[PATCH] fs/remap_range: avoid spurious writeback on zero length request

[PATCH] fs/remap_range: avoid spurious writeback on zero length request

[Brian Foster]

generic_remap_checks() can reduce the effective request length (i.e.,
after the reflink extend to EOF case is handled) down to zero. If this
occurs, __generic_remap_file_range_prep() proceeds through dio
serialization, file mapping flush calls, and may invoke file_modified()
before returning back to the filesystem caller, all of which immediately
check for len == 0 and return.

While this is mostly harmless, it is spurious and not completely
without side effect. A filemap write call can submit I/O (but not
wait on it) when the specified end byte precedes the start but
happens to land on the same aligned page boundary, which can occur
from __generic_remap_file_range_prep() when len is 0.

The dedupe path already has a len == 0 check to break out before doing
range comparisons. Lift this check a bit earlier in the function to
cover the general case of len == 0 and avoid the unnecessary work.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 fs/remap_range.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/remap_range.c b/fs/remap_range.c
index 654912d06862..32ea992f9acc 100644
--- a/fs/remap_range.c
+++ b/fs/remap_range.c
@@ -306,6 +306,8 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 			remap_flags);
 	if (ret)
 		return ret;
+	if (*len == 0)
+		return 0;
 
 	/* Wait for the completion of any pending IOs on both files */
 	inode_dio_wait(inode_in);
@@ -328,9 +330,6 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 	if (remap_flags & REMAP_FILE_DEDUP) {
 		bool		is_same = false;
 
-		if (*len == 0)
-			return 0;
-
 		if (!IS_DAX(inode_in))
 			ret = vfs_dedupe_file_range_compare(file_in, pos_in,
 					file_out, pos_out, *len, &is_same);
-- 
2.37.3

Re: [PATCH] fs/remap_range: avoid spurious writeback on zero length request

[Darrick J. Wong]

On Mon, Nov 28, 2022 at 11:08:13AM -0500, Brian Foster wrote:
> generic_remap_checks() can reduce the effective request length (i.e.,
> after the reflink extend to EOF case is handled) down to zero. If this
> occurs, __generic_remap_file_range_prep() proceeds through dio
> serialization, file mapping flush calls, and may invoke file_modified()
> before returning back to the filesystem caller, all of which immediately
> check for len == 0 and return.
> 
> While this is mostly harmless, it is spurious and not completely
> without side effect. A filemap write call can submit I/O (but not
> wait on it) when the specified end byte precedes the start but
> happens to land on the same aligned page boundary, which can occur
> from __generic_remap_file_range_prep() when len is 0.
> 
> The dedupe path already has a len == 0 check to break out before doing
> range comparisons. Lift this check a bit earlier in the function to
> cover the general case of len == 0 and avoid the unnecessary work.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>

Looks correct,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Should there be an(other) "if (!*len) return 0;" after the
generic_remap_check_len call to skip the mtime update if the remap
request gets shortened to avoid remapping an unaligned eofblock into the
middle of the destination file?

--D

> ---
>  fs/remap_range.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/remap_range.c b/fs/remap_range.c
> index 654912d06862..32ea992f9acc 100644
> --- a/fs/remap_range.c
> +++ b/fs/remap_range.c
> @@ -306,6 +306,8 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
>  			remap_flags);
>  	if (ret)
>  		return ret;
> +	if (*len == 0)
> +		return 0;
>  
>  	/* Wait for the completion of any pending IOs on both files */
>  	inode_dio_wait(inode_in);
> @@ -328,9 +330,6 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
>  	if (remap_flags & REMAP_FILE_DEDUP) {
>  		bool		is_same = false;
>  
> -		if (*len == 0)
> -			return 0;
> -
>  		if (!IS_DAX(inode_in))
>  			ret = vfs_dedupe_file_range_compare(file_in, pos_in,
>  					file_out, pos_out, *len, &is_same);
> -- 
> 2.37.3
> 

Re: [PATCH] fs/remap_range: avoid spurious writeback on zero length request

[Brian Foster]

On Mon, Nov 28, 2022 at 09:22:53AM -0800, Darrick J. Wong wrote:
> On Mon, Nov 28, 2022 at 11:08:13AM -0500, Brian Foster wrote:
> > generic_remap_checks() can reduce the effective request length (i.e.,
> > after the reflink extend to EOF case is handled) down to zero. If this
> > occurs, __generic_remap_file_range_prep() proceeds through dio
> > serialization, file mapping flush calls, and may invoke file_modified()
> > before returning back to the filesystem caller, all of which immediately
> > check for len == 0 and return.
> > 
> > While this is mostly harmless, it is spurious and not completely
> > without side effect. A filemap write call can submit I/O (but not
> > wait on it) when the specified end byte precedes the start but
> > happens to land on the same aligned page boundary, which can occur
> > from __generic_remap_file_range_prep() when len is 0.
> > 
> > The dedupe path already has a len == 0 check to break out before doing
> > range comparisons. Lift this check a bit earlier in the function to
> > cover the general case of len == 0 and avoid the unnecessary work.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> 
> Looks correct,
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> 
> Should there be an(other) "if (!*len) return 0;" after the
> generic_remap_check_len call to skip the mtime update if the remap
> request gets shortened to avoid remapping an unaligned eofblock into the
> middle of the destination file?
> 

Looks sensible to me, though I guess I would do something like the
appended diff. Do you want to just fold that into this patch?

Brian

--- 8< ---

diff --git a/fs/remap_range.c b/fs/remap_range.c
index 32ea992f9acc..2f236c9c5802 100644
--- a/fs/remap_range.c
+++ b/fs/remap_range.c
@@ -347,7 +347,7 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 
 	ret = generic_remap_check_len(inode_in, inode_out, pos_out, len,
 			remap_flags);
-	if (ret)
+	if (ret || *len == 0)
 		return ret;
 
 	/* If can't alter the file contents, we're done. */

Re: [PATCH] fs/remap_range: avoid spurious writeback on zero length request

[Darrick J. Wong]

On Tue, Nov 29, 2022 at 06:29:51AM -0500, Brian Foster wrote:
> On Mon, Nov 28, 2022 at 09:22:53AM -0800, Darrick J. Wong wrote:
> > On Mon, Nov 28, 2022 at 11:08:13AM -0500, Brian Foster wrote:
> > > generic_remap_checks() can reduce the effective request length (i.e.,
> > > after the reflink extend to EOF case is handled) down to zero. If this
> > > occurs, __generic_remap_file_range_prep() proceeds through dio
> > > serialization, file mapping flush calls, and may invoke file_modified()
> > > before returning back to the filesystem caller, all of which immediately
> > > check for len == 0 and return.
> > > 
> > > While this is mostly harmless, it is spurious and not completely
> > > without side effect. A filemap write call can submit I/O (but not
> > > wait on it) when the specified end byte precedes the start but
> > > happens to land on the same aligned page boundary, which can occur
> > > from __generic_remap_file_range_prep() when len is 0.
> > > 
> > > The dedupe path already has a len == 0 check to break out before doing
> > > range comparisons. Lift this check a bit earlier in the function to
> > > cover the general case of len == 0 and avoid the unnecessary work.
> > > 
> > > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > 
> > Looks correct,
> > Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> > 
> > Should there be an(other) "if (!*len) return 0;" after the
> > generic_remap_check_len call to skip the mtime update if the remap
> > request gets shortened to avoid remapping an unaligned eofblock into the
> > middle of the destination file?
> > 
> 
> Looks sensible to me, though I guess I would do something like the
> appended diff. Do you want to just fold that into this patch?

Yes, could you fold it in and send a v2 with my rvb on it, please?

--D

> Brian
> 
> --- 8< ---
> 
> diff --git a/fs/remap_range.c b/fs/remap_range.c
> index 32ea992f9acc..2f236c9c5802 100644
> --- a/fs/remap_range.c
> +++ b/fs/remap_range.c
> @@ -347,7 +347,7 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
>  
>  	ret = generic_remap_check_len(inode_in, inode_out, pos_out, len,
>  			remap_flags);
> -	if (ret)
> +	if (ret || *len == 0)
>  		return ret;
>  
>  	/* If can't alter the file contents, we're done. */
> 

Re: [PATCH] fs/remap_range: avoid spurious writeback on zero length request

[Brian Foster]

On Tue, Nov 29, 2022 at 10:18:53AM -0800, Darrick J. Wong wrote:
> On Tue, Nov 29, 2022 at 06:29:51AM -0500, Brian Foster wrote:
> > On Mon, Nov 28, 2022 at 09:22:53AM -0800, Darrick J. Wong wrote:
> > > On Mon, Nov 28, 2022 at 11:08:13AM -0500, Brian Foster wrote:
> > > > generic_remap_checks() can reduce the effective request length (i.e.,
> > > > after the reflink extend to EOF case is handled) down to zero. If this
> > > > occurs, __generic_remap_file_range_prep() proceeds through dio
> > > > serialization, file mapping flush calls, and may invoke file_modified()
> > > > before returning back to the filesystem caller, all of which immediately
> > > > check for len == 0 and return.
> > > > 
> > > > While this is mostly harmless, it is spurious and not completely
> > > > without side effect. A filemap write call can submit I/O (but not
> > > > wait on it) when the specified end byte precedes the start but
> > > > happens to land on the same aligned page boundary, which can occur
> > > > from __generic_remap_file_range_prep() when len is 0.
> > > > 
> > > > The dedupe path already has a len == 0 check to break out before doing
> > > > range comparisons. Lift this check a bit earlier in the function to
> > > > cover the general case of len == 0 and avoid the unnecessary work.
> > > > 
> > > > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > > 
> > > Looks correct,
> > > Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> > > 
> > > Should there be an(other) "if (!*len) return 0;" after the
> > > generic_remap_check_len call to skip the mtime update if the remap
> > > request gets shortened to avoid remapping an unaligned eofblock into the
> > > middle of the destination file?
> > > 
> > 
> > Looks sensible to me, though I guess I would do something like the
> > appended diff. Do you want to just fold that into this patch?
> 
> Yes, could you fold it in and send a v2 with my rvb on it, please?
> 

Sure. I'll change both lines to do 'if (ret || *len == 0),' not sure why
I didn't do that the first time..

Brian

> --D
> 
> > Brian
> > 
> > --- 8< ---
> > 
> > diff --git a/fs/remap_range.c b/fs/remap_range.c
> > index 32ea992f9acc..2f236c9c5802 100644
> > --- a/fs/remap_range.c
> > +++ b/fs/remap_range.c
> > @@ -347,7 +347,7 @@ __generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
> >  
> >  	ret = generic_remap_check_len(inode_in, inode_out, pos_out, len,
> >  			remap_flags);
> > -	if (ret)
> > +	if (ret || *len == 0)
> >  		return ret;
> >  
> >  	/* If can't alter the file contents, we're done. */
> > 
>