Re: [PATCH 1/3 v6] mm(v4.1): New pfn_mkwrite same as page_mkwrite for VM_PFNMAP

[Boaz Harrosh <boaz@plexistor.com>]

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