Re: [PATCH 26/27] fsdax: switch the fault handlers to use iomap_iter

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Jul 19, 2021 at 12:35:19PM +0200, Christoph Hellwig wrote:
> Avoid the open coded calls to ->iomap_begin and ->iomap_end and call
> iomap_iter instead.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Finally this nightmare is over...
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  fs/dax.c | 193 +++++++++++++++++++++----------------------------------
>  1 file changed, 75 insertions(+), 118 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 6d0c6d28be83b1..118c9e2923f5f8 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1010,7 +1010,7 @@ static sector_t dax_iomap_sector(const struct iomap *iomap, loff_t pos)
>  	return (iomap->addr + (pos & PAGE_MASK) - iomap->offset) >> 9;
>  }
>  
> -static int dax_iomap_pfn(struct iomap *iomap, loff_t pos, size_t size,
> +static int dax_iomap_pfn(const struct iomap *iomap, loff_t pos, size_t size,
>  			 pfn_t *pfnp)
>  {
>  	const sector_t sector = dax_iomap_sector(iomap, pos);
> @@ -1068,7 +1068,7 @@ static vm_fault_t dax_load_hole(struct xa_state *xas,
>  
>  #ifdef CONFIG_FS_DAX_PMD
>  static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
> -		struct iomap *iomap, void **entry)
> +		const struct iomap *iomap, void **entry)
>  {
>  	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
>  	unsigned long pmd_addr = vmf->address & PMD_MASK;
> @@ -1120,7 +1120,7 @@ static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
>  }
>  #else
>  static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
> -		struct iomap *iomap, void **entry)
> +		const struct iomap *iomap, void **entry)
>  {
>  	return VM_FAULT_FALLBACK;
>  }
> @@ -1309,7 +1309,7 @@ static vm_fault_t dax_fault_return(int error)
>   * flushed on write-faults (non-cow), but not read-faults.
>   */
>  static bool dax_fault_is_synchronous(unsigned long flags,
> -		struct vm_area_struct *vma, struct iomap *iomap)
> +		struct vm_area_struct *vma, const struct iomap *iomap)
>  {
>  	return (flags & IOMAP_WRITE) && (vma->vm_flags & VM_SYNC)
>  		&& (iomap->flags & IOMAP_F_DIRTY);
> @@ -1329,22 +1329,22 @@ static vm_fault_t dax_fault_synchronous_pfnp(pfn_t *pfnp, pfn_t pfn)
>  	return VM_FAULT_NEEDDSYNC;
>  }
>  
> -static vm_fault_t dax_fault_cow_page(struct vm_fault *vmf, struct iomap *iomap,
> -		loff_t pos)
> +static vm_fault_t dax_fault_cow_page(struct vm_fault *vmf,
> +		const struct iomap_iter *iter)
>  {
> -	sector_t sector = dax_iomap_sector(iomap, pos);
> +	sector_t sector = dax_iomap_sector(&iter->iomap, iter->pos);
>  	unsigned long vaddr = vmf->address;
>  	vm_fault_t ret;
>  	int error = 0;
>  
> -	switch (iomap->type) {
> +	switch (iter->iomap.type) {
>  	case IOMAP_HOLE:
>  	case IOMAP_UNWRITTEN:
>  		clear_user_highpage(vmf->cow_page, vaddr);
>  		break;
>  	case IOMAP_MAPPED:
> -		error = copy_cow_page_dax(iomap->bdev, iomap->dax_dev, sector,
> -					  vmf->cow_page, vaddr);
> +		error = copy_cow_page_dax(iter->iomap.bdev, iter->iomap.dax_dev,
> +					  sector, vmf->cow_page, vaddr);
>  		break;
>  	default:
>  		WARN_ON_ONCE(1);
> @@ -1363,29 +1363,31 @@ static vm_fault_t dax_fault_cow_page(struct vm_fault *vmf, struct iomap *iomap,
>  }
>  
>  /**
> - * dax_fault_actor - Common actor to handle pfn insertion in PTE/PMD fault.
> + * dax_fault_iter - Common actor to handle pfn insertion in PTE/PMD fault.
>   * @vmf:	vm fault instance
> + * @iter:	iomap iter
>   * @pfnp:	pfn to be returned
>   * @xas:	the dax mapping tree of a file
>   * @entry:	an unlocked dax entry to be inserted
>   * @pmd:	distinguish whether it is a pmd fault
> - * @flags:	iomap flags
> - * @iomap:	from iomap_begin()
> - * @srcmap:	from iomap_begin(), not equal to iomap if it is a CoW
>   */
> -static vm_fault_t dax_fault_actor(struct vm_fault *vmf, pfn_t *pfnp,
> -		struct xa_state *xas, void **entry, bool pmd,
> -		unsigned int flags, struct iomap *iomap, struct iomap *srcmap)
> +static vm_fault_t dax_fault_iter(struct vm_fault *vmf,
> +		const struct iomap_iter *iter, pfn_t *pfnp,
> +		struct xa_state *xas, void **entry, bool pmd)
>  {
>  	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
> +	const struct iomap *iomap = &iter->iomap;
>  	size_t size = pmd ? PMD_SIZE : PAGE_SIZE;
>  	loff_t pos = (loff_t)xas->xa_index << PAGE_SHIFT;
>  	bool write = vmf->flags & FAULT_FLAG_WRITE;
> -	bool sync = dax_fault_is_synchronous(flags, vmf->vma, iomap);
> +	bool sync = dax_fault_is_synchronous(iter->flags, vmf->vma, iomap);
>  	unsigned long entry_flags = pmd ? DAX_PMD : 0;
>  	int err = 0;
>  	pfn_t pfn;
>  
> +	if (!pmd && vmf->cow_page)
> +		return dax_fault_cow_page(vmf, iter);
> +
>  	/* if we are reading UNWRITTEN and HOLE, return a hole. */
>  	if (!write &&
>  	    (iomap->type == IOMAP_UNWRITTEN || iomap->type == IOMAP_HOLE)) {
> @@ -1399,7 +1401,7 @@ static vm_fault_t dax_fault_actor(struct vm_fault *vmf, pfn_t *pfnp,
>  		return pmd ? VM_FAULT_FALLBACK : VM_FAULT_SIGBUS;
>  	}
>  
> -	err = dax_iomap_pfn(iomap, pos, size, &pfn);
> +	err = dax_iomap_pfn(&iter->iomap, pos, size, &pfn);
>  	if (err)
>  		return pmd ? VM_FAULT_FALLBACK : dax_fault_return(err);
>  
> @@ -1422,32 +1424,31 @@ static vm_fault_t dax_fault_actor(struct vm_fault *vmf, pfn_t *pfnp,
>  static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
>  			       int *iomap_errp, const struct iomap_ops *ops)
>  {
> -	struct vm_area_struct *vma = vmf->vma;
> -	struct address_space *mapping = vma->vm_file->f_mapping;
> +	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
>  	XA_STATE(xas, &mapping->i_pages, vmf->pgoff);
> -	struct inode *inode = mapping->host;
> -	loff_t pos = (loff_t)vmf->pgoff << PAGE_SHIFT;
> -	struct iomap iomap = { .type = IOMAP_HOLE };
> -	struct iomap srcmap = { .type = IOMAP_HOLE };
> -	unsigned flags = IOMAP_FAULT;
> -	int error;
> -	bool write = vmf->flags & FAULT_FLAG_WRITE;
> -	vm_fault_t ret = 0, major = 0;
> +	struct iomap_iter iter = {
> +		.inode		= mapping->host,
> +		.pos		= (loff_t)vmf->pgoff << PAGE_SHIFT,
> +		.len		= PAGE_SIZE,
> +		.flags		= IOMAP_FAULT,
> +	};
> +	vm_fault_t ret = 0;
>  	void *entry;
> +	int error;
>  
> -	trace_dax_pte_fault(inode, vmf, ret);
> +	trace_dax_pte_fault(iter.inode, vmf, ret);
>  	/*
>  	 * Check whether offset isn't beyond end of file now. Caller is supposed
>  	 * to hold locks serializing us with truncate / punch hole so this is
>  	 * a reliable test.
>  	 */
> -	if (pos >= i_size_read(inode)) {
> +	if (iter.pos >= i_size_read(iter.inode)) {
>  		ret = VM_FAULT_SIGBUS;
>  		goto out;
>  	}
>  
> -	if (write && !vmf->cow_page)
> -		flags |= IOMAP_WRITE;
> +	if ((vmf->flags & FAULT_FLAG_WRITE) && !vmf->cow_page)
> +		iter.flags |= IOMAP_WRITE;
>  
>  	entry = grab_mapping_entry(&xas, mapping, 0);
>  	if (xa_is_internal(entry)) {
> @@ -1466,59 +1467,34 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
>  		goto unlock_entry;
>  	}
>  
> -	/*
> -	 * Note that we don't bother to use iomap_iter here: DAX required
> -	 * the file system block size to be equal the page size, which means
> -	 * that we never have to deal with more than a single extent here.
> -	 */
> -	error = ops->iomap_begin(inode, pos, PAGE_SIZE, flags, &iomap, &srcmap);
> -	if (iomap_errp)
> -		*iomap_errp = error;
> -	if (error) {
> -		ret = dax_fault_return(error);
> -		goto unlock_entry;
> -	}
> -	if (WARN_ON_ONCE(iomap.offset + iomap.length < pos + PAGE_SIZE)) {
> -		ret = VM_FAULT_SIGBUS;	/* fs corruption? */
> -		goto finish_iomap;
> -	}
> -
> -	if (vmf->cow_page) {
> -		ret = dax_fault_cow_page(vmf, &iomap, pos);
> -		goto finish_iomap;
> -	}
> +	while ((error = iomap_iter(&iter, ops)) > 0) {
> +		if (WARN_ON_ONCE(iomap_length(&iter) < PAGE_SIZE)) {
> +			iter.processed = -EIO;	/* fs corruption? */
> +			continue;
> +		}
>  
> -	ret = dax_fault_actor(vmf, pfnp, &xas, &entry, false, flags,
> -			      &iomap, &srcmap);
> -	if (ret == VM_FAULT_SIGBUS)
> -		goto finish_iomap;
> +		ret = dax_fault_iter(vmf, &iter, pfnp, &xas, &entry, false);
> +		if (ret != VM_FAULT_SIGBUS &&
> +		    (iter.iomap.flags & IOMAP_F_NEW)) {
> +			count_vm_event(PGMAJFAULT);
> +			count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
> +			ret |= VM_FAULT_MAJOR;
> +		}
>  
> -	/* read/write MAPPED, CoW UNWRITTEN */
> -	if (iomap.flags & IOMAP_F_NEW) {
> -		count_vm_event(PGMAJFAULT);
> -		count_memcg_event_mm(vma->vm_mm, PGMAJFAULT);
> -		major = VM_FAULT_MAJOR;
> +		if (!(ret & VM_FAULT_ERROR))
> +			iter.processed = PAGE_SIZE;
>  	}
>  
> -finish_iomap:
> -	if (ops->iomap_end) {
> -		int copied = PAGE_SIZE;
> +	if (iomap_errp)
> +		*iomap_errp = error;
> +	if (!ret && error)
> +		ret = dax_fault_return(error);
>  
> -		if (ret & VM_FAULT_ERROR)
> -			copied = 0;
> -		/*
> -		 * The fault is done by now and there's no way back (other
> -		 * thread may be already happily using PTE we have installed).
> -		 * Just ignore error from ->iomap_end since we cannot do much
> -		 * with it.
> -		 */
> -		ops->iomap_end(inode, pos, PAGE_SIZE, copied, flags, &iomap);
> -	}
>  unlock_entry:
>  	dax_unlock_entry(&xas, entry);
>  out:
> -	trace_dax_pte_fault_done(inode, vmf, ret);
> -	return ret | major;
> +	trace_dax_pte_fault_done(iter.inode, vmf, ret);
> +	return ret;
>  }
>  
>  #ifdef CONFIG_FS_DAX_PMD
> @@ -1558,28 +1534,29 @@ static bool dax_fault_check_fallback(struct vm_fault *vmf, struct xa_state *xas,
>  static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
>  			       const struct iomap_ops *ops)
>  {
> -	struct vm_area_struct *vma = vmf->vma;
> -	struct address_space *mapping = vma->vm_file->f_mapping;
> +	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
>  	XA_STATE_ORDER(xas, &mapping->i_pages, vmf->pgoff, PMD_ORDER);
> -	bool write = vmf->flags & FAULT_FLAG_WRITE;
> -	unsigned int flags = (write ? IOMAP_WRITE : 0) | IOMAP_FAULT;
> -	struct inode *inode = mapping->host;
> +	struct iomap_iter iter = {
> +		.inode		= mapping->host,
> +		.len		= PMD_SIZE,
> +		.flags		= IOMAP_FAULT,
> +	};
>  	vm_fault_t ret = VM_FAULT_FALLBACK;
> -	struct iomap iomap = { .type = IOMAP_HOLE };
> -	struct iomap srcmap = { .type = IOMAP_HOLE };
>  	pgoff_t max_pgoff;
>  	void *entry;
> -	loff_t pos;
>  	int error;
>  
> +	if (vmf->flags & FAULT_FLAG_WRITE)
> +		iter.flags |= IOMAP_WRITE;
> +
>  	/*
>  	 * Check whether offset isn't beyond end of file now. Caller is
>  	 * supposed to hold locks serializing us with truncate / punch hole so
>  	 * this is a reliable test.
>  	 */
> -	max_pgoff = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> +	max_pgoff = DIV_ROUND_UP(i_size_read(iter.inode), PAGE_SIZE);
>  
> -	trace_dax_pmd_fault(inode, vmf, max_pgoff, 0);
> +	trace_dax_pmd_fault(iter.inode, vmf, max_pgoff, 0);
>  
>  	if (xas.xa_index >= max_pgoff) {
>  		ret = VM_FAULT_SIGBUS;
> @@ -1613,45 +1590,25 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
>  		goto unlock_entry;
>  	}
>  
> -	/*
> -	 * Note that we don't use iomap_iter here.  We aren't doing I/O, only
> -	 * setting up a mapping, so really we're using iomap_begin() as a way
> -	 * to look up our filesystem block.
> -	 */
> -	pos = (loff_t)xas.xa_index << PAGE_SHIFT;
> -	error = ops->iomap_begin(inode, pos, PMD_SIZE, flags, &iomap, &srcmap);
> -	if (error)
> -		goto unlock_entry;
> -
> -	if (iomap.offset + iomap.length < pos + PMD_SIZE)
> -		goto finish_iomap;
> +	iter.pos = (loff_t)xas.xa_index << PAGE_SHIFT;
> +	while ((error = iomap_iter(&iter, ops)) > 0) {
> +		if (iomap_length(&iter) < PMD_SIZE)
> +			continue; /* actually breaks out of the loop */
>  
> -	ret = dax_fault_actor(vmf, pfnp, &xas, &entry, true, flags,
> -			      &iomap, &srcmap);
> -
> -finish_iomap:
> -	if (ops->iomap_end) {
> -		int copied = PMD_SIZE;
> -
> -		if (ret == VM_FAULT_FALLBACK)
> -			copied = 0;
> -		/*
> -		 * The fault is done by now and there's no way back (other
> -		 * thread may be already happily using PMD we have installed).
> -		 * Just ignore error from ->iomap_end since we cannot do much
> -		 * with it.
> -		 */
> -		ops->iomap_end(inode, pos, PMD_SIZE, copied, flags, &iomap);
> +		ret = dax_fault_iter(vmf, &iter, pfnp, &xas, &entry, true);
> +		if (ret != VM_FAULT_FALLBACK)
> +			iter.processed = PMD_SIZE;
>  	}
> +
>  unlock_entry:
>  	dax_unlock_entry(&xas, entry);
>  fallback:
>  	if (ret == VM_FAULT_FALLBACK) {
> -		split_huge_pmd(vma, vmf->pmd, vmf->address);
> +		split_huge_pmd(vmf->vma, vmf->pmd, vmf->address);
>  		count_vm_event(THP_FAULT_FALLBACK);
>  	}
>  out:
> -	trace_dax_pmd_fault_done(inode, vmf, max_pgoff, ret);
> +	trace_dax_pmd_fault_done(iter.inode, vmf, max_pgoff, ret);
>  	return ret;
>  }
>  #else
> -- 
> 2.30.2
>