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From: Boaz Harrosh <boaz@plexistor.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Dave Chinner <david@fromorbit.com>,
	Matthew Wilcox <matthew.r.wilcox@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Jan Kara <jack@suse.cz>, Hugh Dickins <hughd@google.com>,
	Mel Gorman <mgorman@suse.de>,
	linux-mm@kvack.org, linux-nvdimm <linux-nvdimm@ml01.01.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Eryu Guan <eguan@redhat.com>,
	Christoph Hellwig <hch@infradead.org>,
	Stable Tree <stable@vger.kernel.org>
Subject: Re: [PATCH 1/3 v6] mm(v4.1): New pfn_mkwrite same as page_mkwrite for VM_PFNMAP
Date: Tue, 07 Apr 2015 16:37:07 +0300	[thread overview]
Message-ID: <5523DD83.4050609@plexistor.com> (raw)
In-Reply-To: <20150407132601.GA14252@node.dhcp.inet.fi>

On 04/07/2015 04:26 PM, Kirill A. Shutemov wrote:
> On Tue, Apr 07, 2015 at 04:17:00PM +0300, Kirill A. Shutemov wrote:
>> On Tue, Apr 07, 2015 at 03:57:32PM +0300, Boaz Harrosh wrote:
>>> +/*
>>> + * Handle write page faults for VM_MIXEDMAP or VM_PFNMAP for a VM_SHARED
>>> + * mapping
>>> + */
>>> +static int wp_pfn_shared(struct mm_struct *mm,
>>> +			struct vm_area_struct *vma, unsigned long address,
>>> +			pte_t *page_table, spinlock_t *ptl, pte_t orig_pte,
>>> +			pmd_t *pmd)
>>> +{
>>> +	if (vma->vm_ops && vma->vm_ops->pfn_mkwrite) {
>>> +		struct vm_fault vmf = {
>>> +			.page = NULL,
>>> +			.pgoff = linear_page_index(vma, address),
>>> +			.virtual_address = (void __user *)(address & PAGE_MASK),
>>> +			.flags = FAULT_FLAG_WRITE | FAULT_FLAG_MKWRITE,
>>> +		};
>>> +		int ret;
>>> +
>>> +		pte_unmap_unlock(page_table, ptl);
>>> +		ret = vma->vm_ops->pfn_mkwrite(vma, &vmf);
>>> +		if (ret & VM_FAULT_ERROR)
>>> +			return ret;
>>> +		page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
>>> +		/* Did pfn_mkwrite already fixed up the pte */
> 
> Oh. I guess you've missunderstood why we need pte_same() check below.
> It's not about ->pfn_mkwrite() changing the pte (generatlly, it should
> not). It's requited to address race with parallel page fault to the pte.
> 
>>> +		if (!pte_same(*page_table, orig_pte)) {
>>> +			pte_unmap_unlock(page_table, ptl);
>>> +			return ret;
>>
>> This should be "return 0;", shouldn't it?
>>
>> VM_FAULT_NOPAGE would imply you've installed new pte, but you did not.

Changing this to "return 0" would be very scary for me. Because I'm running
with this code for 1/2 a year now. And it is stable. You see since the original
code it was always doing just that pte_unmap_unlock && return ret. (See the patch
based on 4.0)

I did not understand if you want that I keep it "return ret".

I gather that you would like the comment changed, about the changed pte.
Both here and at Documentation/.../locking.

I'll send a new patch just tell me if you want the reurn thing

Thank you
Boaz


WARNING: multiple messages have this Message-ID
From: Boaz Harrosh <boaz@plexistor.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Dave Chinner <david@fromorbit.com>,
	Matthew Wilcox <matthew.r.wilcox@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Jan Kara <jack@suse.cz>, Hugh Dickins <hughd@google.com>,
	Mel Gorman <mgorman@suse.de>,
	linux-mm@kvack.org, linux-nvdimm <linux-nvdimm@ml01.01.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Eryu Guan <eguan@redhat.com>,
	Christoph Hellwig <hch@infradead.org>,
	Stable Tree <stable@vger.kernel.org>
Subject: Re: [PATCH 1/3 v6] mm(v4.1): New pfn_mkwrite same as page_mkwrite for VM_PFNMAP
Date: Tue, 07 Apr 2015 16:37:07 +0300	[thread overview]
Message-ID: <5523DD83.4050609@plexistor.com> (raw)
In-Reply-To: <20150407132601.GA14252@node.dhcp.inet.fi>

On 04/07/2015 04:26 PM, Kirill A. Shutemov wrote:
> On Tue, Apr 07, 2015 at 04:17:00PM +0300, Kirill A. Shutemov wrote:
>> On Tue, Apr 07, 2015 at 03:57:32PM +0300, Boaz Harrosh wrote:
>>> +/*
>>> + * Handle write page faults for VM_MIXEDMAP or VM_PFNMAP for a VM_SHARED
>>> + * mapping
>>> + */
>>> +static int wp_pfn_shared(struct mm_struct *mm,
>>> +			struct vm_area_struct *vma, unsigned long address,
>>> +			pte_t *page_table, spinlock_t *ptl, pte_t orig_pte,
>>> +			pmd_t *pmd)
>>> +{
>>> +	if (vma->vm_ops && vma->vm_ops->pfn_mkwrite) {
>>> +		struct vm_fault vmf = {
>>> +			.page = NULL,
>>> +			.pgoff = linear_page_index(vma, address),
>>> +			.virtual_address = (void __user *)(address & PAGE_MASK),
>>> +			.flags = FAULT_FLAG_WRITE | FAULT_FLAG_MKWRITE,
>>> +		};
>>> +		int ret;
>>> +
>>> +		pte_unmap_unlock(page_table, ptl);
>>> +		ret = vma->vm_ops->pfn_mkwrite(vma, &vmf);
>>> +		if (ret & VM_FAULT_ERROR)
>>> +			return ret;
>>> +		page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
>>> +		/* Did pfn_mkwrite already fixed up the pte */
> 
> Oh. I guess you've missunderstood why we need pte_same() check below.
> It's not about ->pfn_mkwrite() changing the pte (generatlly, it should
> not). It's requited to address race with parallel page fault to the pte.
> 
>>> +		if (!pte_same(*page_table, orig_pte)) {
>>> +			pte_unmap_unlock(page_table, ptl);
>>> +			return ret;
>>
>> This should be "return 0;", shouldn't it?
>>
>> VM_FAULT_NOPAGE would imply you've installed new pte, but you did not.

Changing this to "return 0" would be very scary for me. Because I'm running
with this code for 1/2 a year now. And it is stable. You see since the original
code it was always doing just that pte_unmap_unlock && return ret. (See the patch
based on 4.0)

I did not understand if you want that I keep it "return ret".

I gather that you would like the comment changed, about the changed pte.
Both here and at Documentation/.../locking.

I'll send a new patch just tell me if you want the reurn thing

Thank you
Boaz

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  reply	other threads:[~2015-04-07 13:37 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-04-07  8:33 [PATCH 0/3 v5] dax: some dax fixes and cleanups Boaz Harrosh
2015-04-07  8:33 ` Boaz Harrosh
2015-04-07  8:40 ` [PATCH 1/3] mm(v4.1): New pfn_mkwrite same as page_mkwrite for VM_PFNMAP Boaz Harrosh
2015-04-07  8:51   ` Boaz Harrosh
2015-04-07  8:51     ` Boaz Harrosh
2015-04-07  9:03   ` Kirill A. Shutemov
2015-04-07  9:22     ` Boaz Harrosh
2015-04-07 12:57   ` [PATCH 1/3 v6] " Boaz Harrosh
2015-04-07 12:57     ` Boaz Harrosh
2015-04-07 13:17     ` Kirill A. Shutemov
2015-04-07 13:26       ` Kirill A. Shutemov
2015-04-07 13:37         ` Boaz Harrosh [this message]
2015-04-07 13:37           ` Boaz Harrosh
2015-04-07 13:47           ` Kirill A. Shutemov
2015-04-07 14:09             ` Boaz Harrosh
2015-04-07 14:09               ` Boaz Harrosh
2015-04-07 14:06   ` [PATCH 1/3 v7] " Boaz Harrosh
2015-04-07 14:06     ` Boaz Harrosh
2015-04-07 14:12     ` Kirill A. Shutemov
2015-04-07  8:43 ` [PATCH 2/3] dax: use pfn_mkwrite to update c/mtime + freeze protection Boaz Harrosh
2015-04-07  8:43   ` Boaz Harrosh
2015-04-07 16:28   ` Jan Kara
2015-04-07 16:28     ` Jan Kara
2015-04-07  8:45 ` [PATCH 3/3] dax: Unify ext2/4_{dax,}_file_operations Boaz Harrosh
2015-04-07  8:45   ` Boaz Harrosh
2015-04-07 16:26   ` Jan Kara
2015-04-08 15:56 ` [PATCH 1/3 @stable] mm(v4.0): New pfn_mkwrite same as page_mkwrite for VM_PFNMAP Boaz Harrosh
2015-04-08 15:56   ` Boaz Harrosh
2015-04-08 16:00   ` Boaz Harrosh
2015-04-08 16:00     ` Boaz Harrosh
2015-04-08 20:26     ` Greg KH
2015-04-12  7:49       ` Boaz Harrosh
2015-04-12  7:49         ` Boaz Harrosh

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