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    SubjectRe: [PATCH v2 3/3] x86: Support huge vmalloc mappings
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    Le 28/12/2021 à 17:14, Dave Hansen a écrit :
    > 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.

    That's not correct from my point of view.

    When powerpc added that, a clear comment explains why:


    + /*
    + * Don't do huge page allocations for modules yet until more testing
    + * is done. STRICT_MODULE_RWX may require extra work to support this
    + * too.
    + */

    So as you can see, this is something specific to powerpc and temporary.

    >
    > 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.
    > */

    Why ? Not everybody use STRICT_MODULES_RWX.
    Even if you do so, you can still benefit from huge pages for modules.

    Why make what was initially a temporary precaution for powerpc become a
    definitive default limitation for all ?

    > return VM_NO_HUGE_VMAP;
    > }
    >
    > 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.
    >
    > VM_FLUSH_RESET_PERMS could probably be dealt with in the same way.
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