[PATCH v4 0/2] btrfs: trim: Fix a bug certain range may not be trimmed properly

[PATCH v4 0/2] btrfs: trim: Fix a bug certain range may not be trimmed properly

[Qu Wenruo]

There is a bug report about discard mount option not trimming some range
properly, and causing unexpected space usage for thin device.

It turns out to be that, if there are pinned extents across block group
boundary, we will only trim to the end of current block group, skipping
the remaining.

This patchset will fix it by ensuring btrfs_discard_extent() will
iterate the full range before exiting.

The first patch is just a tiny readability improvement found during the
fix.
The second patch is the main body of the fix.

Meanwhile I'm still looking into how to craft such test case for btrfs,
so the test case may be late for several days.

Changelog:
v2:
- Fold the __btrfs_map_block_for_discard() @length change into the main
  patch
  Since the @length parameter change itself doesn't make much sense,
  folding it into the fix looks more reasonable.

- Split the tiny readability improvement patch into its own patch

v3:
- Reset @num_bytes for btrfs_map_block() to limit the trim range inside
  the original range.
  Or we can trim into existing extent to corrupt data.

v4:
- Use better ASCII graph to show the bug
  No code change at all.

Qu Wenruo (2):
  btrfs: volumes: Use more straightforward way to calculate map length
  btrfs: extent-tree: Ensure we trim ranges across block group boundary

 fs/btrfs/extent-tree.c | 41 +++++++++++++++++++++++++++++++----------
 fs/btrfs/volumes.c     |  8 +++++---
 2 files changed, 36 insertions(+), 13 deletions(-)

-- 
2.23.0

[PATCH v4 1/2] btrfs: volumes: Use more straightforward way to calculate map length

[Qu Wenruo]

The old code goes:

 	offset = logical - em->start;
	length = min_t(u64, em->len - offset, length);

Where @length calculate is dependent on offset, it can take reader
several more seconds to find it's just the same code as:

 	offset = logical - em->start;
	length = min_t(u64, em->start + em->len - logical, length);

Use above code to make the length calculate independent from other
variable, thus slightly increase the readability.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/volumes.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index cdd7af424033..a6db11e821a5 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -5616,7 +5616,7 @@ static int __btrfs_map_block_for_discard(struct btrfs_fs_info *fs_info,
 	}
 
 	offset = logical - em->start;
-	length = min_t(u64, em->len - offset, length);
+	length = min_t(u64, em->start + em->len - logical, length);
 
 	stripe_len = map->stripe_len;
 	/*
-- 
2.23.0

[PATCH v4 2/2] btrfs: extent-tree: Ensure we trim ranges across block group boundary

[Qu Wenruo]

[BUG]
When deleting large files (which cross block group boundary) with discard
mount option, we find some btrfs_discard_extent() calls only trimmed part
of its space, not the whole range:

  btrfs_discard_extent: type=0x1 start=19626196992 len=2144530432 trimmed=1073741824 ratio=50%

type:		bbio->map_type, in above case, it's SINGLE DATA.
start:		Logical address of this trim
len:		Logical length of this trim
trimmed:	Physically trimmed bytes
ratio:		trimmed / len

Thus leading some unused space not discarded.

[CAUSE]
When discard mount option is specified, after a transaction is fully
committed (super block written to disk), we begin to cleanup pinned
extents in the following call chain:

btrfs_commit_transaction()
|- btrfs_finish_extent_commit()
   |- find_first_extent_bit(unpin, 0, &start, &end, EXTENT_DIRTY);
   |- btrfs_discard_extent()

However pinned extents are recorded in an extent_io_tree, which can
merge adjacent extent states.

When a large file get deleted and it has adjacent file extents across
block group boundary, we will get a large merged range, like this:

      |<---    BG1    --->|<---      BG2     --->|
      |//////|<--   Range to discard   --->|/////|

To discard that range, we have the following calls:
btrfs_discard_extent()
|- btrfs_map_block()
|  Returned bbio will end at BG1's end. As btrfs_map_block()
|  never returns result across block group boundary.
|- btrfs_issuse_discard()
   Issue discard for each stripe.

So we will only discard the range in BG1, not the remaining part in BG2.

Furthermore, this bug is not that reliably observed, for above case, if
there is no other extent in BG2, BG2 will be empty and btrfs will trim
all space of BG2, covering up the bug.

[FIX]
- Allow __btrfs_map_block_for_discard() to modify @length parameter
  btrfs_map_block() uses its @length paramter to notify the caller how
  many bytes are mapped in current call.
  With __btrfs_map_block_for_discard() also modifing the @length,
  btrfs_discard_extent() now understands when to do extra trim.

- Call btrfs_map_block() in a loop until we hit the range end
  Since we now know how many bytes are mapped each time, we can iterate
  through each block group boundary and issue correct trim for each
  range.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/extent-tree.c | 41 +++++++++++++++++++++++++++++++----------
 fs/btrfs/volumes.c     |  6 ++++--
 2 files changed, 35 insertions(+), 12 deletions(-)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 49cb26fa7c63..ff2838bd677d 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -1306,8 +1306,10 @@ static int btrfs_issue_discard(struct block_device *bdev, u64 start, u64 len,
 int btrfs_discard_extent(struct btrfs_fs_info *fs_info, u64 bytenr,
 			 u64 num_bytes, u64 *actual_bytes)
 {
-	int ret;
+	int ret = 0;
 	u64 discarded_bytes = 0;
+	u64 end = bytenr + num_bytes;
+	u64 cur = bytenr;
 	struct btrfs_bio *bbio = NULL;
 
 
@@ -1316,15 +1318,23 @@ int btrfs_discard_extent(struct btrfs_fs_info *fs_info, u64 bytenr,
 	 * associated to its stripes that don't go away while we are discarding.
 	 */
 	btrfs_bio_counter_inc_blocked(fs_info);
-	/* Tell the block device(s) that the sectors can be discarded */
-	ret = btrfs_map_block(fs_info, BTRFS_MAP_DISCARD, bytenr, &num_bytes,
-			      &bbio, 0);
-	/* Error condition is -ENOMEM */
-	if (!ret) {
-		struct btrfs_bio_stripe *stripe = bbio->stripes;
+	while (cur < end) {
+		struct btrfs_bio_stripe *stripe;
 		int i;
 
+		num_bytes = end - cur;
+		/* Tell the block device(s) that the sectors can be discarded */
+		ret = btrfs_map_block(fs_info, BTRFS_MAP_DISCARD, cur,
+				      &num_bytes, &bbio, 0);
+		/*
+		 * Error can be -ENOMEM, -ENOENT (no such chunk mapping) or
+		 * -EOPNOTSUPP. For any such error, @num_bytes is not updated,
+		 * thus we can't continue anyway.
+		 */
+		if (ret < 0)
+			goto out;
 
+		stripe = bbio->stripes;
 		for (i = 0; i < bbio->num_stripes; i++, stripe++) {
 			u64 bytes;
 			struct request_queue *req_q;
@@ -1341,10 +1351,19 @@ int btrfs_discard_extent(struct btrfs_fs_info *fs_info, u64 bytenr,
 						  stripe->physical,
 						  stripe->length,
 						  &bytes);
-			if (!ret)
+			if (!ret) {
 				discarded_bytes += bytes;
-			else if (ret != -EOPNOTSUPP)
-				break; /* Logic errors or -ENOMEM, or -EIO but I don't know how that could happen JDM */
+			} else if (ret != -EOPNOTSUPP) {
+				/*
+				 * Logic errors or -ENOMEM, or -EIO but I don't
+				 * know how that could happen JDM
+				 *
+				 * Ans since there are two loops, explicitly
+				 * goto out to avoid confusion.
+				 */
+				btrfs_put_bbio(bbio);
+				goto out;
+			}
 
 			/*
 			 * Just in case we get back EOPNOTSUPP for some reason,
@@ -1354,7 +1373,9 @@ int btrfs_discard_extent(struct btrfs_fs_info *fs_info, u64 bytenr,
 			ret = 0;
 		}
 		btrfs_put_bbio(bbio);
+		cur += num_bytes;
 	}
+out:
 	btrfs_bio_counter_dec(fs_info);
 
 	if (actual_bytes)
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index a6db11e821a5..f66bd0d03f44 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -5578,12 +5578,13 @@ void btrfs_put_bbio(struct btrfs_bio *bbio)
  * replace.
  */
 static int __btrfs_map_block_for_discard(struct btrfs_fs_info *fs_info,
-					 u64 logical, u64 length,
+					 u64 logical, u64 *length_ret,
 					 struct btrfs_bio **bbio_ret)
 {
 	struct extent_map *em;
 	struct map_lookup *map;
 	struct btrfs_bio *bbio;
+	u64 length = *length_ret;
 	u64 offset;
 	u64 stripe_nr;
 	u64 stripe_nr_end;
@@ -5617,6 +5618,7 @@ static int __btrfs_map_block_for_discard(struct btrfs_fs_info *fs_info,
 
 	offset = logical - em->start;
 	length = min_t(u64, em->start + em->len - logical, length);
+	*length_ret = length;
 
 	stripe_len = map->stripe_len;
 	/*
@@ -6031,7 +6033,7 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info,
 
 	if (op == BTRFS_MAP_DISCARD)
 		return __btrfs_map_block_for_discard(fs_info, logical,
-						     *length, bbio_ret);
+						     length, bbio_ret);
 
 	ret = btrfs_get_io_geometry(fs_info, op, logical, *length, &geom);
 	if (ret < 0)
-- 
2.23.0

Re: [PATCH v4 1/2] btrfs: volumes: Use more straightforward way to calculate map length

[Josef Bacik]

On Fri, Oct 25, 2019 at 04:59:55PM +0800, Qu Wenruo wrote:
> The old code goes:
> 
>  	offset = logical - em->start;
> 	length = min_t(u64, em->len - offset, length);
> 
> Where @length calculate is dependent on offset, it can take reader
> several more seconds to find it's just the same code as:
> 
>  	offset = logical - em->start;
> 	length = min_t(u64, em->start + em->len - logical, length);
> 
> Use above code to make the length calculate independent from other
> variable, thus slightly increase the readability.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef

Re: [PATCH v4 2/2] btrfs: extent-tree: Ensure we trim ranges across block group boundary

[Josef Bacik]

On Fri, Oct 25, 2019 at 04:59:56PM +0800, Qu Wenruo wrote:
> [BUG]
> When deleting large files (which cross block group boundary) with discard
> mount option, we find some btrfs_discard_extent() calls only trimmed part
> of its space, not the whole range:
> 
>   btrfs_discard_extent: type=0x1 start=19626196992 len=2144530432 trimmed=1073741824 ratio=50%
> 
> type:		bbio->map_type, in above case, it's SINGLE DATA.
> start:		Logical address of this trim
> len:		Logical length of this trim
> trimmed:	Physically trimmed bytes
> ratio:		trimmed / len
> 
> Thus leading some unused space not discarded.
> 
> [CAUSE]
> When discard mount option is specified, after a transaction is fully
> committed (super block written to disk), we begin to cleanup pinned
> extents in the following call chain:
> 
> btrfs_commit_transaction()
> |- btrfs_finish_extent_commit()
>    |- find_first_extent_bit(unpin, 0, &start, &end, EXTENT_DIRTY);
>    |- btrfs_discard_extent()
> 
> However pinned extents are recorded in an extent_io_tree, which can
> merge adjacent extent states.
> 
> When a large file get deleted and it has adjacent file extents across
> block group boundary, we will get a large merged range, like this:
> 
>       |<---    BG1    --->|<---      BG2     --->|
>       |//////|<--   Range to discard   --->|/////|
> 
> To discard that range, we have the following calls:
> btrfs_discard_extent()
> |- btrfs_map_block()
> |  Returned bbio will end at BG1's end. As btrfs_map_block()
> |  never returns result across block group boundary.
> |- btrfs_issuse_discard()
>    Issue discard for each stripe.
> 
> So we will only discard the range in BG1, not the remaining part in BG2.
> 
> Furthermore, this bug is not that reliably observed, for above case, if
> there is no other extent in BG2, BG2 will be empty and btrfs will trim
> all space of BG2, covering up the bug.
> 
> [FIX]
> - Allow __btrfs_map_block_for_discard() to modify @length parameter
>   btrfs_map_block() uses its @length paramter to notify the caller how
>   many bytes are mapped in current call.
>   With __btrfs_map_block_for_discard() also modifing the @length,
>   btrfs_discard_extent() now understands when to do extra trim.
> 
> - Call btrfs_map_block() in a loop until we hit the range end
>   Since we now know how many bytes are mapped each time, we can iterate
>   through each block group boundary and issue correct trim for each
>   range.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/extent-tree.c | 41 +++++++++++++++++++++++++++++++----------
>  fs/btrfs/volumes.c     |  6 ++++--
>  2 files changed, 35 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 49cb26fa7c63..ff2838bd677d 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -1306,8 +1306,10 @@ static int btrfs_issue_discard(struct block_device *bdev, u64 start, u64 len,
>  int btrfs_discard_extent(struct btrfs_fs_info *fs_info, u64 bytenr,
>  			 u64 num_bytes, u64 *actual_bytes)
>  {
> -	int ret;
> +	int ret = 0;
>  	u64 discarded_bytes = 0;
> +	u64 end = bytenr + num_bytes;
> +	u64 cur = bytenr;
>  	struct btrfs_bio *bbio = NULL;
>  
>  
> @@ -1316,15 +1318,23 @@ int btrfs_discard_extent(struct btrfs_fs_info *fs_info, u64 bytenr,
>  	 * associated to its stripes that don't go away while we are discarding.
>  	 */
>  	btrfs_bio_counter_inc_blocked(fs_info);
> -	/* Tell the block device(s) that the sectors can be discarded */
> -	ret = btrfs_map_block(fs_info, BTRFS_MAP_DISCARD, bytenr, &num_bytes,
> -			      &bbio, 0);
> -	/* Error condition is -ENOMEM */
> -	if (!ret) {
> -		struct btrfs_bio_stripe *stripe = bbio->stripes;
> +	while (cur < end) {
> +		struct btrfs_bio_stripe *stripe;
>  		int i;
>  
> +		num_bytes = end - cur;
> +		/* Tell the block device(s) that the sectors can be discarded */
> +		ret = btrfs_map_block(fs_info, BTRFS_MAP_DISCARD, cur,
> +				      &num_bytes, &bbio, 0);
> +		/*
> +		 * Error can be -ENOMEM, -ENOENT (no such chunk mapping) or
> +		 * -EOPNOTSUPP. For any such error, @num_bytes is not updated,
> +		 * thus we can't continue anyway.
> +		 */
> +		if (ret < 0)
> +			goto out;
>  
> +		stripe = bbio->stripes;
>  		for (i = 0; i < bbio->num_stripes; i++, stripe++) {
>  			u64 bytes;
>  			struct request_queue *req_q;
> @@ -1341,10 +1351,19 @@ int btrfs_discard_extent(struct btrfs_fs_info *fs_info, u64 bytenr,
>  						  stripe->physical,
>  						  stripe->length,
>  						  &bytes);
> -			if (!ret)
> +			if (!ret) {
>  				discarded_bytes += bytes;
> -			else if (ret != -EOPNOTSUPP)
> -				break; /* Logic errors or -ENOMEM, or -EIO but I don't know how that could happen JDM */
> +			} else if (ret != -EOPNOTSUPP) {
> +				/*
> +				 * Logic errors or -ENOMEM, or -EIO but I don't
> +				 * know how that could happen JDM
> +				 *
> +				 * Ans since there are two loops, explicitly

Hate for there to be a v5 at this point, but it should be "and".  Anyway

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef

Re: [PATCH v4 2/2] btrfs: extent-tree: Ensure we trim ranges across block group boundary

[David Sterba]

On Fri, Oct 25, 2019 at 08:23:54AM -0400, Josef Bacik wrote:
> > @@ -1341,10 +1351,19 @@ int btrfs_discard_extent(struct btrfs_fs_info *fs_info, u64 bytenr,
> >  						  stripe->physical,
> >  						  stripe->length,
> >  						  &bytes);
> > -			if (!ret)
> > +			if (!ret) {
> >  				discarded_bytes += bytes;
> > -			else if (ret != -EOPNOTSUPP)
> > -				break; /* Logic errors or -ENOMEM, or -EIO but I don't know how that could happen JDM */
> > +			} else if (ret != -EOPNOTSUPP) {
> > +				/*
> > +				 * Logic errors or -ENOMEM, or -EIO but I don't
> > +				 * know how that could happen JDM
> > +				 *
> > +				 * Ans since there are two loops, explicitly
> 
> Hate for there to be a v5 at this point, but it should be "and".

Fixed and the first sentence of the comment needed to be updated anyway.