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    SubjectRe: [PATCH bpf-next 1/3] mm/vmalloc: introduce vmalloc_exec which allocates RO+X memory
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    Hi Peter,

    > On Jul 13, 2022, at 3:20 AM, Peter Zijlstra <peterz@infradead.org> wrote:
    >

    [...]

    >
    > So how about instead we separate them? Then much of the problem goes
    > away, you don't need to track these 2M chunks at all.
    >
    > Start by adding VM_TOPDOWN_VMAP, which instead of returning the lowest
    > (leftmost) vmap_area that fits, picks the higests (rightmost).
    >
    > Then add module_alloc_data() that uses VM_TOPDOWN_VMAP and make
    > ARCH_WANTS_MODULE_DATA_IN_VMALLOC use that instead of vmalloc (with a
    > weak function doing the vmalloc).
    >
    > This gets you bottom of module range is RO+X only, top is shattered
    > between different !X types.
    >
    > Then track the boundary between X and !X and ensure module_alloc_data()
    > and module_alloc() never cross over and stay strictly separated.
    >
    > Then change all module_alloc() users to expect RO+X memory, instead of
    > RW.
    >
    > Then make sure any extention of the X range is 2M aligned.
    >
    > And presto, *everybody* always uses 2M TLB for text, modules, bpf,
    > ftrace, the lot and nobody is tracking chunks.
    >
    > Maybe migration can be eased by instead providing module_alloc_text()
    > and ARCH_WANTS_MODULE_ALLOC_TEXT.

    I finally got some time to look into the code. A few questions:

    1. AFAICT, vmap_area tree only works with PAGE_SIZE aligned addresses.
    For the sharing to be more efficient, I think we need to go with
    smaller granularity. Will this work? Shall we pick a smaller
    granularity, say 64 bytes? Or shall we go all the way to 1 byte?

    2. I think we will need multiple vmap_area's sharing the same vm_struct.
    Do we need to add refcount to vm_struct?

    Thanks,
    Song


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