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

[Ruan Shiyang]

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@xxxxxxxx>
> > Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxxxxx>
> > ---
> >   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