linux-kernel - Re: [PATCHv5] x86/kdump: bugfix, make the behavior of crashkernel=X consistent with kaslr

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Date:   Thu, 10 Jan 2019 09:56:53 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     Baoquan He <bhe@...hat.com>, linux-mm@...ck.org,
        kexec@...ts.infradead.org, Tang Chen <tangchen@...fujitsu.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Michal Hocko <mhocko@...e.com>,
        Jonathan Corbet <corbet@....net>,
        Yaowei Bai <baiyaowei@...s.chinamobile.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Daniel Vacek <neelx@...hat.com>,
        Mathieu Malaterre <malat@...ian.org>,
        Stefan Agner <stefan@...er.ch>, Dave Young <dyoung@...hat.com>,
        yinghai@...nel.org, vgoyal@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv5] x86/kdump: bugfix, make the behavior of crashkernel=X
 consistent with kaslr

Hi Pingfan,

On Wed, Jan 09, 2019 at 09:02:41PM +0800, Pingfan Liu wrote:
> On Tue, Jan 8, 2019 at 11:49 PM Mike Rapoport <rppt@...ux.ibm.com> wrote:
> >
> > On Tue, Jan 08, 2019 at 05:01:38PM +0800, Baoquan He wrote:
> > > Hi Mike,
> > >
> > > On 01/08/19 at 10:05am, Mike Rapoport wrote:
> > > > I'm not thrilled by duplicating this code (yet again).
> > > > I liked the v3 of this patch [1] more, assuming we allow bottom-up mode to
> > > > allocate [0, kernel_start) unconditionally.
> > > > I'd just replace you first patch in v3 [2] with something like:
> > >
> > > In initmem_init(), we will restore the top-down allocation style anyway.
> > > While reserve_crashkernel() is called after initmem_init(), it's not
> > > appropriate to adjust memblock_find_in_range_node(), and we really want
> > > to find region bottom up for crashkernel reservation, no matter where
> > > kernel is loaded, better call __memblock_find_range_bottom_up().
> > >
> > > Create a wrapper to do the necessary handling, then call
> > > __memblock_find_range_bottom_up() directly, looks better.
> >
> > What bothers me is 'the necessary handling' which is already done in
> > several places in memblock in a similar, but yet slightly different way.
> >
> > memblock_find_in_range() and memblock_phys_alloc_nid() retry with different
> > MEMBLOCK_MIRROR, but memblock_phys_alloc_try_nid() does that only when
> > allocating from the specified node and does not retry when it falls back to
> > any node. And memblock_alloc_internal() has yet another set of fallbacks.
> >
> > So what should be the necessary handling in the wrapper for
> > __memblock_find_range_bottom_up() ?
> >
> Well, it is a hard choice.
> > BTW, even without any memblock modifications, retrying allocation in
> > reserve_crashkerenel() for different ranges, like the proposal at [1] would
> > also work, wouldn't it?
> >
> Yes, it can work. Then is it worth to expose the bottom-up allocation
> style beside for hotmovable purpose?

Some architectures use bottom-up as a "compatability" mode with bootmem.
And, I believe, powerpc and s390 use bottom-up to make some of the
allocations close to the kernel.
 
> Thanks,
> Pingfan
> > [1] http://lists.infradead.org/pipermail/kexec/2017-October/019571.html
> >
> > > Thanks
> > > Baoquan
> > >
> > > >
> > > > diff --git a/mm/memblock.c b/mm/memblock.c
> > > > index 7df468c..d1b30b9 100644
> > > > --- a/mm/memblock.c
> > > > +++ b/mm/memblock.c
> > > > @@ -274,24 +274,14 @@ phys_addr_t __init_memblock memblock_find_in_range_node(phys_addr_t size,
> > > >      * try bottom-up allocation only when bottom-up mode
> > > >      * is set and @end is above the kernel image.
> > > >      */
> > > > -   if (memblock_bottom_up() && end > kernel_end) {
> > > > -           phys_addr_t bottom_up_start;
> > > > -
> > > > -           /* make sure we will allocate above the kernel */
> > > > -           bottom_up_start = max(start, kernel_end);
> > > > -
> > > > +   if (memblock_bottom_up()) {
> > > >             /* ok, try bottom-up allocation first */
> > > > -           ret = __memblock_find_range_bottom_up(bottom_up_start, end,
> > > > +           ret = __memblock_find_range_bottom_up(start, end,
> > > >                                                   size, align, nid, flags);
> > > >             if (ret)
> > > >                     return ret;
> > > >
> > > >             /*
> > > > -            * we always limit bottom-up allocation above the kernel,
> > > > -            * but top-down allocation doesn't have the limit, so
> > > > -            * retrying top-down allocation may succeed when bottom-up
> > > > -            * allocation failed.
> > > > -            *
> > > >              * bottom-up allocation is expected to be fail very rarely,
> > > >              * so we use WARN_ONCE() here to see the stack trace if
> > > >              * fail happens.
> > > >
> > > > [1] https://lore.kernel.org/lkml/1545966002-3075-3-git-send-email-kernelfans@gmail.com/
> > > > [2] https://lore.kernel.org/lkml/1545966002-3075-2-git-send-email-kernelfans@gmail.com/
> > > >
> > > > > +
> > > > > + return ret;
> > > > > +}
> > > > > +
> > > > >  /**
> > > > >   * __memblock_find_range_top_down - find free area utility, in top-down
> > > > >   * @start: start of candidate range
> > > > > --
> > > > > 2.7.4
> > > > >
> > > >
> > > > --
> > > > Sincerely yours,
> > > > Mike.
> > > >
> > >
> >
> > --
> > Sincerely yours,
> > Mike.
> >
> 

-- 
Sincerely yours,
Mike.