Re: [PATCH v5 1/1] mm: refactor initialization of struct page for holes in memory layout
From: Mike Rapoport <rppt@kernel.org>
Date: 2021-02-15 21:25:37
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On Mon, Feb 15, 2021 at 10:00:31AM +0100, Michal Hocko wrote:
On Sun 14-02-21 20:00:16, Mike Rapoport wrote:quoted
On Fri, Feb 12, 2021 at 02:18:20PM +0100, Michal Hocko wrote:quoted
We can correctly set the zone links for the reserved pages for holes in the middle of a zone based on the architecture constraints and with only the holes in the beginning/end of the memory will be not spanned by any node/zone which in practice does not seem to be a problem as the VM_BUG_ON in set_pfnblock_flags_mask() never triggered on pfn 0.I really fail to see what you mean by correct zone/node for a memory range which is not associated with any real node.
We know architectural zone constraints, so we can have always have 1:1 match from pfn to zone. Node indeed will be a guess.
quoted
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I am sorry, I haven't followed previous discussions. Has the removal of the VM_BUG_ON been considered as an immediate workaround?It was never discussed, but I'm not sure it's a good idea. Judging by the commit message that introduced the VM_BUG_ON (commit 86051ca5eaf5 ("mm: fix usemap initialization")) there was yet another inconsistency in the memory map that required a special care.Can we actually explore that path before adding yet additional complexity and potentially a very involved fix for a subtle problem?
This patch was intended as a fix for inconsistency of the memory map that is the root cause for triggering this VM_BUG_ON and other corner case problems. The previous version [1] is less involved as it does not extend node/zone spans. [1] https://lore.kernel.org/lkml/20210130221035.4169-3-rppt@kernel.org (local) -- Sincerely yours, Mike.