linux-kernel - Re: [PATCH v8 11/12] mm/vmalloc: Hugepage vmalloc mappings

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Date:   Fri, 4 Dec 2020 18:33:54 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "npiggin@...il.com" <npiggin@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
CC:     "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "lizefan@...wei.com" <lizefan@...wei.com>,
        "christophe.leroy@...roup.eu" <christophe.leroy@...roup.eu>,
        "Jonathan.Cameron@...wei.com" <Jonathan.Cameron@...wei.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 11/12] mm/vmalloc: Hugepage vmalloc mappings

On Fri, 2020-12-04 at 18:12 +1000, Nicholas Piggin wrote:
> Excerpts from Edgecombe, Rick P's message of December 1, 2020 6:21
> am:
> > On Sun, 2020-11-29 at 01:25 +1000, Nicholas Piggin wrote:
> > > Support huge page vmalloc mappings. Config option
> > > HAVE_ARCH_HUGE_VMALLOC
> > > enables support on architectures that define HAVE_ARCH_HUGE_VMAP
> > > and
> > > supports PMD sized vmap mappings.
> > > 
> > > vmalloc will attempt to allocate PMD-sized pages if allocating
> > > PMD
> > > size
> > > or larger, and fall back to small pages if that was unsuccessful.
> > > 
> > > Allocations that do not use PAGE_KERNEL prot are not permitted to
> > > use
> > > huge pages, because not all callers expect this (e.g., module
> > > allocations vs strict module rwx).
> > 
> > Several architectures (x86, arm64, others?) allocate modules
> > initially
> > with PAGE_KERNEL and so I think this test will not exclude module
> > allocations in those cases.
> 
> Ah, thanks. I guess archs must additionally ensure that their
> PAGE_KERNEL allocations are suitable for huge page mappings before
> enabling the option.
> 
> If there is interest from those archs to support this, I have an
> early (un-posted) patch that adds an explicit VM_HUGE flag that could
> override the pessemistic arch default. It's not much trouble to add
> this 
> to the large system hash allocations. It's very out of date now but
> I 
> can at least give what I have to anyone doing an arch support that
> wants it.

Ahh, sorry, I totally missed that this was only enabled for powerpc.

That patch might be useful for me actually. Or maybe a VM_NOHUGE, since
there are only a few places where executable vmallocs are created? I'm
not sure what the other issues are.

I am endeavoring to have small module allocations share large pages, so
this infrastructure is a big help already.
https://lore.kernel.org/lkml/20201120202426.18009-1-rick.p.edgecombe@intel.com/

Thanks!