linux-kernel - Re: [PATCH 02/12] x86/mm/hotplug: Remove pgd

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Message-ID: <557B61B7.4080301@hp.com>
Date:	Fri, 12 Jun 2015 18:48:23 -0400
From:	Waiman Long <waiman.long@...com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	linux-kernel@...r.kernel.org, linux-mml@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 02/12] x86/mm/hotplug: Remove pgd_list use from the memory
 hotplug code

On 06/11/2015 10:07 AM, Ingo Molnar wrote:
> The memory hotplug code uses sync_global_pgds() to synchronize updates
> to the global (&init_mm) kernel PGD and the task PGDs. It does this
> by iterating over the pgd_list - which list closely tracks task
> creation/destruction via fork/clone.
>
> But we want to remove this list, so that it does not have to be
> maintained from fork()/exit(), so convert the memory hotplug code
> to use the task list to iterate over all pgds in the system.
>
> Also improve the comments a bit, to make this function easier
> to understand.
>
> Only lightly tested, as I don't have a memory hotplug setup.
>
> Cc: Andrew Morton<akpm@...ux-foundation.org>
> Cc: Andy Lutomirski<luto@...capital.net>
> Cc: Borislav Petkov<bp@...en8.de>
> Cc: Brian Gerst<brgerst@...il.com>
> Cc: Denys Vlasenko<dvlasenk@...hat.com>
> Cc: H. Peter Anvin<hpa@...or.com>
> Cc: Linus Torvalds<torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra<peterz@...radead.org>
> Cc: Thomas Gleixner<tglx@...utronix.de>
> Cc: Waiman Long<Waiman.Long@...com>
> Signed-off-by: Ingo Molnar<mingo@...nel.org>
> ---
>   arch/x86/mm/init_64.c | 30 +++++++++++++++++-------------
>   1 file changed, 17 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 3fba623e3ba5..1921acbd49fd 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -160,8 +160,8 @@ static int __init nonx32_setup(char *str)
>   __setup("noexec32=", nonx32_setup);
>
>   /*
> - * When memory was added/removed make sure all the processes MM have
> - * suitable PGD entries in the local PGD level page.
> + * When memory was added/removed make sure all the process MMs have
> + * matching PGD entries in the local PGD level page as well.
>    */
>   void sync_global_pgds(unsigned long start, unsigned long end, int removed)
>   {
> @@ -169,29 +169,33 @@ void sync_global_pgds(unsigned long start, unsigned long end, int removed)
>
>   	for (address = start; address<= end; address += PGDIR_SIZE) {
>   		const pgd_t *pgd_ref = pgd_offset_k(address);
> -		struct page *page;
> +		struct task_struct *g, *p;
>
>   		/*
> -		 * When it is called after memory hot remove, pgd_none()
> -		 * returns true. In this case (removed == 1), we must clear
> -		 * the PGD entries in the local PGD level page.
> +		 * When this function is called after memory hot remove,
> +		 * pgd_none() already returns true, but only the reference
> +		 * kernel PGD has been cleared, not the process PGDs.
> +		 *
> +		 * So clear the affected entries in every process PGD as well:
>   		 */
>   		if (pgd_none(*pgd_ref)&&  !removed)
>   			continue;
>
>   		spin_lock(&pgd_lock);
> -		list_for_each_entry(page,&pgd_list, lru) {
> -			pgd_t *pgd;
> +
> +		for_each_process_thread(g, p) {
> +			pgd_t *pgd = p->mm->pgd;
>   			spinlock_t *pgt_lock;
>
> -			pgd = (pgd_t *)page_address(page) + pgd_index(address);
> -			/* the pgt_lock only for Xen */
> -			pgt_lock =&pgd_page_get_mm(page)->page_table_lock;
> +			if (!p->mm)
> +				continue;

pgd was initialized to p->mm->pgd before the "p->mm" check is done. 
Shouldn't the initialization be moved after that.

Cheers,
Longman
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