Re: [PATCH 09/15] btrfs: add dax mmap support

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Mar 26, 2019 at 02:02:55PM -0500, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> 
> Add a new vm_operations struct btrfs_dax_vm_ops
> specifically for dax files.
> 
> Since we will be removing(nulling) readpages/writepages for dax
> return ENOEXEC only for non-dax files.
> 
> dax_insert_entry() looks ugly. Do you think we should break it
> into dax_insert_cow_entry() and dax_insert_entry()?

I would (or replace the two bools with flags), but people seem not to
like my stylistic choices. :)

> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> ---
>  fs/btrfs/ctree.h |  1 +
>  fs/btrfs/dax.c   | 11 +++++++++++
>  fs/btrfs/file.c  | 18 ++++++++++++++++--
>  fs/dax.c         | 17 ++++++++++-------
>  4 files changed, 38 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 3bcd2a4959c1..0e5060933bde 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -3802,6 +3802,7 @@ int btree_readahead_hook(struct extent_buffer *eb, int err);
>  /* dax.c */
>  ssize_t btrfs_file_dax_read(struct kiocb *iocb, struct iov_iter *to);
>  ssize_t btrfs_file_dax_write(struct kiocb *iocb, struct iov_iter *from);
> +vm_fault_t btrfs_dax_fault(struct vm_fault *vmf);
>  #else
>  static inline ssize_t btrfs_file_dax_write(struct kiocb *iocb, struct iov_iter *from)
>  {
> diff --git a/fs/btrfs/dax.c b/fs/btrfs/dax.c
> index 49619fe3f94f..927f962d1e88 100644
> --- a/fs/btrfs/dax.c
> +++ b/fs/btrfs/dax.c
> @@ -157,4 +157,15 @@ ssize_t btrfs_file_dax_write(struct kiocb *iocb, struct iov_iter *iter)
>  	}
>  	return ret;
>  }
> +
> +vm_fault_t btrfs_dax_fault(struct vm_fault *vmf)
> +{
> +	vm_fault_t ret;
> +	pfn_t pfn;
> +	ret = dax_iomap_fault(vmf, PE_SIZE_PTE, &pfn, NULL, &btrfs_iomap_ops);
> +	if (ret & VM_FAULT_NEEDDSYNC)
> +		ret = dax_finish_sync_fault(vmf, PE_SIZE_PTE, pfn);
> +
> +	return ret;
> +}
>  #endif /* CONFIG_FS_DAX */
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 3b320d0ab495..196c8f37ff9d 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -2214,15 +2214,29 @@ static const struct vm_operations_struct btrfs_file_vm_ops = {
>  	.page_mkwrite	= btrfs_page_mkwrite,
>  };
>  
> +#ifdef CONFIG_FS_DAX
> +static const struct vm_operations_struct btrfs_dax_vm_ops = {
> +	.fault          = btrfs_dax_fault,
> +	.page_mkwrite   = btrfs_dax_fault,
> +	.pfn_mkwrite    = btrfs_dax_fault,
> +};
> +#else
> +#define btrfs_dax_vm_ops btrfs_file_vm_ops
> +#endif
> +
>  static int btrfs_file_mmap(struct file	*filp, struct vm_area_struct *vma)
>  {
>  	struct address_space *mapping = filp->f_mapping;
> +	struct inode *inode = file_inode(filp);
>  
> -	if (!mapping->a_ops->readpage)
> +	if (!IS_DAX(inode) && !mapping->a_ops->readpage)
>  		return -ENOEXEC;
>  
>  	file_accessed(filp);
> -	vma->vm_ops = &btrfs_file_vm_ops;
> +	if (IS_DAX(inode))
> +		vma->vm_ops = &btrfs_dax_vm_ops;
> +	else
> +		vma->vm_ops = &btrfs_file_vm_ops;
>  
>  	return 0;
>  }
> diff --git a/fs/dax.c b/fs/dax.c
> index 21ee3df6f02c..41061da42771 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c

Whoah, waitaminute, I thought this was a "twiddle stuff inside btrfs
only" patch...

> @@ -708,14 +708,15 @@ static int copy_user_dax(struct block_device *bdev, struct dax_device *dax_dev,
>   */
>  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 dirty,
> +		bool cow)
>  {
>  	void *new_entry = dax_make_entry(pfn, flags);
>  
>  	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))
> @@ -732,7 +733,7 @@ static void *dax_insert_entry(struct xa_state *xas,
>  		dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address);
>  	}
>  
> -	if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
> +	if (cow || dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
>  		/*
>  		 * Only swap our new entry into the page cache if the current
>  		 * entry is a zero page or an empty entry.  If a normal PTE or
> @@ -1031,7 +1032,7 @@ static vm_fault_t dax_load_hole(struct xa_state *xas,
>  	vm_fault_t ret;
>  
>  	*entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
> -			DAX_ZERO_PAGE, false);
> +			DAX_ZERO_PAGE, false, false);
>  
>  	ret = vmf_insert_mixed(vmf->vma, vaddr, pfn);
>  	trace_dax_load_hole(inode, vmf, ret);
> @@ -1408,7 +1409,8 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
>  			goto error_finish_iomap;
>  
>  		entry = dax_insert_entry(&xas, mapping, vmf, entry, pfn,
> -						 0, write && !sync);
> +						 0, write && !sync,
> +						(iomap.flags & IOMAP_F_COW) != 0);

Assuming you stick with bool cow, you don't need the != 0 test.

>  
>  		/*
>  		 * If we are doing synchronous page fault and inode needs fsync,
> @@ -1487,7 +1489,7 @@ static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
>  
>  	pfn = page_to_pfn_t(zero_page);
>  	*entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
> -			DAX_PMD | DAX_ZERO_PAGE, false);
> +			DAX_PMD | DAX_ZERO_PAGE, false, false);
>  
>  	ptl = pmd_lock(vmf->vma->vm_mm, vmf->pmd);
>  	if (!pmd_none(*(vmf->pmd))) {
> @@ -1610,7 +1612,8 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
>  			goto finish_iomap;
>  
>  		entry = dax_insert_entry(&xas, mapping, vmf, entry, pfn,
> -						DAX_PMD, write && !sync);
> +						DAX_PMD, write && !sync,
> +						false);

Why don't PMD faults support COW?

--D

>  
>  		/*
>  		 * If we are doing synchronous page fault and inode needs fsync,
> -- 
> 2.16.4
>