linux-kernel - [RFC v3 08/11] ext4: add forcealign support of mballoc

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Message-ID: <18c0e6352a9d20bea37447e6500ecec4cae73614.1742800203.git.ojaswin@linux.ibm.com>
Date: Mon, 24 Mar 2025 13:07:06 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>
Cc: John Garry <john.g.garry@...cle.com>, dchinner@...hat.com,
        "Darrick J . Wong" <djwong@...nel.org>,
        Ritesh Harjani <ritesh.list@...il.com>, linux-kernel@...r.kernel.org
Subject: [RFC v3 08/11] ext4: add forcealign support of mballoc

Introduce an EXT4_MB_FORCE_ALIGN flag that essentially enforces the same
behavior as EXT4_MB_HINT_ALIGNED however the alignment requirements are no
longer a hint but must be respected. If the allocator can't return aligned
blocks, then ENOSPC will be thrown.

This will be eventually used to guarantee aligned blocks to perform H/W
accelerated atomic writes.

Signed-off-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
---
 fs/ext4/ext4.h              |  2 ++
 fs/ext4/mballoc.c           | 31 +++++++++++++++++++++++--------
 include/trace/events/ext4.h |  1 +
 3 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index ab4f10f9031a..9b9d7a354736 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -224,6 +224,8 @@ enum criteria {
 #define EXT4_MB_CR_BEST_AVAIL_LEN_OPTIMIZED		0x00020000
 /* mballoc will try to align physical start to length (aka natural alignment) */
 #define EXT4_MB_HINT_ALIGNED	      	0x40000
+/* Same as HINT_ALIGNED but fail allocation if alginment can't be guaranteed */
+#define EXT4_MB_FORCE_ALIGN		0x80000
 
 struct ext4_allocation_request {
 	/* target inode for block we're allocating */
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index db7c593873a9..412aa80bc6e7 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2872,12 +2872,21 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 		ac->ac_criteria = cr;
 
 		if (ac->ac_criteria > CR_POWER2_ALIGNED &&
-		    ac->ac_flags & EXT4_MB_HINT_ALIGNED &&
 		    ac->ac_g_ex.fe_len > 1) {
-			ext4_warning_inode(
-				ac->ac_inode,
-				"Aligned allocation not possible, using unaligned allocation");
-			ac->ac_flags &= ~EXT4_MB_HINT_ALIGNED;
+			if (ac->ac_flags & EXT4_MB_FORCE_ALIGN) {
+				ext4_warning_inode(
+					ac->ac_inode,
+					"Aligned allocation not possible, failing allocation");
+				ac->ac_status = AC_STATUS_BREAK;
+				goto exit;
+			}
+
+			if (ac->ac_flags & EXT4_MB_HINT_ALIGNED) {
+				ext4_warning_inode(
+					ac->ac_inode,
+					"Aligned allocation not possible, using unaligned allocation");
+				ac->ac_flags &= ~EXT4_MB_HINT_ALIGNED;
+			}
 		}
 
 		/*
@@ -3023,9 +3032,15 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 			goto exit;
 		}
 
-		WARN_ON_ONCE(!is_power_of_2(len));
-		WARN_ON_ONCE(start % len);
-		WARN_ON_ONCE(ac->ac_b_ex.fe_len < ac->ac_o_ex.fe_len);
+		if (WARN_ON_ONCE(!is_power_of_2(len)) ||
+		    WARN_ON_ONCE(start % len) ||
+		    WARN_ON_ONCE(ac->ac_b_ex.fe_len < ac->ac_o_ex.fe_len)) {
+			/* FORCE_ALIGN should error out if aligned blocks can't be found */
+			if (ac->ac_flags & EXT4_MB_FORCE_ALIGN) {
+				ac->ac_status = AC_STATUS_BREAK;
+				goto exit;
+			}
+		}
 	}
 
  exit:
diff --git a/include/trace/events/ext4.h b/include/trace/events/ext4.h
index d9464ee764af..ebc1fb5ad57b 100644
--- a/include/trace/events/ext4.h
+++ b/include/trace/events/ext4.h
@@ -37,6 +37,7 @@ struct partial_cluster;
 	{ EXT4_MB_USE_ROOT_BLOCKS,	"USE_ROOT_BLKS" },	\
 	{ EXT4_MB_USE_RESERVED,		"USE_RESV" },		\
 	{ EXT4_MB_HINT_ALIGNED,		"HINT_ALIGNED" }, \
+	{ EXT4_MB_FORCE_ALIGN,		"FORCE_ALIGN" }, \
 	{ EXT4_MB_STRICT_CHECK,		"STRICT_CHECK" })
 
 #define show_map_flags(flags) __print_flags(flags, "|",			\
-- 
2.48.1