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Re: [PATCH] x86/mm: ensure L2 is always freed if empty
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 31 Oct 2024 10:59:36 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 31 Oct 2024 09:59:48 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 30.10.2024 12:48, Roger Pau Monne wrote:
> The current logic in modify_xen_mappings() allows for fully empty L2 tables to
> not be freed and unhooked from the parent L3 if the last L2 slot is not
> populated.
>
> Ensure that even when an L2 slot is empty the logic to check whether the whole
> L2 can be removed is not skipped.
>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> I've attempted to find a Fixes tag for this one, but I'm afraid there have
> been
> many changes in the function, and it's possibly the code that introduced the
> L2
> freeing (4376c05c31132) the one that failed to originally adjust this case.
Yes, that looks to be the one. It has been a long while, but I think it was
(wrong) similarity considerations with the corresponding L3E handling (near
the top of the main loop) that resulted in me not touching that "continue".
We certainly don't want to ever free L3 tables (vacating L4 entries); since
the 32-bit case still mattered back then, the "continue" also couldn't have
been simply replaced (as there that same consideration applied to L2 tables
and L3 entries, for further extended reasons imposed by PAE).
Jan
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