Re: [PATCH v2 3/3] x86: Support huge vmalloc mappings

[Kefeng Wang]

On 2022/1/19 12:17, Nicholas Piggin wrote:
> Excerpts from Dave Hansen's message of January 19, 2022 3:28 am:
> > On 1/17/22 6:46 PM, Nicholas Piggin wrote:
> > > > 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.
> > > This is documented in the Kconfig.
> > >
> > >   #
> > >   #  Archs that select this would be capable of PMD-sized vmaps (i.e.,
> > >   #  arch_vmap_pmd_supported() returns true), and they must make no assumptions
> > >   #  that vmalloc memory is mapped with PAGE_SIZE ptes. The VM_NO_HUGE_VMAP flag
> > >   #  can be used to prohibit arch-specific allocations from using hugepages to
> > >   #  help with this (e.g., modules may require it).
> > >   #
> > >   config HAVE_ARCH_HUGE_VMALLOC
> > >           depends on HAVE_ARCH_HUGE_VMAP
> > >           bool
> > >
> > > Is it really fair to say it's *very* fragile? Surely it's reasonable to
> > > read the (not very long) documentation ad understand the consequences for
> > > the arch code before enabling it.
> > Very fragile or not, I think folks are likely to get it wrong.  It would
> > be nice to have it default *everyone* to safe and slow and make *sure*
> It's not safe to enable though. That's the problem. If it was just
> modules then you'd have a point but it could be anything.
>
> > they go look at the architecture modules code itself before enabling
> > this for modules.
> This is required not just for modules for the whole arch code, it
> has to be looked at and decided this will work.
>
> > Just from that Kconfig text, I don't think I'd know off the top of my
> > head what do do for x86, or what code I needed to go touch.
> You have to make sure arch/x86 makes no assumptions that vmalloc memory
> is backed by PAGE_SIZE ptes. If you can't do that then you shouldn't
> enable the option. The option can not explain it any more because any
> arch could do anything with its mappings. The module code is an example,
> not the recipe.

Hi Nick, Dave and Christophe，thanks for your review,  a little 
confused,   I think,

1) for ppc/arm64 module_alloc(),  it must set VM_NO_HUGE_VMAP because the

arch's set_memory_* funcitons can only support PAGE_SIZE mapping, due to the

limit of apply_to_page_range().

2) but for x86's module_alloc(), add VM_NO_HUGE_VMAP is to avoid 
fragmentation,

x86's __change_page_attr functions will split the huge mapping. this 
flags is not a must.


and the behavior above occurred when STRICT_MODULE_RWX enabled, so

1) add a unified function to set vm flags(suggested by Dave ) or

2) add vm flags with some comments to per-arch's module_alloc()

are both acceptable, for the way of unified function ,  we could make 
this a default recipe

with STRICT_MODULE_RWX, also make two more vm flags into it, eg,

+unsigned long module_alloc_vm_flags(bool need_flush_reset_perms)
+{
+       unsigned long vm_flags = VM_DEFER_KMEMLEAK;
+
+       if (need_flush_reset_perms)
+               vm_flags |= VM_FLUSH_RESET_PERMS;
+       /*
+        * Modules use a single, large vmalloc(). Different permissions
+        * are applied later and will fragment huge mappings or even
+        * fails in set_memory_* on some architectures. Avoid using
+        * huge pages for modules.
+        */
+       if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
+               vm_flags |= VM_NO_HUGE_VMAP;
+
+       return vm_flags;
+}

then called each arch's module_alloc().

Any suggestion, many thanks.


> Thanks,
> Nick
> .