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buffer: Remove b_end_io

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This shrinks buffer_head by 8 bytes, letting us pack more buffer heads
per slab.  With a Debian config, it shrinks from 104 bytes to 96 bytes
which is 42 objects per 4KiB page rather than 39, a 7% reduction in the
amount of memory used.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20260528173150.1093780-33-willy@infradead.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Matthew Wilcox (Oracle) 2026-05-28 18:31:45 +01:00 committed by Christian Brauner
parent 2911b93582
commit ac75b922bb
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4 changed files with 4 additions and 21 deletions
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14
Documentation/filesystems/locking.rst
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@ -416,20 +416,6 @@ lm_open_conflict yes no no
lm_breaker_timedout yes no no
====================== ============= ================= =========
buffer_head
===========
prototypes::
void (*b_end_io)(struct buffer_head *bh, int uptodate);
locking rules:
called from interrupts. In other words, extreme care is needed here.
bh is locked, but that's all warranties we have here. Currently only RAID1,
highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
call this method upon the IO completion.
block_device_operations
=======================
prototypes::

6
drivers/md/raid5.h
View File

@ -38,7 +38,7 @@
* Clean -> Dirty - on compute_parity to satisfy write/sync (RECONSTRUCT or RMW)
*
* The Want->Empty, Want->Clean, Dirty->Clean, transitions
* all happen in b_end_io at interrupt time.
* all happen in end_io at interrupt time.
* Each sets the Uptodate bit before releasing the Lock bit.
* This leaves one multi-stage transition:
* Want->Dirty->Clean
@ -64,7 +64,7 @@
* together, but we are not guaranteed of that so we allow for more.
*
* If a buffer is on the read list when the associated cache buffer is
* Uptodate, the data is copied into the read buffer and it's b_end_io
* Uptodate, the data is copied into the read buffer and it's end_io
* routine is called. This may happen in the end_request routine only
* if the buffer has just successfully been read. end_request should
* remove the buffers from the list and then set the Uptodate bit on
@ -76,7 +76,7 @@
* into the cache buffer, which is then marked dirty, and moved onto a
* third list, the written list (bh_written). Once both the parity
* block and the cached buffer are successfully written, any buffer on
* a written list can be returned with b_end_io.
* a written list can be returned with end_io.
*
* The write list and read list both act as fifos. The read list,
* write list and written list are protected by the device_lock.

1
fs/buffer.c
View File

@ -946,7 +946,6 @@ static sector_t folio_init_buffers(struct folio *folio,
do {
if (!buffer_mapped(bh)) {
bh->b_end_io = NULL;
bh->b_private = NULL;
bh->b_bdev = bdev;
bh->b_blocknr = block;

4
include/linux/buffer_head.h
View File

@ -46,7 +46,6 @@ enum bh_state_bits {
struct page;
struct buffer_head;
struct address_space;
typedef void (bh_end_io_t)(struct buffer_head *bh, int uptodate);
/*
* Historically, a buffer_head was used to map a single block
@ -70,8 +69,7 @@ struct buffer_head {
char *b_data; /* pointer to data within the page */
struct block_device *b_bdev;
bh_end_io_t *b_end_io; /* I/O completion */
void *b_private; /* reserved for b_end_io */
void *b_private; /* reserved for bio_end_io */
struct list_head b_assoc_buffers; /* associated with another mapping */
struct mapping_metadata_bhs *b_mmb; /* head of the list of metadata bhs
* this buffer is associated with */