Btrfs devel - Oct 2013 - [PATCH] Btrfs: limit delalloc pages outside of find_delalloc_range

Liu fixed part of this problem and unfortunately I steered him in slightly the
wrong direction and so didn''t completely fix the problem.  The problem
is we
limit the size of the delalloc range we are looking for to max bytes and then we
try to lock that range.  If we fail to lock the pages in that range we will
shrink the max bytes to a single page and re loop.  However if our first page is
inside of the delalloc range then we will end up limiting the end of the range
to a period before our first page.  This is illustrated below

[0 -------- delalloc range --------- 256mb]
                                  [page]

So find_delalloc_range will return with delalloc_start as 0 and end as 128mb,
and then we will notice that delalloc_start < *start and adjust it up, but
not
adjust delalloc_end up, so things go sideways.  To fix this we need to not limit
the max bytes in find_delalloc_range, but in find_lock_delalloc_range and that
way we don''t end up with this confusion.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
---
 fs/btrfs/extent_io.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index b48942f..2bf6f46 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -1491,10 +1491,8 @@ static noinline u64 find_delalloc_range(struct
extent_io_tree *tree,
 		cur_start = state->end + 1;
 		node = rb_next(node);
 		total_bytes += state->end - state->start + 1;
-		if (total_bytes >= max_bytes) {
-			*end = *start + max_bytes - 1;
+		if (total_bytes >= max_bytes)
 			break;
-		}
 		if (!node)
 			break;
 	}
@@ -1636,10 +1634,9 @@ again:
 
 	/*
 	 * make sure to limit the number of pages we try to lock down
-	 * if we''re looping.
 	 */
-	if (delalloc_end + 1 - delalloc_start > max_bytes && loops)
-		delalloc_end = delalloc_start + PAGE_CACHE_SIZE - 1;
+	if (delalloc_end + 1 - delalloc_start > max_bytes)
+		delalloc_end = delalloc_start + max_bytes - 1;
 
 	/* step two, lock all the pages after the page that has start */
 	ret = lock_delalloc_pages(inode, locked_page,
@@ -1650,8 +1647,7 @@ again:
 		 */
 		free_extent_state(cached_state);
 		if (!loops) {
-			unsigned long offset = (*start) & (PAGE_CACHE_SIZE - 1);
-			max_bytes = PAGE_CACHE_SIZE - offset;
+			max_bytes = PAGE_CACHE_SIZE;
 			loops = 1;
 			goto again;
 		} else {
-- 
1.8.3.1

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On Tue, Oct 08, 2013 at 01:31:57PM -0400, Josef Bacik
wrote:
> Liu fixed part of this problem and unfortunately I steered him in slightly
the
> wrong direction and so didn''t completely fix the problem.  The
problem is we
> limit the size of the delalloc range we are looking for to max bytes and
then we
> try to lock that range.  If we fail to lock the pages in that range we will
> shrink the max bytes to a single page and re loop.  However if our first
page is
> inside of the delalloc range then we will end up limiting the end of the
range
> to a period before our first page.  This is illustrated below
> 
> [0 -------- delalloc range --------- 256mb]
>                                   [page]
> 
> So find_delalloc_range will return with delalloc_start as 0 and end as
128mb,
> and then we will notice that delalloc_start < *start and adjust it up,
but not
> adjust delalloc_end up, so things go sideways.  To fix this we need to not
limit
This makes me more confused...
so do you mean that ''delalloc_end < delalloc_start'' leads
to the not locked pages?

-liubo
thanks,
> the max bytes in find_delalloc_range, but in find_lock_delalloc_range and
that
> way we don''t end up with this confusion.  Thanks,
> 
> Signed-off-by: Josef Bacik <jbacik@fusionio.com>
> ---
>  fs/btrfs/extent_io.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index b48942f..2bf6f46 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -1491,10 +1491,8 @@ static noinline u64 find_delalloc_range(struct
extent_io_tree *tree,
>  		cur_start = state->end + 1;
>  		node = rb_next(node);
>  		total_bytes += state->end - state->start + 1;
> -		if (total_bytes >= max_bytes) {
> -			*end = *start + max_bytes - 1;
> +		if (total_bytes >= max_bytes)
>  			break;
> -		}
>  		if (!node)
>  			break;
>  	}
> @@ -1636,10 +1634,9 @@ again:
>  
>  	/*
>  	 * make sure to limit the number of pages we try to lock down
> -	 * if we''re looping.
>  	 */
> -	if (delalloc_end + 1 - delalloc_start > max_bytes && loops)
> -		delalloc_end = delalloc_start + PAGE_CACHE_SIZE - 1;
> +	if (delalloc_end + 1 - delalloc_start > max_bytes)
> +		delalloc_end = delalloc_start + max_bytes - 1;
>  
>  	/* step two, lock all the pages after the page that has start */
>  	ret = lock_delalloc_pages(inode, locked_page,
> @@ -1650,8 +1647,7 @@ again:
>  		 */
>  		free_extent_state(cached_state);
>  		if (!loops) {
> -			unsigned long offset = (*start) & (PAGE_CACHE_SIZE - 1);
> -			max_bytes = PAGE_CACHE_SIZE - offset;
> +			max_bytes = PAGE_CACHE_SIZE;
>  			loops = 1;
>  			goto again;
>  		} else {
> -- 
> 1.8.3.1
> 
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On Wed, Oct 09, 2013 at 06:07:20PM +0800, Liu Bo wrote:
> On Tue, Oct 08, 2013 at 01:31:57PM -0400, Josef Bacik wrote:
> > Liu fixed part of this problem and unfortunately I steered him in
slightly the
> > wrong direction and so didn''t completely fix the problem. 
The problem is we
> > limit the size of the delalloc range we are looking for to max bytes
and then we
> > try to lock that range.  If we fail to lock the pages in that range we
will
> > shrink the max bytes to a single page and re loop.  However if our
first page is
> > inside of the delalloc range then we will end up limiting the end of
the range
> > to a period before our first page.  This is illustrated below
> > 
> > [0 -------- delalloc range --------- 256mb]
> >                                   [page]
> > 
> > So find_delalloc_range will return with delalloc_start as 0 and end as
128mb,
> > and then we will notice that delalloc_start < *start and adjust it
up, but not
> > adjust delalloc_end up, so things go sideways.  To fix this we need to
not limit
> 
> This makes me more confused...
> so do you mean that ''delalloc_end < delalloc_start''
leads to the not locked pages?
> 
No, it leads to an infinite loop, I still have no idea how that guy was seeing
unlocked pages with you patch :(.  Thanks,

Josef
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On Wed, Oct 09, 2013 at 08:57:12AM -0400, Josef Bacik
wrote:
> On Wed, Oct 09, 2013 at 06:07:20PM +0800, Liu Bo wrote:
> > On Tue, Oct 08, 2013 at 01:31:57PM -0400, Josef Bacik wrote:
> > > Liu fixed part of this problem and unfortunately I steered him in
slightly the
> > > wrong direction and so didn''t completely fix the
problem.  The problem is we
> > > limit the size of the delalloc range we are looking for to max
bytes and then we
> > > try to lock that range.  If we fail to lock the pages in that
range we will
> > > shrink the max bytes to a single page and re loop.  However if
our first page is
> > > inside of the delalloc range then we will end up limiting the end
of the range
> > > to a period before our first page.  This is illustrated below
> > > 
> > > [0 -------- delalloc range --------- 256mb]
> > >                                   [page]
> > > 
> > > So find_delalloc_range will return with delalloc_start as 0 and
end as 128mb,
> > > and then we will notice that delalloc_start < *start and
adjust it up, but not
> > > adjust delalloc_end up, so things go sideways.  To fix this we
need to not limit
> > 
> > This makes me more confused...
> > so do you mean that ''delalloc_end <
delalloc_start'' leads to the not locked pages?
> > 
> 
> No, it leads to an infinite loop, I still have no idea how that guy was
seeing
> unlocked pages with you patch :(.  Thanks,
Oh infinite loop yah, that makes sense :)

-liubo
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