Re: [PATCH 4/4] x86/mm: replace GFP_ATOMIC with GFP_KERNEL for direct map allocations

[Dave Hansen]

On 11/11/21 3:02 AM, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> 
> The allocations of the direct map pages are mostly happen very early during
> the system boot and they use either the page table cache in brk area of bss
> or memblock.
> 
> The few callers that effectively use page allocator for the direct map
> updates are gart_iommu_init() and memory hotplug. Neither of them happen in
> an atomic context so there is no reason to use GFP_ATOMIC for these
> allocations.
> 
> Replace GFP_ATOMIC with GFP_KERNEL to avoid using atomic reserves for
> allocations that do not require that.

I usually think of the biggest downside of GFP_ATOMIC as being that it
fails more often.  But, since we tend not to be low on memory in early
boot, GFP_ATOMIC and GFP_KERNEL end up being pretty close in actual
behavior.

These allocations also get exposed via init_extra_mapping_*().  But,
those are used via early_initcall()s where GFP_KERNEL is fine too.
Those are a bit worrying because they're in somewhat nice code, like the
Numascale APIC code.  I'm not sure how much use it sees these days.

I guess if this goes wrong somehow, we'll get some nice splats to tell
us what happened.

Was this motivated by anything in particular?  Or is it a pure cleanup?