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SubjectRe: [PATCH 3/3] md: bitmap: Support circular buffer list.
On Tue, Jan 02, 2018 at 02:54:49PM +0800, Sean Fu wrote:
> Modify write_page free_buffers and read_page to support circular buffer
> list.
>
> Signed-off-by: Sean Fu <fxinrong@gmail.com>
> ---
> drivers/md/md-bitmap.c | 36 +++++++++++++++++++-----------------
> 1 file changed, 19 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/md/md-bitmap.c b/drivers/md/md-bitmap.c
> index 239c7bb..b8412c2 100644
> --- a/drivers/md/md-bitmap.c
> +++ b/drivers/md/md-bitmap.c
> @@ -286,7 +286,7 @@ static void bitmap_file_kick(struct bitmap *bitmap);
> */
> static void write_page(struct bitmap *bitmap, struct page *page, int wait)
> {
> - struct buffer_head *bh;
> + struct buffer_head *bh, *head;
>
> if (bitmap->storage.file == NULL) {
> switch (write_sb_page(bitmap, page, wait)) {
> @@ -295,15 +295,16 @@ static void write_page(struct bitmap *bitmap, struct page *page, int wait)
> }
> } else {
>
> - bh = page_buffers(page);
> + bh = head = page_buffers(page);
>
> - while (bh && bh->b_blocknr) {
> - atomic_inc(&bitmap->pending_writes);
> - set_buffer_locked(bh);
> - set_buffer_mapped(bh);
> - submit_bh(REQ_OP_WRITE, REQ_SYNC, bh);
> - bh = bh->b_this_page;
> - }
> + do {
> + if (bh && bh->b_blocknr) {
> + atomic_inc(&bitmap->pending_writes);
> + set_buffer_locked(bh);
> + set_buffer_mapped(bh);
> + submit_bh(REQ_OP_WRITE, REQ_SYNC, bh);
> + }
> + } while ((bh = bh->b_this_page) != head);
>
> if (wait)
> wait_event(bitmap->write_wait,
> @@ -333,17 +334,18 @@ __clear_page_buffers(struct page *page)
> }
> static void free_buffers(struct page *page)
> {
> - struct buffer_head *bh;
> + struct buffer_head *bh, *head;
>
> if (!PagePrivate(page))
> return;
>
> - bh = page_buffers(page);
> - while (bh) {
> + bh = head = page_buffers(page);
> + do {
> struct buffer_head *next = bh->b_this_page;
> free_buffer_head(bh);
> bh = next;
> - }
> + } while (bh != head);
> +
> __clear_page_buffers(page);
> put_page(page);
> }
> @@ -362,20 +364,20 @@ static int read_page(struct file *file, unsigned long index,
> {
> int ret = 0;
> struct inode *inode = file_inode(file);
> - struct buffer_head *bh;
> + struct buffer_head *bh, *head;
> sector_t block;
>
> pr_debug("read bitmap file (%dB @ %llu)\n", (int)PAGE_SIZE,
> (unsigned long long)index << PAGE_SHIFT);
>
> - bh = alloc_page_buffers(page, 1<<inode->i_blkbits, false);
> + bh = head = alloc_page_buffers(page, 1<<inode->i_blkbits, false);
> if (!bh) {
> ret = -ENOMEM;
> goto out;
> }
> attach_page_buffers(page, bh);
> block = index << (PAGE_SHIFT - inode->i_blkbits);
> - while (bh) {
> + do {
> if (count == 0)
> bh->b_blocknr = 0;
> else {
> @@ -400,7 +402,7 @@ static int read_page(struct file *file, unsigned long index,
> }
> block++;
> bh = bh->b_this_page;
> - }
> + } while (bh != head);
> page->index = index;
>
> wait_event(bitmap->write_wait,
> --
> 2.6.2
>
Before
sean@linux-zmni:~/sda5/source/linus_repo/linux> size fs/buffer.o
text data bss dec hex filename
33693 1466 16 35175 8967 fs/buffer.o
sean@linux-zmni:~/sda5/source/linus_repo/linux> size drivers/md/md-bitmap.o
text data bss dec hex filename
28149 2168 0 30317 766d drivers/md/md-bitmap.o
sean@linux-zmni:~/sda5/source/linus_repo/linux> size fs/ntfs/mft.o
text data bss dec hex filename
2133 36 0 2169 879 fs/ntfs/mft.o
sean@linux-zmni:~/sda5/source/linus_repo/linux> size fs/ntfs/aops.o
text data bss dec hex filename
6125 168 0 6293 1895 fs/ntfs/aops.o
sean@linux-zmni:~/sda5/source/linus_repo/linux> size vmlinux
text data bss dec hex filename
11480260 5730762 1646084 18857106 11fbc92 vmlinux
sean@linux-zmni:~/sda5/source/linus_repo/linux> size ./arch/x86/boot/bzImage
size: ./arch/x86/boot/bzImage: Warning: Ignoring section flag IMAGE_SCN_MEM_NOT_PAGED in section .bss
text data bss dec hex filename
6571744 0 16975648 23547392 1674e00 ./arch/x86/boot/bzImage


After
sean@linux-zmni:~/sda5/source/linus_repo/linux> size fs/buffer.o
text data bss dec hex filename
33687 1466 16 35169 8961 fs/buffer.o
sean@linux-zmni:~/sda5/source/linus_repo/linux> size drivers/md/md-bitmap.o
text data bss dec hex filename
28221 2168 0 30389 76b5 drivers/md/md-bitmap.o
sean@linux-zmni:~/sda5/source/linus_repo/linux> size fs/ntfs/mft.o
text data bss dec hex filename
2133 36 0 2169 879 fs/ntfs/mft.o
sean@linux-zmni:~/sda5/source/linus_repo/linux> size fs/ntfs/aops.o
text data bss dec hex filename
6125 168 0 6293 1895 fs/ntfs/aops.o
sean@linux-zmni:~/sda5/source/linus_repo/linux> size vmlinux
text data bss dec hex filename
11480270 5730762 1646084 18857116 11fbc9c vmlinux
sean@linux-zmni:~/sda5/source/linus_repo/linux> size ./arch/x86/boot/bzImage
size: ./arch/x86/boot/bzImage: Warning: Ignoring section flag IMAGE_SCN_MEM_NOT_PAGED in section .bss
text data bss dec hex filename
6571488 0 16975904 23547392 1674e00 ./arch/x86/boot/bzImage

Only patch #3 increases the text section size of drivers/md/md-bitmap.o
The actual text section increment should be several bytes. Duo to compiler align-functions option, the total increment is increased to 72 bytes.

Why is circular list traversal more complex than linear list traversal in this situation?
How to optimize it?

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