Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 8 Feb 2018 07:00:25 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     Laurent Dufour <ldufour@linux.vnet.ibm.com>
Cc:     paulmck@linux.vnet.ibm.com, peterz@infradead.org,
        akpm@linux-foundation.org, kirill@shutemov.name,
        ak@linux.intel.com, mhocko@kernel.org, dave@stgolabs.net,
        jack@suse.cz, benh@kernel.crashing.org, mpe@ellerman.id.au,
        paulus@samba.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, hpa@zytor.com,
        Will Deacon <will.deacon@arm.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        kemi.wang@intel.com, sergey.senozhatsky.work@gmail.com,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        haren@linux.vnet.ibm.com, khandual@linux.vnet.ibm.com,
        npiggin@gmail.com, bsingharora@gmail.com,
        Tim Chen <tim.c.chen@linux.intel.com>,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org
Subject: Re: [PATCH v7 04/24] mm: Dont assume page-table invariance during
 faults
Message-ID: <20180208150025.GD15846@bombadil.infradead.org>
References: <1517935810-31177-1-git-send-email-ldufour@linux.vnet.ibm.com>
 <1517935810-31177-5-git-send-email-ldufour@linux.vnet.ibm.com>
 <20180206202831.GB16511@bombadil.infradead.org>
 <484242d8-e632-9e39-5c99-2e1b4b3b69a5@linux.vnet.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <484242d8-e632-9e39-5c99-2e1b4b3b69a5@linux.vnet.ibm.com>
User-Agent: Mutt/1.9.1 (2017-09-22)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Feb 08, 2018 at 03:35:58PM +0100, Laurent Dufour wrote:
> I reviewed that part of code, and I think I could now change the way
> pte_unmap_safe() is checking for the pte's value. Since we now have all the
> needed details in the vm_fault structure, I will pass it to
> pte_unamp_same() and deal with the VMA checks when locking for the pte as
> it is done in the other part of the page fault handler by calling
> pte_spinlock().

This does indeed look much better!  Thank you!

> This means that this patch will be dropped, and pte_unmap_same() will become :
> 
> static inline int pte_unmap_same(struct vm_fault *vmf, int *same)
> {
> 	int ret = 0;
> 
> 	*same = 1;
> #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
> 	if (sizeof(pte_t) > sizeof(unsigned long)) {
> 		if (pte_spinlock(vmf)) {
> 			*same = pte_same(*vmf->pte, vmf->orig_pte);
> 			spin_unlock(vmf->ptl);
> 		}
> 		else
> 			ret = VM_FAULT_RETRY;
> 	}
> #endif
> 	pte_unmap(vmf->pte);
> 	return ret;
> }

I'm not a huge fan of auxiliary return values.  Perhaps we could do this
instead:

	ret = pte_unmap_same(vmf);
	if (ret != VM_FAULT_NOTSAME) {
		if (page)
			put_page(page);
		goto out;
	}
	ret = 0;

(we have a lot of unused bits in VM_FAULT_, so adding a new one shouldn't
be a big deal)