[PATCH 0/2] ext4: align preallocation size to stripe width

[PATCH 0/2] ext4: align preallocation size to stripe width

[Yu Kuai]

Yu Kuai (2):
  ext4: refactor size prediction into helper functions
  ext4: align preallocation size to stripe width

 fs/ext4/mballoc.c | 159 ++++++++++++++++++++++++++++++++++------------
 1 file changed, 117 insertions(+), 42 deletions(-)

-- 
2.51.0

[PATCH 1/2] ext4: refactor size prediction into helper functions

[Yu Kuai]

The ext4_mb_normalize_request() function contains a large if-else
ladder for predicting file size and uses a macro NRL_CHECK_SIZE.
Factor these out into proper helper functions to improve code
readability and maintainability.

This patch introduces:
- ext4_mb_check_size(): static inline function replacing NRL_CHECK_SIZE macro
- ext4_mb_predict_file_size(): extracts size prediction logic

No functional change.

Signed-off-by: Yu Kuai <yukuai@fnnas.com>
---
 fs/ext4/mballoc.c | 101 +++++++++++++++++++++++++++-------------------
 1 file changed, 60 insertions(+), 41 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 9087183602e4..eb46a4f5fb4f 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4489,6 +4489,63 @@ ext4_mb_pa_adjust_overlap(struct ext4_allocation_context *ac,
 	*end = new_end;
 }
 
+/*
+ * Check if request size allows for chunk-based allocation
+ */
+static inline bool ext4_mb_check_size(loff_t req, loff_t size,
+				      int max, int chunk_size)
+{
+	return (req <= size) || (max <= chunk_size);
+}
+
+/*
+ * Predict file size for preallocation. Returns the predicted size
+ * in bytes and sets start_off if alignment is needed for large files.
+ */
+static loff_t ext4_mb_predict_file_size(struct ext4_sb_info *sbi,
+					struct ext4_allocation_context *ac,
+					loff_t size, loff_t *start_off)
+{
+	int bsbits = ac->ac_sb->s_blocksize_bits;
+	int max = 2 << bsbits;
+
+	*start_off = 0;
+
+	if (size <= 16 * 1024) {
+		size = 16 * 1024;
+	} else if (size <= 32 * 1024) {
+		size = 32 * 1024;
+	} else if (size <= 64 * 1024) {
+		size = 64 * 1024;
+	} else if (size <= 128 * 1024) {
+		size = 128 * 1024;
+	} else if (size <= 256 * 1024) {
+		size = 256 * 1024;
+	} else if (size <= 512 * 1024) {
+		size = 512 * 1024;
+	} else if (size <= 1024 * 1024) {
+		size = 1024 * 1024;
+	} else if (ext4_mb_check_size(size, 4 * 1024 * 1024, max, 2 * 1024)) {
+		*start_off = ((loff_t)ac->ac_o_ex.fe_logical >>
+						(21 - bsbits)) << 21;
+		size = 2 * 1024 * 1024;
+	} else if (ext4_mb_check_size(size, 8 * 1024 * 1024, max, 4 * 1024)) {
+		*start_off = ((loff_t)ac->ac_o_ex.fe_logical >>
+							(22 - bsbits)) << 22;
+		size = 4 * 1024 * 1024;
+	} else if (ext4_mb_check_size(EXT4_C2B(sbi, ac->ac_o_ex.fe_len),
+					(8<<20)>>bsbits, max, 8 * 1024)) {
+		*start_off = ((loff_t)ac->ac_o_ex.fe_logical >>
+							(23 - bsbits)) << 23;
+		size = 8 * 1024 * 1024;
+	} else {
+		*start_off = (loff_t)ac->ac_o_ex.fe_logical << bsbits;
+		size = (loff_t)EXT4_C2B(sbi, ac->ac_o_ex.fe_len) << bsbits;
+	}
+
+	return size;
+}
+
 /*
  * Normalization means making request better in terms of
  * size and alignment
@@ -4500,7 +4557,7 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
 	struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
 	struct ext4_super_block *es = sbi->s_es;
 	int bsbits, max;
-	loff_t size, start_off, end;
+	loff_t size, start_off = 0, end;
 	loff_t orig_size __maybe_unused;
 	ext4_lblk_t start;
 
@@ -4533,47 +4590,9 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
 		size = i_size_read(ac->ac_inode);
 	orig_size = size;
 
-	/* max size of free chunks */
-	max = 2 << bsbits;
+	/* Predict file size for preallocation */
+	size = ext4_mb_predict_file_size(sbi, ac, size, &start_off);
 
-#define NRL_CHECK_SIZE(req, size, max, chunk_size)	\
-		(req <= (size) || max <= (chunk_size))
-
-	/* first, try to predict filesize */
-	/* XXX: should this table be tunable? */
-	start_off = 0;
-	if (size <= 16 * 1024) {
-		size = 16 * 1024;
-	} else if (size <= 32 * 1024) {
-		size = 32 * 1024;
-	} else if (size <= 64 * 1024) {
-		size = 64 * 1024;
-	} else if (size <= 128 * 1024) {
-		size = 128 * 1024;
-	} else if (size <= 256 * 1024) {
-		size = 256 * 1024;
-	} else if (size <= 512 * 1024) {
-		size = 512 * 1024;
-	} else if (size <= 1024 * 1024) {
-		size = 1024 * 1024;
-	} else if (NRL_CHECK_SIZE(size, 4 * 1024 * 1024, max, 2 * 1024)) {
-		start_off = ((loff_t)ac->ac_o_ex.fe_logical >>
-						(21 - bsbits)) << 21;
-		size = 2 * 1024 * 1024;
-	} else if (NRL_CHECK_SIZE(size, 8 * 1024 * 1024, max, 4 * 1024)) {
-		start_off = ((loff_t)ac->ac_o_ex.fe_logical >>
-							(22 - bsbits)) << 22;
-		size = 4 * 1024 * 1024;
-	} else if (NRL_CHECK_SIZE(EXT4_C2B(sbi, ac->ac_o_ex.fe_len),
-					(8<<20)>>bsbits, max, 8 * 1024)) {
-		start_off = ((loff_t)ac->ac_o_ex.fe_logical >>
-							(23 - bsbits)) << 23;
-		size = 8 * 1024 * 1024;
-	} else {
-		start_off = (loff_t) ac->ac_o_ex.fe_logical << bsbits;
-		size	  = (loff_t) EXT4_C2B(sbi,
-					      ac->ac_o_ex.fe_len) << bsbits;
-	}
 	size = size >> bsbits;
 	start = start_off >> bsbits;
 
-- 
2.51.0

[PATCH 2/2] ext4: align preallocation size to stripe width

[Yu Kuai]

When stripe width (io_opt) is configured, align the predicted
preallocation size to stripe boundaries. This ensures optimal I/O
performance on RAID and other striped storage devices by avoiding
partial stripe operations.

The current implementation uses hardcoded size predictions (16KB, 32KB,
64KB, etc.) that are not stripe-aware. This causes physical block
offsets on disk to be misaligned to stripe boundaries, leading to
read-modify-write penalties on RAID arrays and reduced performance.

This patch makes size prediction stripe-aware by using multiples of
stripe size (1x, 2x, 4x, 8x, 16x, 32x) when s_stripe is set.
Additionally, the start offset is aligned to stripe boundaries using
rounddown(), which works correctly for both power-of-2 and non-power-of-2
stripe sizes. For devices without stripe configuration, the original
behavior is preserved.

The predicted size is limited to max free chunk size (2 << bsbits) to
ensure reasonable allocation requests, with the limit rounded down to
maintain stripe alignment.

Test case:
  Device: 32-disk RAID5, 64KB chunk size
  Stripe: 496 blocks (31 data disks × 16 blocks/disk)

  Before patch (misaligned physical offsets):
    ext:  logical_offset:  physical_offset: length:
      0:        0.. 63487:      34816.. 98303:  63488
      1:    63488..126975:     100352..163839:  63488
      2:   126976..190463:     165888..229375:  63488
      3:   190464..253951:     231424..294911:  63488
      4:   253952..262143:     296960..305151:   8192

    Physical offsets: 34816 % 496 = 96 (misaligned)
                     100352 % 496 = 160 (misaligned)
                     165888 % 496 = 224 (misaligned)
    → Causes partial stripe writes on RAID

  After patch (aligned physical offsets):
    ext:  logical_offset:  physical_offset: length:
      0:        0.. 17855:       9920.. 27775:  17856
      1:    17856.. 42159:      34224.. 58527:  24304
      2:    42160.. 73407:      65968.. 97215:  31248
      3:    73408.. 97711:      99696..123999:  24304
      ... (all extents aligned until EOF)

    Physical offsets: 9920 % 496 = 0 (aligned)
                     34224 % 496 = 0 (aligned)
                     65968 % 496 = 0 (aligned)
    Extent lengths: 17856=496×36, 24304=496×49, 31248=496×63
    → Optimal RAID performance, no partial stripe writes

Benefits:
- Eliminates read-modify-write operations on RAID arrays
- Improves sequential write performance on striped devices
- Maintains proper alignment throughout file lifetime
- Works with any stripe size (power-of-2 or not)

Signed-off-by: Yu Kuai <yukuai@fnnas.com>
---
 fs/ext4/mballoc.c | 60 +++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 58 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index eb46a4f5fb4f..dbd0b239cc96 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4500,7 +4500,10 @@ static inline bool ext4_mb_check_size(loff_t req, loff_t size,
 
 /*
  * Predict file size for preallocation. Returns the predicted size
- * in bytes and sets start_off if alignment is needed for large files.
+ * in bytes. When stripe width (io_opt) is configured, returns sizes
+ * that are multiples of stripe for optimal RAID performance.
+ *
+ * Sets start_off if alignment is needed for large files.
  */
 static loff_t ext4_mb_predict_file_size(struct ext4_sb_info *sbi,
 					struct ext4_allocation_context *ac,
@@ -4511,6 +4514,59 @@ static loff_t ext4_mb_predict_file_size(struct ext4_sb_info *sbi,
 
 	*start_off = 0;
 
+	/*
+	 * For RAID/striped devices, align preallocation size to stripe
+	 * width (io_opt) for optimal I/O performance. Use power-of-2
+	 * multiples of stripe size for size prediction.
+	 */
+	if (sbi->s_stripe) {
+		loff_t stripe_bytes = (loff_t)sbi->s_stripe << bsbits;
+		loff_t max_size = (loff_t)max << bsbits;
+
+		/*
+		 * TODO: If stripe is larger than max chunk size, we can't
+		 * do stripe-aligned allocation. Fall back to traditional
+		 * size prediction. This can happen with very large stripe
+		 * configurations on small block sizes.
+		 */
+		if (stripe_bytes > max_size)
+			goto no_stripe;
+
+		if (size <= stripe_bytes) {
+			size = stripe_bytes;
+		} else if (size <= stripe_bytes * 2) {
+			size = stripe_bytes * 2;
+		} else if (size <= stripe_bytes * 4) {
+			size = stripe_bytes * 4;
+		} else if (size <= stripe_bytes * 8) {
+			size = stripe_bytes * 8;
+		} else if (size <= stripe_bytes * 16) {
+			size = stripe_bytes * 16;
+		} else if (size <= stripe_bytes * 32) {
+			size = stripe_bytes * 32;
+		} else {
+			size = roundup(size, stripe_bytes);
+		}
+
+		/*
+		 * Limit size to max free chunk size, rounded down to
+		 * stripe alignment.
+		 */
+		if (size > max_size)
+			size = rounddown(max_size, stripe_bytes);
+
+		/*
+		 * Align start offset to stripe boundary for large allocations
+		 * to ensure both start and size are stripe-aligned.
+		 */
+		*start_off = rounddown((loff_t)ac->ac_o_ex.fe_logical << bsbits,
+				       stripe_bytes);
+
+		return size;
+	}
+
+no_stripe:
+	/* No stripe: use traditional hardcoded size prediction */
 	if (size <= 16 * 1024) {
 		size = 16 * 1024;
 	} else if (size <= 32 * 1024) {
@@ -4556,7 +4612,7 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
 {
 	struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
 	struct ext4_super_block *es = sbi->s_es;
-	int bsbits, max;
+	int bsbits;
 	loff_t size, start_off = 0, end;
 	loff_t orig_size __maybe_unused;
 	ext4_lblk_t start;
-- 
2.51.0

Re: [PATCH 2/2] ext4: align preallocation size to stripe width

[kernel test robot]

Hi Yu,

kernel test robot noticed the following build errors:

[auto build test ERROR on tytso-ext4/dev]
[also build test ERROR on linus/master v6.18 next-20251210]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Yu-Kuai/ext4-refactor-size-prediction-into-helper-functions/20251208-163553
base:   https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git dev
patch link:    https://lore.kernel.org/r/20251208083246.320965-3-yukuai%40fnnas.com
patch subject: [PATCH 2/2] ext4: align preallocation size to stripe width
config: i386-randconfig-001-20251210 (https://download.01.org/0day-ci/archive/20251210/202512102331.yweFnVTU-lkp@intel.com/config)
compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251210/202512102331.yweFnVTU-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202512102331.yweFnVTU-lkp@intel.com/

All errors (new ones prefixed by >>):

   ld: fs/ext4/mballoc.o: in function `ext4_mb_predict_file_size':
>> fs/ext4/mballoc.c:4570:(.text+0xc37): undefined reference to `__moddi3'
>> ld: fs/ext4/mballoc.c:4578:(.text+0xc7c): undefined reference to `__moddi3'
   ld: fs/ext4/mballoc.c:4584:(.text+0xcf2): undefined reference to `__moddi3'


vim +4570 fs/ext4/mballoc.c

  4522	
  4523	/*
  4524	 * Predict file size for preallocation. Returns the predicted size
  4525	 * in bytes. When stripe width (io_opt) is configured, returns sizes
  4526	 * that are multiples of stripe for optimal RAID performance.
  4527	 *
  4528	 * Sets start_off if alignment is needed for large files.
  4529	 */
  4530	static loff_t ext4_mb_predict_file_size(struct ext4_sb_info *sbi,
  4531						struct ext4_allocation_context *ac,
  4532						loff_t size, loff_t *start_off)
  4533	{
  4534		int bsbits = ac->ac_sb->s_blocksize_bits;
  4535		int max = 2 << bsbits;
  4536	
  4537		*start_off = 0;
  4538	
  4539		/*
  4540		 * For RAID/striped devices, align preallocation size to stripe
  4541		 * width (io_opt) for optimal I/O performance. Use power-of-2
  4542		 * multiples of stripe size for size prediction.
  4543		 */
  4544		if (sbi->s_stripe) {
  4545			loff_t stripe_bytes = (loff_t)sbi->s_stripe << bsbits;
  4546			loff_t max_size = (loff_t)max << bsbits;
  4547	
  4548			/*
  4549			 * TODO: If stripe is larger than max chunk size, we can't
  4550			 * do stripe-aligned allocation. Fall back to traditional
  4551			 * size prediction. This can happen with very large stripe
  4552			 * configurations on small block sizes.
  4553			 */
  4554			if (stripe_bytes > max_size)
  4555				goto no_stripe;
  4556	
  4557			if (size <= stripe_bytes) {
  4558				size = stripe_bytes;
  4559			} else if (size <= stripe_bytes * 2) {
  4560				size = stripe_bytes * 2;
  4561			} else if (size <= stripe_bytes * 4) {
  4562				size = stripe_bytes * 4;
  4563			} else if (size <= stripe_bytes * 8) {
  4564				size = stripe_bytes * 8;
  4565			} else if (size <= stripe_bytes * 16) {
  4566				size = stripe_bytes * 16;
  4567			} else if (size <= stripe_bytes * 32) {
  4568				size = stripe_bytes * 32;
  4569			} else {
> 4570				size = roundup(size, stripe_bytes);
  4571			}
  4572	
  4573			/*
  4574			 * Limit size to max free chunk size, rounded down to
  4575			 * stripe alignment.
  4576			 */
  4577			if (size > max_size)
> 4578				size = rounddown(max_size, stripe_bytes);
  4579	
  4580			/*
  4581			 * Align start offset to stripe boundary for large allocations
  4582			 * to ensure both start and size are stripe-aligned.
  4583			 */
  4584			*start_off = rounddown((loff_t)ac->ac_o_ex.fe_logical << bsbits,
  4585					       stripe_bytes);
  4586	
  4587			return size;
  4588		}
  4589	
  4590	no_stripe:
  4591		/* No stripe: use traditional hardcoded size prediction */
  4592		if (size <= 16 * 1024) {
  4593			size = 16 * 1024;
  4594		} else if (size <= 32 * 1024) {
  4595			size = 32 * 1024;
  4596		} else if (size <= 64 * 1024) {
  4597			size = 64 * 1024;
  4598		} else if (size <= 128 * 1024) {
  4599			size = 128 * 1024;
  4600		} else if (size <= 256 * 1024) {
  4601			size = 256 * 1024;
  4602		} else if (size <= 512 * 1024) {
  4603			size = 512 * 1024;
  4604		} else if (size <= 1024 * 1024) {
  4605			size = 1024 * 1024;
  4606		} else if (ext4_mb_check_size(size, 4 * 1024 * 1024, max, 2 * 1024)) {
  4607			*start_off = ((loff_t)ac->ac_o_ex.fe_logical >>
  4608							(21 - bsbits)) << 21;
  4609			size = 2 * 1024 * 1024;
  4610		} else if (ext4_mb_check_size(size, 8 * 1024 * 1024, max, 4 * 1024)) {
  4611			*start_off = ((loff_t)ac->ac_o_ex.fe_logical >>
  4612								(22 - bsbits)) << 22;
  4613			size = 4 * 1024 * 1024;
  4614		} else if (ext4_mb_check_size(EXT4_C2B(sbi, ac->ac_o_ex.fe_len),
  4615						(8<<20)>>bsbits, max, 8 * 1024)) {
  4616			*start_off = ((loff_t)ac->ac_o_ex.fe_logical >>
  4617								(23 - bsbits)) << 23;
  4618			size = 8 * 1024 * 1024;
  4619		} else {
  4620			*start_off = (loff_t)ac->ac_o_ex.fe_logical << bsbits;
  4621			size = (loff_t)EXT4_C2B(sbi, ac->ac_o_ex.fe_len) << bsbits;
  4622		}
  4623	
  4624		return size;
  4625	}
  4626	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH 2/2] ext4: align preallocation size to stripe width

[kernel test robot]

Hi Yu,

kernel test robot noticed the following build errors:

[auto build test ERROR on tytso-ext4/dev]
[also build test ERROR on linus/master v6.18 next-20251211]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Yu-Kuai/ext4-refactor-size-prediction-into-helper-functions/20251208-163553
base:   https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git dev
patch link:    https://lore.kernel.org/r/20251208083246.320965-3-yukuai%40fnnas.com
patch subject: [PATCH 2/2] ext4: align preallocation size to stripe width
config: arm-randconfig-r072-20251210 (https://download.01.org/0day-ci/archive/20251212/202512120613.mM5COVWV-lkp@intel.com/config)
compiler: arm-linux-gnueabi-gcc (GCC) 12.5.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251212/202512120613.mM5COVWV-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202512120613.mM5COVWV-lkp@intel.com/

All errors (new ones prefixed by >>):

   arm-linux-gnueabi-ld: fs/ext4/mballoc.o: in function `ext4_mb_predict_file_size':
>> mballoc.c:(.text+0x242): undefined reference to `__aeabi_ldivmod'
   arm-linux-gnueabi-ld: (__aeabi_ldivmod): Unknown destination type (ARM/Thumb) in fs/ext4/mballoc.o
>> mballoc.c:(.text+0x242): dangerous relocation: unsupported relocation
>> arm-linux-gnueabi-ld: mballoc.c:(.text+0x268): undefined reference to `__aeabi_ldivmod'
   arm-linux-gnueabi-ld: (__aeabi_ldivmod): Unknown destination type (ARM/Thumb) in fs/ext4/mballoc.o
   mballoc.c:(.text+0x268): dangerous relocation: unsupported relocation
   arm-linux-gnueabi-ld: mballoc.c:(.text+0x29c): undefined reference to `__aeabi_ldivmod'
   arm-linux-gnueabi-ld: (__aeabi_ldivmod): Unknown destination type (ARM/Thumb) in fs/ext4/mballoc.o
   mballoc.c:(.text+0x29c): dangerous relocation: unsupported relocation

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki