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

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Date:	Sat, 13 Jun 2015 21:24:54 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.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>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH 02/12] x86/mm/hotplug: Remove pgd_list use from the
	memory hotplug code

On 06/13, Ingo Molnar wrote:
>
> @@ -169,29 +169,40 @@ 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) {
> +		spin_lock(&pgd_lock); /* Implies rcu_read_lock() for the task list iteration: */
                                         ^^^^^^^^^^^^^^^^^^^^^^^

Hmm, but it doesn't if PREEMPT_RCU? No, no, I do not pretend I understand
how it actually works ;) But, say, rcu_check_callbacks() can be called from
irq and since spin_lock() doesn't increment current->rcu_read_lock_nesting
this can lead to rcu_preempt_qs()?

> +		for_each_process_thread(g, p) {
> +			struct mm_struct *mm;
>  			pgd_t *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;
> +			task_lock(p);
> +			mm = p->mm;
> +			if (!mm) {
> +				task_unlock(p);
> +				continue;
> +			}

Again, you can simplify this code and avoid for_each_process_thread() if
you use for_each_process() + find_lock_task_mm().

Oleg.

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