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

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Date:	Sat, 13 Jun 2015 09:46:55 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Waiman Long <waiman.long@...com>
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


* Waiman Long <waiman.long@...com> wrote:

> >@@ -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.

Yes, already found this bug in testing and fixed it - will send out a new series 
with all the feedback so far addressed.

Thanks,

	Ingo
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