linux-kernel - Re: [PATCH v2 4/8] fsdax,xfs: set the shared flag when file extent is shared

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Date:   Thu, 1 Dec 2022 16:05:09 -0800
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        nvdimm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
        david@...morbit.com, dan.j.williams@...el.com,
        akpm@...ux-foundation.org
Subject: Re: [PATCH v2 4/8] fsdax,xfs: set the shared flag when file extent
 is shared

On Thu, Dec 01, 2022 at 03:28:54PM +0000, Shiyang Ruan wrote:
> If a dax page is shared, mapread at different offsets can also trigger
> page fault on same dax page.  So, change the flag from "cow" to
> "shared".  And get the shared flag from filesystem when read.
> 
> Signed-off-by: Shiyang Ruan <ruansy.fnst@...itsu.com>

Makes sense.
Reviewed-by: Darrick J. Wong <djwong@...nel.org>

--D

> ---
>  fs/dax.c           | 19 +++++++------------
>  fs/xfs/xfs_iomap.c |  2 +-
>  2 files changed, 8 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 6b6e07ad8d80..f1eb59bee0b5 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -846,12 +846,6 @@ static bool dax_fault_is_synchronous(const struct iomap_iter *iter,
>  		(iter->iomap.flags & IOMAP_F_DIRTY);
>  }
>  
> -static bool dax_fault_is_cow(const struct iomap_iter *iter)
> -{
> -	return (iter->flags & IOMAP_WRITE) &&
> -		(iter->iomap.flags & IOMAP_F_SHARED);
> -}
> -
>  /*
>   * 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
> @@ -865,13 +859,14 @@ static void *dax_insert_entry(struct xa_state *xas, struct vm_fault *vmf,
>  {
>  	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
>  	void *new_entry = dax_make_entry(pfn, flags);
> -	bool dirty = !dax_fault_is_synchronous(iter, vmf->vma);
> -	bool cow = dax_fault_is_cow(iter);
> +	bool write = iter->flags & IOMAP_WRITE;
> +	bool dirty = write && !dax_fault_is_synchronous(iter, vmf->vma);
> +	bool shared = iter->iomap.flags & IOMAP_F_SHARED;
>  
>  	if (dirty)
>  		__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
>  
> -	if (cow || (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE))) {
> +	if (shared || (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))
> @@ -883,12 +878,12 @@ static void *dax_insert_entry(struct xa_state *xas, struct vm_fault *vmf,
>  
>  	xas_reset(xas);
>  	xas_lock_irq(xas);
> -	if (cow || dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
> +	if (shared || dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
>  		void *old;
>  
>  		dax_disassociate_entry(entry, mapping, false);
>  		dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address,
> -				cow);
> +				shared);
>  		/*
>  		 * 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
> @@ -908,7 +903,7 @@ static void *dax_insert_entry(struct xa_state *xas, struct vm_fault *vmf,
>  	if (dirty)
>  		xas_set_mark(xas, PAGECACHE_TAG_DIRTY);
>  
> -	if (cow)
> +	if (write && shared)
>  		xas_set_mark(xas, PAGECACHE_TAG_TOWRITE);
>  
>  	xas_unlock_irq(xas);
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 07da03976ec1..881de99766ca 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -1215,7 +1215,7 @@ xfs_read_iomap_begin(
>  		return error;
>  	error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb, &imap,
>  			       &nimaps, 0);
> -	if (!error && (flags & IOMAP_REPORT))
> +	if (!error && ((flags & IOMAP_REPORT) || IS_DAX(inode)))
>  		error = xfs_reflink_trim_around_shared(ip, &imap, &shared);
>  	xfs_iunlock(ip, lockmode);
>  
> -- 
> 2.38.1
>