linux-kernel - Re: [PATCH] mm: make reserve_bootmem can crossed the nodes

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Message-ID: <86802c440803132009q40291587i7a9f41c686f0f10b@mail.gmail.com>
Date:	Thu, 13 Mar 2008 20:09:16 -0700
From:	"Yinghai Lu" <yhlu.kernel@...il.com>
To:	"Johannes Weiner" <hannes@...urebad.de>
Cc:	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...e.hu>, "Andi Kleen" <ak@...e.de>,
	"Christoph Lameter" <clameter@....com>,
	linux-kernel@...r.kernel.org,
	"Yasunori Goto" <y-goto@...fujitsu.com>,
	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] mm: make reserve_bootmem can crossed the nodes

On Thu, Mar 13, 2008 at 7:50 PM, Johannes Weiner <hannes@...urebad.de> wrote:
>
> Hi,
>
>  "Yinghai Lu" <yhlu.kernel@...il.com> writes:
>
>  > On Thu, Mar 13, 2008 at 5:13 PM, Johannes Weiner <hannes@...urebad.de> wrote:
>  >> Hi,
>  >>
>  >>  "Yinghai Lu" <yhlu.kernel@...il.com> writes:
>  >>
>  >
>  >>  >  static void __init free_bootmem_core(bootmem_data_t *bdata, unsigned long addr,
>  >>  > @@ -407,6 +432,11 @@ unsigned long __init init_bootmem_node(p
>  >>  >  void __init reserve_bootmem_node(pg_data_t *pgdat, unsigned long physaddr,
>  >>  >                                unsigned long size, int flags)
>  >>  >  {
>  >>  > +     int ret;
>  >>  > +
>  >>  > +     ret = can_reserve_bootmem_core(pgdat->bdata, physaddr, size, flags);
>  >>  > +     if (ret < 0)
>  >>  > +             return;
>  >>  >       reserve_bootmem_core(pgdat->bdata, physaddr, size, flags);
>  >>
>  >>  I don't get it.  Sorry.  What is the purpose of
>  >>  can_reserve_bootmem_core()?  It does exactly what reserve_bootmem_core
>  >>  does besides actually setting the bits.  All the pre-checking you wanted
>  >>  to have out of the way is repeated again in reserve_bootmem_core()
>  >>  (well, almost all).
>  >
>  > can_reserve_bootmem_core is check if there is some pages is reserved
>  > already with
>  >>  > +                     if (flags & BOOTMEM_EXCLUSIVE)
>  >>  > +                             return -EBUSY;
>  >
>  > so it will avoid the restoring later.
>
>  Yes, I understood that.  But you skipped the lower part of my email:
>
>  Your current state now is _not_ that you have one function that
>  prechecks the range and another function that reserves it!  You have
>  _two_ functions checking the range and the second reserving it.

Yes

>
>  Why double-check most of the things?  If you want to have a pre-check
>  function, _move_ all the pre-checks into another function, not
>  copy-paste them.

for cross the nodes

>
>  And is the condition of trying to reserve a range twice, the second time
>  exclusively, so common that it is worth iterating twice over the nodes
>  (once for checking, once for reserving) instead of just unwinding the
>  reservation if it fails in between?
>
>  On something else: is there a bug when a memory range is reserved with
>  BOOTMEM_EXCLUSIVE and then again without this flag?  The second call
>  does not return an error then.

Yes. only provide one direction protection.
so always make sure crash_kernel reserve_bootmem as the last. that is doable.

YH
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