From 95465318849f7525f4dce1b720e4627f48963327 Mon Sep 17 00:00:00 2001 From: Bart Van Assche Date: Tue, 25 Oct 2022 12:17:55 -0700 Subject: block: Micro-optimize get_max_segment_size() This patch removes a conditional jump from get_max_segment_size(). The x86-64 assembler code for this function without this patch is as follows: 206 return min_not_zero(mask - offset + 1, 0x0000000000000118 <+72>: not %rax 0x000000000000011b <+75>: and 0x8(%r10),%rax 0x000000000000011f <+79>: add $0x1,%rax 0x0000000000000123 <+83>: je 0x138 0x0000000000000125 <+85>: cmp %rdx,%rax 0x0000000000000128 <+88>: mov %rdx,%r12 0x000000000000012b <+91>: cmovbe %rax,%r12 0x000000000000012f <+95>: test %rdx,%rdx 0x0000000000000132 <+98>: mov %eax,%edx 0x0000000000000134 <+100>: cmovne %r12d,%edx With this patch applied: 206 return min(mask - offset, (unsigned long)lim->max_segment_size - 1) + 1; 0x000000000000003f <+63>: mov 0x28(%rdi),%ebp 0x0000000000000042 <+66>: not %rax 0x0000000000000045 <+69>: and 0x8(%rdi),%rax 0x0000000000000049 <+73>: sub $0x1,%rbp 0x000000000000004d <+77>: cmp %rbp,%rax 0x0000000000000050 <+80>: cmova %rbp,%rax 0x0000000000000054 <+84>: add $0x1,%eax Reviewed-by: Ming Lei Cc: Christoph Hellwig Cc: Keith Busch Cc: Steven Rostedt Cc: Guenter Roeck Signed-off-by: Bart Van Assche Link: https://lore.kernel.org/r/20221025191755.1711437-4-bvanassche@acm.org Reviewed-by: Keith Busch Signed-off-by: Jens Axboe --- block/blk-merge.c | 15 +++++++++++---- 1 file changed, 11 insertions(+), 4 deletions(-) (limited to 'block/blk-merge.c') diff --git a/block/blk-merge.c b/block/blk-merge.c index 58fdc3f8905b..35a8f75cc45d 100644 --- a/block/blk-merge.c +++ b/block/blk-merge.c @@ -186,6 +186,14 @@ static inline unsigned get_max_io_size(struct bio *bio, return max_sectors & ~(lbs - 1); } +/** + * get_max_segment_size() - maximum number of bytes to add as a single segment + * @lim: Request queue limits. + * @start_page: See below. + * @offset: Offset from @start_page where to add a segment. + * + * Returns the maximum number of bytes that can be added as a single segment. + */ static inline unsigned get_max_segment_size(const struct queue_limits *lim, struct page *start_page, unsigned long offset) { @@ -194,11 +202,10 @@ static inline unsigned get_max_segment_size(const struct queue_limits *lim, offset = mask & (page_to_phys(start_page) + offset); /* - * overflow may be triggered in case of zero page physical address - * on 32bit arch, use queue's max segment size when that happens. + * Prevent an overflow if mask = ULONG_MAX and offset = 0 by adding 1 + * after having calculated the minimum. */ - return min_not_zero(mask - offset + 1, - (unsigned long)lim->max_segment_size); + return min(mask - offset, (unsigned long)lim->max_segment_size - 1) + 1; } /** -- cgit v1.2.3