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Date:   Thu, 26 Nov 2020 23:03:32 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Qian Cai <cai@....pw>, Michal Hocko <mhocko@...nel.org>,
        linux-kernel@...r.kernel.org, Baoquan He <bhe@...hat.com>
Subject: Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set
 pageblock_skip on reserved pages

On Thu, Nov 26, 2020 at 03:30:01PM -0500, Andrea Arcangeli wrote:
> On Thu, Nov 26, 2020 at 09:44:26PM +0200, Mike Rapoport wrote:
> > TBH, the whole interaction between e820 and memblock keeps me puzzled
> > and I can only make educated guesses why some ranges here are
> > memblock_reserve()'d and some memblock_add()ed.
> 
> The mixed usage in that interaction between memblock.reserve and
> memblock.memory where sometime it's used to reserve overlapping
> memblock.memory ranges (clearly ok), and sometimes is used on ranges
> with no overlap (not clear even why it's being called), is what makes
> the current code messy.
> 
> We're basically passing down the exact same information (inverted),
> through two different APIs, if there is no overlap.
> 
> > I think what should be there is that e820 entries that are essentially
> > RAM, used by BIOS or not, should be listed in memblock.memory. Then
> > using memblock_reserve() for parts that BIOS claimed for itself would
> > have the same semantics as for memory allocated by kernel.
> >
> > I.e. if there is a DIMM from 0 to, say 512M, memblock.memory will have a
> > range [0, 512M]. And areas such as 0x000-0xfff, 0x9d000-0x9ffff will be
> > in memblock.reserved.
> > 
> > Than in page_alloc.c we'll know that we have a physical memory bank from
> > 0 to 512M but there are some ranges that we cannot use.
> >
> > I suggested it back then when the issue with compaction was reported at
> > the first time, but Baoquan mentioned that there are systems that cannot
> > even tolerate having BIOS reserved areas in the page tables and I didn't
> > continue to pursue this.
> 
> That explains why we can't add the e820 non-RAM regions to
> memblock_add, what's not clear is why it should be required to call
> memblock_reserve on a region that has no overlap with any memblock_add.
> 
> Instead of the patch that adds a walk to the memblock.reserve and that
> requires adding even more memblock_reserve to e820__memblock_setup for
> type 20, we can add a walk for the memblock.memory holes and then we
> can remove the memblock_reserve for E820_TYPE_SOFT_RESERVED too.

This is more or less what I have done here:

https://lore.kernel.org/lkml/20201126174601.GT123287@linux.ibm.com/

just without the removal of the call to memblock_reserve() for
E820_TYPE_SOFT_RESERVED.


> Thanks,
> Andrea
> 

-- 
Sincerely yours,
Mike.

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