linux-kernel - Re: [PATCHv2] arm64:mm: free the useless initial page table

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Message-ID: <20150128121017.GB9000@e104818-lin.cambridge.arm.com>
Date:	Wed, 28 Jan 2015 12:10:17 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	"zhichang.yuan@...aro.org" <zhichang.yuan@...aro.org>,
	Will Deacon <Will.Deacon@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"liguozhu@...wei.com" <liguozhu@...wei.com>,
	"dsaxena@...aro.org" <dsaxena@...aro.org>,
	Laura Abbott <lauraa@...eaurora.org>
Subject: Re: [PATCHv2] arm64:mm: free the useless initial page table

On Fri, Jan 23, 2015 at 05:40:40PM +0000, Ard Biesheuvel wrote:
> On 23 January 2015 at 16:21, Catalin Marinas <catalin.marinas@....com> wrote:
> > On Tue, Dec 09, 2014 at 07:26:47AM +0000, zhichang.yuan@...aro.org wrote:
> >> From: "zhichang.yuan" <zhichang.yuan@...aro.org>
> >>
> >> For 64K page system, after mapping a PMD section, the corresponding initial
> >> page table is not needed any more. That page can be freed.
> >>
> >> Changes since v1:
> >>
> >> * make consistent code between alloc_init_pmd and alloc_init_pud;
> >> * flush the TLB before the unused page table is freed;
> >>
> >> Signed-off-by: Zhichang Yuan <zhichang.yuan@...aro.org>
> >
> > I thought about queuing this patch but I realised that
> > alloc_init_pmd/pud may be called in a late context where memblock_free()
> > would no longer make sense. Cc'ing Laura and Ard for any ideas here but
> > I think we may just end up with a few old page table pages sitting
> > around (not many though). In general we don't go from smaller to larger
> > mappings (that's what this patch targets) but given the API, I'm not
> > sure we have any guarantee.
> >
> > One solution would be to check for alloc == early_alloc or something
> > similar. Cc'ing Laura and Ard as they added the create_mapping_late()
> > code.
> >
> 
> The UEFI page tables are only built up once, based on a series of
> disjoint memory regions, so that will never hit either of the
> memblock_free() branches.
> And AFAICT, the DEBUG_RODATA code does the splitting early, which
> causes the create_mapping_late() code to only change permissions, not
> change the granularity of any regions.
> 
> Perhaps it's sufficient to add a comment and a BUG_ON(alloc !=
> early_alloc) to the memblock_free() branches?

Thanks for confirming, I merged this patch together with BUG_ON(), just
in case.

-- 
Catalin
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