linux-kernel - Re: [PATCH 4/4] x86/mm: replace GFP_ATOMIC with GFP

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Date:   Thu, 11 Nov 2021 07:19:40 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Mike Rapoport <rppt@...nel.org>, x86@...nel.org
Cc:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] x86/mm: replace GFP_ATOMIC with GFP_KERNEL for direct
 map allocations

On 11/11/21 3:02 AM, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@...ux.ibm.com>
> 
> 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?