Subject: Re: [PATCH 2/3] hotplug-memory.c: enhance dlpar_memory_remove* LMB checks

From: David Gibson <[email-masked]>
Date: Tue, 4 May 2021 11:02:44 +1000

On Fri, Apr 30, 2021 at 09:09:16AM -0300, Daniel Henrique Barboza wrote:
> dlpar_memory_remove_by_ic() validates the amount of LMBs to be removed
> by checking !DRCONF_MEM_RESERVED, and in the following loop before
> dlpar_remove_lmb() a check for DRCONF_MEM_ASSIGNED is made before
> removing it. This means that a LMB that is both !DRCONF_MEM_RESERVED and
> !DRCONF_MEM_ASSIGNED will be counted as valid, but then not being
> removed.  The function will end up not removing all 'lmbs_to_remove'
> LMBs while also not reporting any errors.
> 
> Comparing it to dlpar_memory_remove_by_count(), the validation is done
> via lmb_is_removable(), which checks for DRCONF_MEM_ASSIGNED and fadump
> constraints. No additional check is made afterwards, and
> DRCONF_MEM_RESERVED is never checked before dlpar_remove_lmb(). The
> function doesn't have the same 'check A for validation, then B for
> removal' issue as remove_by_ic(), but it's not checking if the LMB is
> reserved.
> 
> There is no reason for these functions to validate the same operation in
> two different manners.

Actually, I think there is: remove_by_ic() is handling a request to
remove a specific range of LMBs.  If any are reserved, they can't be
removed and so this needs to fail.  But if they are !ASSIGNED, that
essentially means they're *already* removed (or never added), so
"removing" them is, correctly, a no-op.

remove_by_count(), in contrast, is being asked to remove a fixed
number of LMBs from wherever they can be found, and for that it needs
to find LMBs that haven't already been removed.

Basically remove_by_ic() is an absolute request: "make this set of
LMBs be not-plugged", whereas remove_by_count() is a relative request
"make N less LMBs be plugged".

So I think remove_by_ic()s existing handling is correct.  I'm less
sure if remove_by_count() ignoring RESERVED is correct - I couldn't
quickly find under what circumstances RESERVED gets set.

> This patch addresses that by changing
> lmb_is_removable() to also check for DRCONF_MEM_RESERVED to tell if a
> lmb is removable, making dlpar_memory_remove_by_count() take the
> reservation state into account when counting the LMBs.
> lmb_is_removable() is then used in the validation step of
> dlpar_memory_remove_by_ic(), which is already checking for both states
> but in different stages, to avoid counting a LMB that is not assigned as
> eligible for removal. We can then skip the check before
> dlpar_remove_lmb() since we're validating all LMBs beforehand.
> 
> Signed-off-by: Daniel Henrique Barboza <[email-masked]>
> ---
>  arch/powerpc/platforms/pseries/hotplug-memory.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c b/arch/powerpc/platforms/pseries/hotplug-memory.c
> index bb98574a84a2..4e6d162c3f1a 100644
> --- a/arch/powerpc/platforms/pseries/hotplug-memory.c
> +++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
> @@ -348,7 +348,8 @@ static int pseries_remove_mem_node(struct device_node *np)
>  
>  static bool lmb_is_removable(struct drmem_lmb *lmb)
>  {
> -	if (!(lmb->flags & DRCONF_MEM_ASSIGNED))
> +	if ((lmb->flags & DRCONF_MEM_RESERVED) ||
> +		!(lmb->flags & DRCONF_MEM_ASSIGNED))
>  		return false;
>  
>  #ifdef CONFIG_FA_DUMP
> @@ -523,7 +524,7 @@ static int dlpar_memory_remove_by_ic(u32 lmbs_to_remove, u32 drc_index)
>  
>  	/* Validate that there are enough LMBs to satisfy the request */
>  	for_each_drmem_lmb_in_range(lmb, start_lmb, end_lmb) {
> -		if (lmb->flags & DRCONF_MEM_RESERVED)
> +		if (!lmb_is_removable(lmb))
>  			break;
>  
>  		lmbs_available++;
> @@ -533,9 +534,6 @@ static int dlpar_memory_remove_by_ic(u32 lmbs_to_remove, u32 drc_index)
>  		return -EINVAL;
>  
>  	for_each_drmem_lmb_in_range(lmb, start_lmb, end_lmb) {
> -		if (!(lmb->flags & DRCONF_MEM_ASSIGNED))
> -			continue;
> -
>  		rc = dlpar_remove_lmb(lmb);
>  		if (rc)
>  			break;

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson