set_memory_*, without this flag maybe crash. Whatever, we need this
flag for module.
> } > > Stick that in some the common module code, next to: > >> void * __weak module_alloc(unsigned long size) >> { >> return __vmalloc_node_range(size, 1, VMALLOC_START, VMALLOC_END, > ... > > Then, put arch_module_vm_flags() in *all* of the module_alloc() > implementations, including the generic one. That way (even with a new > architecture) whoever copies-and-pastes their module_alloc() > implementation is likely to get it right. The next guy who just does a > "select HAVE_ARCH_HUGE_VMALLOC" will hopefully just work.
OK, Let me check the VM_FLUSH_RESET_PERMS and try about this way.
Thanks.
> > VM_FLUSH_RESET_PERMS could probably be dealt with in the same way. > .
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