linux-kernel - Re: [PATCH RFC v3 11/35] mm: Allow an arch to hook into folio allocation when VMA is known

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Date: Fri, 26 Jan 2024 12:00:36 -0800
From: Peter Collingbourne <pcc@...gle.com>
To: Alexandru Elisei <alexandru.elisei@....com>
Cc: catalin.marinas@....com, will@...nel.org, oliver.upton@...ux.dev, 
	maz@...nel.org, james.morse@....com, suzuki.poulose@....com, 
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	linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v3 11/35] mm: Allow an arch to hook into folio
 allocation when VMA is known

On Thu, Jan 25, 2024 at 8:43 AM Alexandru Elisei
<alexandru.elisei@....com> wrote:
>
> arm64 uses VM_HIGH_ARCH_0 and VM_HIGH_ARCH_1 for enabling MTE for a VMA.
> When VM_HIGH_ARCH_0, which arm64 renames to VM_MTE, is set for a VMA, and
> the gfp flag __GFP_ZERO is present, the __GFP_ZEROTAGS gfp flag also gets
> set in vma_alloc_zeroed_movable_folio().
>
> Expand this to be more generic by adding an arch hook that modifes the gfp
> flags for an allocation when the VMA is known.
>
> Note that __GFP_ZEROTAGS is ignored by the page allocator unless __GFP_ZERO
> is also set; from that point of view, the current behaviour is unchanged,
> even though the arm64 flag is set in more places.  When arm64 will have
> support to reuse the tag storage for data allocation, the uses of the
> __GFP_ZEROTAGS flag will be expanded to instruct the page allocator to try
> to reserve the corresponding tag storage for the pages being allocated.
>
> The flags returned by arch_calc_vma_gfp() are or'ed with the flags set by
> the caller; this has been done to keep an architecture from modifying the
> flags already set by the core memory management code; this is similar to
> how do_mmap() -> calc_vm_flag_bits() -> arch_calc_vm_flag_bits() has been
> implemented. This can be revisited in the future if there's a need to do
> so.
>
> Signed-off-by: Alexandru Elisei <alexandru.elisei@....com>

This patch also needs to update the non-CONFIG_NUMA definition of
vma_alloc_folio in include/linux/gfp.h to call arch_calc_vma_gfp. See:
https://r.android.com/2849146

Peter