Ocfs2 devel - Feb 2020 - [PATCH v6 17/19] iomap: Restructure iomap_readpages_actor

From: "Matthew Wilcox (Oracle)" <willy at infradead.org>

By putting the 'have we reached the end of the page' condition at the
end
of the loop instead of the beginning, we can remove the 'submit the last
page' code from iomap_readpages().  Also check that iomap_readpage_actor()
didn't return 0, which would lead to an endless loop.

Signed-off-by: Matthew Wilcox (Oracle) <willy at infradead.org>
---
 fs/iomap/buffered-io.c | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index cb3511eb152a..44303f370b2d 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -400,15 +400,9 @@ iomap_readpages_actor(struct inode *inode, loff_t pos,
loff_t length,
 		void *data, struct iomap *iomap, struct iomap *srcmap)
 {
 	struct iomap_readpage_ctx *ctx = data;
-	loff_t done, ret;
+	loff_t ret, done = 0;
 
-	for (done = 0; done < length; done += ret) {
-		if (ctx->cur_page && offset_in_page(pos + done) == 0) {
-			if (!ctx->cur_page_in_bio)
-				unlock_page(ctx->cur_page);
-			put_page(ctx->cur_page);
-			ctx->cur_page = NULL;
-		}
+	while (done < length) {
 		if (!ctx->cur_page) {
 			ctx->cur_page = iomap_next_page(inode, ctx->pages,
 					pos, length, &done);
@@ -418,6 +412,15 @@ iomap_readpages_actor(struct inode *inode, loff_t pos,
loff_t length,
 		}
 		ret = iomap_readpage_actor(inode, pos + done, length - done,
 				ctx, iomap, srcmap);
+		if (WARN_ON(ret == 0))
+			break;
+		done += ret;
+		if (offset_in_page(pos + done) == 0) {
+			if (!ctx->cur_page_in_bio)
+				unlock_page(ctx->cur_page);
+			put_page(ctx->cur_page);
+			ctx->cur_page = NULL;
+		}
 	}
 
 	return done;
@@ -451,11 +454,7 @@ iomap_readpages(struct address_space *mapping, struct
list_head *pages,
 done:
 	if (ctx.bio)
 		submit_bio(ctx.bio);
-	if (ctx.cur_page) {
-		if (!ctx.cur_page_in_bio)
-			unlock_page(ctx.cur_page);
-		put_page(ctx.cur_page);
-	}
+	BUG_ON(ctx.cur_page);
 
 	/*
 	 * Check that we didn't lose a page due to the arcance calling
-- 
2.25.0

On 2/17/20 10:46 AM, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy at infradead.org>
> 
> By putting the 'have we reached the end of the page' condition at
the end
> of the loop instead of the beginning, we can remove the 'submit the
last
> page' code from iomap_readpages().  Also check that
iomap_readpage_actor()
> didn't return 0, which would lead to an endless loop.

Also added a new WARN_ON() and BUG(), although I'm wondering about the BUG
below...

> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy at infradead.org>
> ---
>  fs/iomap/buffered-io.c | 25 ++++++++++++-------------
>  1 file changed, 12 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index cb3511eb152a..44303f370b2d 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -400,15 +400,9 @@ iomap_readpages_actor(struct inode *inode, loff_t pos,
loff_t length,
>  		void *data, struct iomap *iomap, struct iomap *srcmap)
>  {
>  	struct iomap_readpage_ctx *ctx = data;
> -	loff_t done, ret;
> +	loff_t ret, done = 0;
>  
> -	for (done = 0; done < length; done += ret) {

nit: this "for" loop was perfect just the way it was. :) I'd vote
here for reverting
the change to a "while" loop. Because with this change, now the code
has to
separately initialize "done", separately increment "done",
and the beauty of a
for loop is that the loop init and control is all clearly in one place. For
things
that follow that model (as in this case!), that's a Good Thing.

And I don't see any technical reason (even in the following patch) that
requires
this change.

> -		if (ctx->cur_page && offset_in_page(pos + done) == 0) {
> -			if (!ctx->cur_page_in_bio)
> -				unlock_page(ctx->cur_page);
> -			put_page(ctx->cur_page);
> -			ctx->cur_page = NULL;
> -		}
> +	while (done < length) {
>  		if (!ctx->cur_page) {
>  			ctx->cur_page = iomap_next_page(inode, ctx->pages,
>  					pos, length, &done);
> @@ -418,6 +412,15 @@ iomap_readpages_actor(struct inode *inode, loff_t pos,
loff_t length,
>  		}
>  		ret = iomap_readpage_actor(inode, pos + done, length - done,
>  				ctx, iomap, srcmap);
> +		if (WARN_ON(ret == 0))
> +			break;
> +		done += ret;
> +		if (offset_in_page(pos + done) == 0) {
> +			if (!ctx->cur_page_in_bio)
> +				unlock_page(ctx->cur_page);
> +			put_page(ctx->cur_page);
> +			ctx->cur_page = NULL;
> +		}
>  	}
>  
>  	return done;
> @@ -451,11 +454,7 @@ iomap_readpages(struct address_space *mapping, struct
list_head *pages,
>  done:
>  	if (ctx.bio)
>  		submit_bio(ctx.bio);
> -	if (ctx.cur_page) {
> -		if (!ctx.cur_page_in_bio)
> -			unlock_page(ctx.cur_page);
> -		put_page(ctx.cur_page);
> -	}
> +	BUG_ON(ctx.cur_page);

Is a full BUG_ON() definitely called for here? Seems like a WARN might
suffice...

>  
>  	/*
>  	 * Check that we didn't lose a page due to the arcance calling
> 


thanks,
-- 
John Hubbard
NVIDIA

On Mon, Feb 17, 2020 at 10:46:11AM -0800, Matthew Wilcox
wrote:
> From: "Matthew Wilcox (Oracle)" <willy at infradead.org>
> 
> By putting the 'have we reached the end of the page' condition at
the end
> of the loop instead of the beginning, we can remove the 'submit the
last
> page' code from iomap_readpages().  Also check that
iomap_readpage_actor()
> didn't return 0, which would lead to an endless loop.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy at infradead.org>
> ---
>  fs/iomap/buffered-io.c | 25 ++++++++++++-------------
>  1 file changed, 12 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index cb3511eb152a..44303f370b2d 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -400,15 +400,9 @@ iomap_readpages_actor(struct inode *inode, loff_t pos,
loff_t length,
>  		void *data, struct iomap *iomap, struct iomap *srcmap)
>  {
>  	struct iomap_readpage_ctx *ctx = data;
> -	loff_t done, ret;
> +	loff_t ret, done = 0;
>  
> -	for (done = 0; done < length; done += ret) {
> -		if (ctx->cur_page && offset_in_page(pos + done) == 0) {
> -			if (!ctx->cur_page_in_bio)
> -				unlock_page(ctx->cur_page);
> -			put_page(ctx->cur_page);
> -			ctx->cur_page = NULL;
> -		}
> +	while (done < length) {
>  		if (!ctx->cur_page) {
>  			ctx->cur_page = iomap_next_page(inode, ctx->pages,
>  					pos, length, &done);
> @@ -418,6 +412,15 @@ iomap_readpages_actor(struct inode *inode, loff_t pos,
loff_t length,
>  		}
>  		ret = iomap_readpage_actor(inode, pos + done, length - done,
>  				ctx, iomap, srcmap);
> +		if (WARN_ON(ret == 0))
> +			break;
This error case now leaks ctx->cur_page....
> +		done += ret;
> +		if (offset_in_page(pos + done) == 0) {
> +			if (!ctx->cur_page_in_bio)
> +				unlock_page(ctx->cur_page);
> +			put_page(ctx->cur_page);
> +			ctx->cur_page = NULL;
> +		}
>  	}
>  
>  	return done;
> @@ -451,11 +454,7 @@ iomap_readpages(struct address_space *mapping, struct
list_head *pages,
>  done:
>  	if (ctx.bio)
>  		submit_bio(ctx.bio);
> -	if (ctx.cur_page) {
> -		if (!ctx.cur_page_in_bio)
> -			unlock_page(ctx.cur_page);
> -		put_page(ctx.cur_page);
> -	}
> +	BUG_ON(ctx.cur_page);
And so will now trigger both a warn and a bug....

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com