Re: [PATCH v4 2/7] fsdax: Replace mmap entry in case of CoW

[Darrick J. Wong]

On Thu, Apr 08, 2021 at 08:04:27PM +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@xxxxxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> Reviewed-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
> ---
>  fs/dax.c | 39 ++++++++++++++++++++++++++++-----------
>  1 file changed, 28 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index b4fd3813457a..e6c1354b27a8 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -722,6 +722,10 @@ static int copy_cow_page_dax(struct block_device *bdev, struct dax_device *dax_d
>  	return 0;
>  }
>  
> +/* DAX Insert Flag for the entry we insert */

Might be worth mentioning that these are xarray marks for the inserted
entry, since this comment didn't help much.

> +#define DAX_IF_DIRTY		(1 << 0)
> +#define DAX_IF_COW		(1 << 1)
> +
>  /*
>   * 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
> @@ -729,16 +733,19 @@ static int copy_cow_page_dax(struct block_device *bdev, struct dax_device *dax_d
>   * already in the tree, we will skip the insertion and just dirty the PMD as
>   * appropriate.
>   */
> -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)
> +static void *dax_insert_entry(struct xa_state *xas, struct vm_fault *vmf,
> +		void *entry, pfn_t pfn, unsigned long flags,
> +		unsigned int insert_flags)

Urk, two flags arguments.  Oh, I see.  We insert (shifted) pfn_t values
into the mapping as xarray values, so @flags determines the state flags
of the new entry value, whereas @insert_flags determines what xarray
mark we're going to attach (if any) to the inserted value.

--D

>  {
> +	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
>  	void *new_entry = dax_make_entry(pfn, flags);
> +	bool dirty = insert_flags & DAX_IF_DIRTY;
> +	bool cow = insert_flags & DAX_IF_COW;
>  
>  	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 +757,7 @@ 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 +781,9 @@ 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;
>  }
> @@ -1109,8 +1119,7 @@ static vm_fault_t dax_load_hole(struct xa_state *xas,
>  	pfn_t pfn = pfn_to_pfn_t(my_zero_pfn(vaddr));
>  	vm_fault_t ret;
>  
> -	*entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
> -			DAX_ZERO_PAGE, false);
> +	*entry = dax_insert_entry(xas, vmf, *entry, pfn, DAX_ZERO_PAGE, 0);
>  
>  	ret = vmf_insert_mixed(vmf->vma, vaddr, pfn);
>  	trace_dax_load_hole(inode, vmf, ret);
> @@ -1137,8 +1146,8 @@ static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
>  		goto fallback;
>  
>  	pfn = page_to_pfn_t(zero_page);
> -	*entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
> -			DAX_PMD | DAX_ZERO_PAGE, false);
> +	*entry = dax_insert_entry(xas, vmf, *entry, pfn,
> +				  DAX_PMD | DAX_ZERO_PAGE, 0);
>  
>  	if (arch_needs_pgtable_deposit()) {
>  		pgtable = pte_alloc_one(vma->vm_mm);
> @@ -1444,6 +1453,7 @@ static vm_fault_t dax_fault_actor(struct vm_fault *vmf, pfn_t *pfnp,
>  	bool write = vmf->flags & FAULT_FLAG_WRITE;
>  	bool sync = dax_fault_is_synchronous(flags, vmf->vma, iomap);
>  	unsigned long entry_flags = pmd ? DAX_PMD : 0;
> +	unsigned int insert_flags = 0;
>  	int err = 0;
>  	pfn_t pfn;
>  	void *kaddr;
> @@ -1466,8 +1476,15 @@ static vm_fault_t dax_fault_actor(struct vm_fault *vmf, pfn_t *pfnp,
>  	if (err)
>  		return pmd ? VM_FAULT_FALLBACK : dax_fault_return(err);
>  
> -	*entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn, entry_flags,
> -				  write && !sync);
> +	if (write) {
> +		if (!sync)
> +			insert_flags |= DAX_IF_DIRTY;
> +		if (iomap->flags & IOMAP_F_SHARED)
> +			insert_flags |= DAX_IF_COW;
> +	}
> +
> +	*entry = dax_insert_entry(xas, vmf, *entry, pfn, entry_flags,
> +				  insert_flags);
>  
>  	if (write &&
>  	    srcmap->addr != IOMAP_HOLE && srcmap->addr != iomap->addr) {
> -- 
> 2.31.0
> 
> 
>