On 31.01.23 г. 18:27 ч., Alexander Atanasov wrote:
Hi,
On 31.01.23 14:23, Nikolay Borisov wrote:
Replace all shift rights/shift lefts by divide and multiply operations.
What's wrong with shifts?
I've checked that this doesn't generate worse assembly and that shifts
are indeed continued to be used. PAGE_SHIFT + 3 is actually
BITS_PER_PAGE and is a constant integer expression so the compiler is
perfectly capable of figuring that out and generatin optimal assembly.
Replace an if/else construct with just an if which handles the error
condition. This makes the code somewhat easier to follow in a linear
fashion.
Remove a superfluous continue and make off2 variable local the sole
context it's being used.
Signed-off-by: Nikolay Borisov <nikolay.bori...@virtuozzo.com>
---
block/blk-cbt.c | 21 +++++++++++----------
1 file changed, 11 insertions(+), 10 deletions(-)
diff --git a/block/blk-cbt.c b/block/blk-cbt.c
index 967533a2a7a3..8baeb2804a61 100644
--- a/block/blk-cbt.c
+++ b/block/blk-cbt.c
@@ -758,14 +758,14 @@ static void cbt_flush_cache(struct cbt_info *cbt)
static void cbt_find_next_extent(struct cbt_info *cbt, blkcnt_t
block, struct cbt_extent *ex)
{
- unsigned long off, off2, idx;
+ unsigned long off, idx;
struct page *page;
bool found = 0;
ex->start = cbt->block_max;
ex->len = 0;
- idx = block >> (PAGE_SHIFT + 3);
+ idx = block / BITS_PER_PAGE;
As we discussed about the previous series is that this is valid only
when working with powers of two - what guarantees that BITS_PER_PAGE is
power of two ? Same bellow.
BITS_PER_PAGE is PAGE_SIZE * 8, so long as PAGE_SIZE Is power of 2,
which at this point is pretty much guaranteed for every architecture,
let alone for every arch VHI is used on (x86_64).
- Assembly generation is compiler dependant
Sure, but the macro is a guaranteed integer constant expression which is
power of 2 so shifts are pretty much "guaranteed".
- It is well known that division must be avoided inside the kernel
while (block < cbt->block_max) {
off = block & (BITS_PER_PAGE -1);
page = CBT_PAGE(cbt, idx);
@@ -778,17 +778,19 @@ static void cbt_find_next_extent(struct cbt_info
*cbt, blkcnt_t block, struct cb
/* Find extent start */
if (!found) {
ex->start = find_next_bit(page_address(page),
BITS_PER_PAGE, off);
- if (ex->start != BITS_PER_PAGE) {
- off = ex->start;
- ex->start += idx << (PAGE_SHIFT + 3);
- found = 1;
- } else {
+ /* No set bits in current page, skip to next one */
+ if (ex->start == BITS_PER_PAGE) {
unlock_page(page);
goto next;
}
+
+ off = ex->start;
+ ex->start += idx / BITS_PER_PAGE;
+ found = 1;
}
if (found) {
- off2 = find_next_zero_bit(page_address(page),
BITS_PER_PAGE, off);
+ unsigned long off2 = find_next_zero_bit(page_address(page),
+ BITS_PER_PAGE, off);
ex->len += off2 - off;
if (off2 != BITS_PER_PAGE) {
unlock_page(page);
@@ -798,8 +800,7 @@ static void cbt_find_next_extent(struct cbt_info
*cbt, blkcnt_t block, struct cb
unlock_page(page);
next:
idx++;
- block = idx << (PAGE_SHIFT + 3);
- continue;
+ block = idx * BITS_PER_PAGE;
}
}
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