linux-kernel - Re: [PATCH 4/7] fsdax: Replace mmap entry in case of CoW

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Date:   Wed, 17 Feb 2021 11:06:55 +0800
From:   Ruan Shiyang <ruansy.fnst@...fujitsu.com>
To:     <dsterba@...e.cz>, <linux-kernel@...r.kernel.org>,
        <linux-xfs@...r.kernel.org>, <linux-nvdimm@...ts.01.org>,
        <linux-fsdevel@...r.kernel.org>, <darrick.wong@...cle.com>,
        <dan.j.williams@...el.com>, <willy@...radead.org>, <jack@...e.cz>,
        <viro@...iv.linux.org.uk>, <linux-btrfs@...r.kernel.org>,
        <ocfs2-devel@....oracle.com>, <david@...morbit.com>, <hch@....de>,
        <rgoldwyn@...e.de>, Goldwyn Rodrigues <rgoldwyn@...e.com>
Subject: Re: [PATCH 4/7] fsdax: Replace mmap entry in case of CoW



On 2021/2/16 下午9:11, David Sterba wrote:
> On Mon, Feb 08, 2021 at 01:09:21AM +0800, Shiyang Ruan wrote:
>> We replace the existing entry to the newly allocated one
>> in case of CoW. Also, we mark the entry as PAGECACHE_TAG_TOWRITE
>> so writeback marks this entry as writeprotected. This
>> helps us snapshots so new write pagefaults after snapshots
>> trigger a CoW.
>>
>> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@...e.com>
>> Signed-off-by: Shiyang Ruan <ruansy.fnst@...fujitsu.com>
>> ---
>>   fs/dax.c | 31 +++++++++++++++++++++++--------
>>   1 file changed, 23 insertions(+), 8 deletions(-)
>>
>> diff --git a/fs/dax.c b/fs/dax.c
>> index b2195cbdf2dc..29698a3d2e37 100644
>> --- a/fs/dax.c
>> +++ b/fs/dax.c
>> @@ -722,6 +722,9 @@ static int copy_cow_page_dax(struct block_device *bdev, struct dax_device *dax_d
>>   	return 0;
>>   }
>>   
>> +#define DAX_IF_DIRTY		(1ULL << 0)
>> +#define DAX_IF_COW		(1ULL << 1)
> 
> The constants are ULL, but I see other flags only 'unsigned long'
> 
>> +
>>   /*
>>    * By this point grab_mapping_entry() has ensured that we have a locked entry
>>    * of the appropriate size so we don't have to worry about downgrading PMDs to
>> @@ -731,14 +734,16 @@ static int copy_cow_page_dax(struct block_device *bdev, struct dax_device *dax_d
>>    */
>>   static void *dax_insert_entry(struct xa_state *xas,
>>   		struct address_space *mapping, struct vm_fault *vmf,
>> -		void *entry, pfn_t pfn, unsigned long flags, bool dirty)
>> +		void *entry, pfn_t pfn, unsigned long flags, bool insert_flags)
> 
> insert_flags is bool
> 
>>   {
>>   	void *new_entry = dax_make_entry(pfn, flags);
>> +	bool dirty = insert_flags & DAX_IF_DIRTY;
> 
> "insert_flags & DAX_IF_DIRTY" is "bool & ULL", this can't be right

This is a mistake caused by rebasing my old version patchset.  Thanks 
for pointing out.  I'll fix this in next version.
> 
>> +	bool cow = insert_flags & DAX_IF_COW;
> 
> Same
> 
>>   
>>   	if (dirty)
>>   		__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
>>   
>> -	if (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE)) {
>> +	if (cow || (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE))) {
>>   		unsigned long index = xas->xa_index;
>>   		/* we are replacing a zero page with block mapping */
>>   		if (dax_is_pmd_entry(entry))
>> @@ -750,7 +755,7 @@ static void *dax_insert_entry(struct xa_state *xas,
>>   
>>   	xas_reset(xas);
>>   	xas_lock_irq(xas);
>> -	if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
>> +	if (cow || dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
>>   		void *old;
>>   
>>   		dax_disassociate_entry(entry, mapping, false);
>> @@ -774,6 +779,9 @@ static void *dax_insert_entry(struct xa_state *xas,
>>   	if (dirty)
>>   		xas_set_mark(xas, PAGECACHE_TAG_DIRTY);
>>   
>> +	if (cow)
>> +		xas_set_mark(xas, PAGECACHE_TAG_TOWRITE);
>> +
>>   	xas_unlock_irq(xas);
>>   	return entry;
>>   }
>> @@ -1319,6 +1327,7 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
>>   	void *entry;
>>   	pfn_t pfn;
>>   	void *kaddr;
>> +	unsigned long insert_flags = 0;
>>   
>>   	trace_dax_pte_fault(inode, vmf, ret);
>>   	/*
>> @@ -1444,8 +1453,10 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
>>   
>>   		goto finish_iomap;
>>   	case IOMAP_UNWRITTEN:
>> -		if (write && iomap.flags & IOMAP_F_SHARED)
>> +		if (write && (iomap.flags & IOMAP_F_SHARED)) {
>> +			insert_flags |= DAX_IF_COW;
> 
> Here's an example of 'unsigned long = unsigned long long', though it'll
> work, it would be better to unify all the types.

Yes, I'll fix it.


--
Thanks,
Ruan Shiyang.
> 
>>   			goto cow;
>> +		}
>>   		fallthrough;
>>   	case IOMAP_HOLE:
>>   		if (!write) {
>> @@ -1555,6 +1566,7 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
>>   	int error;
>>   	pfn_t pfn;
>>   	void *kaddr;
>> +	unsigned long insert_flags = 0;
>>   
>>   	/*
>>   	 * Check whether offset isn't beyond end of file now. Caller is
>> @@ -1670,14 +1682,17 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
>>   		result = vmf_insert_pfn_pmd(vmf, pfn, write);
>>   		break;
>>   	case IOMAP_UNWRITTEN:
>> -		if (write && iomap.flags & IOMAP_F_SHARED)
>> +		if (write && (iomap.flags & IOMAP_F_SHARED)) {
>> +			insert_flags |= DAX_IF_COW;
>>   			goto cow;
>> +		}
>>   		fallthrough;
>>   	case IOMAP_HOLE:
>> -		if (WARN_ON_ONCE(write))
>> +		if (!write) {
>> +			result = dax_pmd_load_hole(&xas, vmf, &iomap, &entry);
>>   			break;
>> -		result = dax_pmd_load_hole(&xas, vmf, &iomap, &entry);
>> -		break;
>> +		}
>> +		fallthrough;
>>   	default:
>>   		WARN_ON_ONCE(1);
>>   		break;
>> -- 
>> 2.30.0
>>
>>
> 
>