linux-kernel - RE: [PATCH] x86_64: do not assume head_64.S used 4KB pages when !use

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Message-ID: <1A61D8EA6755AF458F06EA669A4EC8182F05B0F8@JKLMAIL02.ixonos.local>
Date:	Thu, 23 Jun 2011 16:25:34 +0000
From:	Penttilä Mika <mika.penttila@...nos.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] x86_64: do not assume head_64.S used 4KB pages when
 !use_pse

> From: Stefano Stabellini [mailto:stefano.stabellini@...citrix.com]
> Sent: 23. kesäkuuta 2011 18:20
> To: Penttilä Mika
> Cc: Stefano Stabellini; linux-kernel@...r.kernel.org
> Subject: RE: [PATCH] x86_64: do not assume head_64.S used 4KB pages
> when !use_pse
> 
> On Thu, 23 Jun 2011, Penttilä Mika wrote:
> > > > And arch/x86/mm/init.c also has:
> > > >
> > > > #if defined(CONFIG_DEBUG_PAGEALLOC) || defined(CONFIG_KMEMCHECK)
> > > >         /*
> > > >          * For CONFIG_DEBUG_PAGEALLOC, identity mapping will use
> > > small pages.
> > > >          * This will simplify cpa(), which otherwise needs to
> support
> > > splitting
> > > >          * large pages into small in interrupt context, etc.
> > > >          */
> > > >         use_pse = use_gbpages = 0;
> > > > #else
> > > >         use_pse = cpu_has_pse;
> > > >         use_gbpages = direct_gbpages;
> > > > #endif
> > > >
> > > >
> > > > So big pages are also not used for DEBUG_PAGEALLOC and KMEMCHECK
> > > configs even if head_32.S did.
> > >
> > > Right, but that is not a problem because head_32.S always uses 4KB
> > > pages.
> >
> > We use large pages FOR PAE kernels on x86-32 there
> >
> 
> Do you mean we use large pages for PAE kernels on x86_32 in
> arch/x86/mm/init.c:init_memory_mapping?
> That wouldn't be a problem for this patch.
> 
> The problem I am trying to solve occurs when head_64.S doesn't allocate
> any pte pages because it is using 2MB pages, while init_memory_mapping
> wants to use 4KB pages (for example because the user set
> CONFIG_DEBUG_PAGEALLOC).
> 
> So on x86_32 is not going to be an issue because head_32.S doesn't use
> 2MB or 4MB pages as far as I can tell, even if the hardware supports
> them.
> Please correct me if I am wrong but I don't see any _PAGE_PSE in
> head_32.S.


Yes you're right head_32.S doesn't do large pages, I remembered it would..

--Mika