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SubjectRe: [PATCH 04/12] mm: alloc_contig_freed_pages() added
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On Fri, 2011-04-01 at 00:51 +0200, Michal Nazarewicz wrote:
> On Fri, 01 Apr 2011 00:26:51 +0200, Dave Hansen <dave@linux.vnet.ibm.com>
> wrote:
> >> Bug in the above place does not mean that we could not allocate
> >> memory. It means caller is broken.
> >
> > Could you explain that a bit?
> >
> > Is this a case where a device is mapped to a very *specific* range of
> > physical memory and no where else? What are the reasons for not marking
> > it off limits at boot? I also saw some bits of isolation and migration
> > in those patches. Can't the migration fail?
>
> The function is called from alloc_contig_range() (see patch 05/12) which
> makes sure that the PFN is valid. Situation where there is not enough
> space is caught earlier in alloc_contig_range().
>
> alloc_contig_freed_pages() must be given a valid PFN range such that all
> the pages in that range are free (as in are within the region tracked by
> page allocator) and of MIGRATETYPE_ISOLATE so that page allocator won't
> touch them.

OK, so it really is a low-level function only. How about a comment that
explicitly says this? "Only called from $FOO with the area already
isolated." It probably also deserves an __ prefix.

> That's why invalid PFN is a bug in the caller and not an exception that
> has to be handled.
>
> Also, the function is not called during boot time. It is called while
> system is already running.

What kind of success have you had running this in practice? I'd be
worried that some silly task or a sticky dentry would end up in the
range that you want to allocate in.


-- Dave



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