linux-kernel - Re: [PATCHv7 02/19] mm/zsmalloc: Prepare to variable MAX_PHYSMEM

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Message-ID: <20170928094423.c75fatvl6rnqzt5n@gmail.com>
Date:   Thu, 28 Sep 2017 11:44:23 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Minchan Kim <minchan@...nel.org>, Nitin Gupta <ngupta@...are.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Borislav Petkov <bp@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv7 02/19] mm/zsmalloc: Prepare to variable MAX_PHYSMEM_BITS


* Kirill A. Shutemov <kirill@...temov.name> wrote:

> On Thu, Sep 28, 2017 at 10:10:34AM +0200, Ingo Molnar wrote:
> > 
> > * Kirill A. Shutemov <kirill.shutemov@...ux.intel.com> wrote:
> > 
> > > With boot-time switching between paging mode we will have variable
> > > MAX_PHYSMEM_BITS.
> > > 
> > > Let's use the maximum variable possible for CONFIG_X86_5LEVEL=y
> > > configuration to define zsmalloc data structures.
> > > 
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > > Cc: Minchan Kim <minchan@...nel.org>
> > > Cc: Nitin Gupta <ngupta@...are.org>
> > > Cc: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
> > > ---
> > >  mm/zsmalloc.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > > 
> > > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > > index 7c38e850a8fc..fe22661f2fe5 100644
> > > --- a/mm/zsmalloc.c
> > > +++ b/mm/zsmalloc.c
> > > @@ -93,7 +93,13 @@
> > >  #define MAX_PHYSMEM_BITS BITS_PER_LONG
> > >  #endif
> > >  #endif
> > > +
> > > +#ifdef CONFIG_X86_5LEVEL
> > > +/* MAX_PHYSMEM_BITS is variable, use maximum value here */
> > > +#define _PFN_BITS		(52 - PAGE_SHIFT)
> > > +#else
> > >  #define _PFN_BITS		(MAX_PHYSMEM_BITS - PAGE_SHIFT)
> > > +#endif
> > 
> > This is a totally ugly hack, polluting generic MM code with an x86-ism and an 
> > arbitrary hard-coded constant that would silently lose validity when x86 paging 
> > gets extended again ...
> 
> Well, yes it's ugly. And I would be glad to find better solution. But I
> don't see one.
> 
> And it won't break silently on x86 paging expanding as it won't use
> CONFIG_X86_5LEVEL, so we would fallback to MAX_PHYSMEM_BITS - PAGE_SHIFT.
>
> I worth noting that the code already has x86 hack. See PAE special case
> for MAX_PHYSMEM_BITS.

Old mistakes don't justify new ones.

It's possible to do better: for example if we provide a MAX_POSSIBLE_PHYSMEM_BITS 
define that is the higher value then code which needs this for sizing can use it?

That could eliminate the PAE dependency as well perhaps.

Thanks,

	Ingo