[v2,2/2] btrfs: extent-tree: Ensure we trim ranges across block group boundary
diff mbox series

Message ID 20191023125648.30840-3-wqu@suse.com
State New
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Series
  • btrfs: trim: Fix a bug certain range may not be trimmed properly
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Commit Message

Qu WenRuo Oct. 23, 2019, 12:56 p.m. UTC
[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()
|- write_all_supers()
|- 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.

Then when we pass the large range into btrfs_discard_extent(),
btrfs_discard_extent() will just trim the first part, without trimming
the remaining part.

Furthermore, this bug is not that reliably observed, as if the whole
block group is empty, there will be another trim for that block group.

So the most obvious way to find this missing trim needs to delete large
extents at block group boundary without empting involved block groups.

[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 if it needs to do next 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 | 40 ++++++++++++++++++++++++++++++----------
 fs/btrfs/volumes.c     |  6 ++++--
 2 files changed, 34 insertions(+), 12 deletions(-)

Comments

Nikolay Borisov Oct. 23, 2019, 1:37 p.m. UTC | #1
On 23.10.19 г. 15:56 ч., 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()
> |- write_all_supers()
> |- 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.
> 
> Then when we pass the large range into btrfs_discard_extent(),
> btrfs_discard_extent() will just trim the first part, without trimming
> the remaining part.
> 
> Furthermore, this bug is not that reliably observed, as if the whole
> block group is empty, there will be another trim for that block group.
> 
> So the most obvious way to find this missing trim needs to delete large
> extents at block group boundary without empting involved block groups.
> 
> [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 if it needs to do next 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 | 40 ++++++++++++++++++++++++++++++----------
>  fs/btrfs/volumes.c     |  6 ++++--
>  2 files changed, 34 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 49cb26fa7c63..45df45fa775b 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 +1318,22 @@ 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;
>  
> +		/* 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 +1350,19 @@ 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 +1372,9 @@ int btrfs_discard_extent(struct btrfs_fs_info *fs_info, u64 bytenr,
>  			ret = 0;
>  		}
>  		btrfs_put_bbio(bbio);
> +		cur += num_bytes;

Shouldn't  you keep the number of bytes requested to discard and deduct
from the actual amount being discarded.

Say you want to discard 1G but for whatever reason this has to be done
in, say, 1 chunk of 750mb and a second chunk of 250 mb. On the first go
you will discard 750mb and add them to cur, numbytes at this point are
750mb then you call btrfs_map_block with num_bytes being 750mb whereas
they should have been 250mb. And if the code manages to map the 2nd
750mb it could potentially trim more than requested and corrupt data.

>  	}
> +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)
>
Qu Wenruo Oct. 23, 2019, 1:48 p.m. UTC | #2
On 2019/10/23 下午9:37, Nikolay Borisov wrote:
>
>
> On 23.10.19 г. 15:56 ч., 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()
>> |- write_all_supers()
>> |- 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.
>>
>> Then when we pass the large range into btrfs_discard_extent(),
>> btrfs_discard_extent() will just trim the first part, without trimming
>> the remaining part.
>>
>> Furthermore, this bug is not that reliably observed, as if the whole
>> block group is empty, there will be another trim for that block group.
>>
>> So the most obvious way to find this missing trim needs to delete large
>> extents at block group boundary without empting involved block groups.
>>
>> [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 if it needs to do next 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 | 40 ++++++++++++++++++++++++++++++----------
>>  fs/btrfs/volumes.c     |  6 ++++--
>>  2 files changed, 34 insertions(+), 12 deletions(-)
>>
>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>> index 49cb26fa7c63..45df45fa775b 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 +1318,22 @@ 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;
>>
>> +		/* 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 +1350,19 @@ 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 +1372,9 @@ int btrfs_discard_extent(struct btrfs_fs_info *fs_info, u64 bytenr,
>>  			ret = 0;
>>  		}
>>  		btrfs_put_bbio(bbio);
>> +		cur += num_bytes;
>
> Shouldn't  you keep the number of bytes requested to discard and deduct
> from the actual amount being discarded.
>
> Say you want to discard 1G but for whatever reason this has to be done
> in, say, 1 chunk of 750mb and a second chunk of 250 mb. On the first go
> you will discard 750mb and add them to cur, numbytes at this point are
> 750mb then you call btrfs_map_block with num_bytes being 750mb whereas
> they should have been 250mb. And if the code manages to map the 2nd
> 750mb it could potentially trim more than requested and corrupt data.

Oh! Great find!

That's indeed a problem! So every time before we call btrfs_map_block()
we should adjust @num_bytes to @end - @cur.

Thanks for catching this big bug!
Qu
>
>>  	}
>> +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)
>>

Patch
diff mbox series

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 49cb26fa7c63..45df45fa775b 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 +1318,22 @@  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;
 
+		/* 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 +1350,19 @@  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 +1372,9 @@  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)