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Message-ID: <20130411133815.GB3448@gmail.com>
Date: Thu, 11 Apr 2013 21:38:15 +0800
From: Zheng Liu <gnehzuil.liu@...il.com>
To: Jan Kara <jack@...e.cz>
Cc: Ted Tso <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 01/29] ext4: Make ext4_bio_write_page() use
BH_Async_Write flags instead page pointers from ext4_io_end
On Mon, Apr 08, 2013 at 11:32:06PM +0200, Jan Kara wrote:
> So far ext4_bio_write_page() attached all the pages to ext4_io_end structure.
> This makes that structure pretty heavy (1 KB for pointers + 16 bytes per
> page attached to the bio). Also later we would like to share ext4_io_end
> structure among several bios in case IO to a single extent needs to be split
> among several bios and pointing to pages from ext4_io_end makes this complex.
>
> We remove page pointers from ext4_io_end and use pointers from bio itself
> instead. This isn't as easy when blocksize < pagesize because then we can have
> several bios in flight for a single page and we have to be careful when to call
> end_page_writeback(). However this is a known problem already solved by
> block_write_full_page() / end_buffer_async_write() so we mimic its behavior
> here. We mark buffers going to disk with BH_Async_Write flag and in
> ext4_bio_end_io() we check whether there are any buffers with BH_Async_Write
> flag left. If there are not, we can call end_page_writeback().
>
> Signed-off-by: Jan Kara <jack@...e.cz>
Reviewed-by: Zheng Liu <wenqing.lz@...bao.com>
Regards,
- Zheng
> ---
> fs/ext4/ext4.h | 14 -----
> fs/ext4/page-io.c | 163 +++++++++++++++++++++++++----------------------------
> 2 files changed, 77 insertions(+), 100 deletions(-)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 4a01ba3..3c70547 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -196,19 +196,8 @@ struct mpage_da_data {
> #define EXT4_IO_END_ERROR 0x0002
> #define EXT4_IO_END_DIRECT 0x0004
>
> -struct ext4_io_page {
> - struct page *p_page;
> - atomic_t p_count;
> -};
> -
> -#define MAX_IO_PAGES 128
> -
> /*
> * For converting uninitialized extents on a work queue.
> - *
> - * 'page' is only used from the writepage() path; 'pages' is only used for
> - * buffered writes; they are used to keep page references until conversion
> - * takes place. For AIO/DIO, neither field is filled in.
> */
> typedef struct ext4_io_end {
> struct list_head list; /* per-file finished IO list */
> @@ -218,15 +207,12 @@ typedef struct ext4_io_end {
> ssize_t size; /* size of the extent */
> struct kiocb *iocb; /* iocb struct for AIO */
> int result; /* error value for AIO */
> - int num_io_pages; /* for writepages() */
> - struct ext4_io_page *pages[MAX_IO_PAGES]; /* for writepages() */
> } ext4_io_end_t;
>
> struct ext4_io_submit {
> int io_op;
> struct bio *io_bio;
> ext4_io_end_t *io_end;
> - struct ext4_io_page *io_page;
> sector_t io_next_block;
> };
>
> diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
> index 809b310..73bc011 100644
> --- a/fs/ext4/page-io.c
> +++ b/fs/ext4/page-io.c
> @@ -29,25 +29,19 @@
> #include "xattr.h"
> #include "acl.h"
>
> -static struct kmem_cache *io_page_cachep, *io_end_cachep;
> +static struct kmem_cache *io_end_cachep;
>
> int __init ext4_init_pageio(void)
> {
> - io_page_cachep = KMEM_CACHE(ext4_io_page, SLAB_RECLAIM_ACCOUNT);
> - if (io_page_cachep == NULL)
> - return -ENOMEM;
> io_end_cachep = KMEM_CACHE(ext4_io_end, SLAB_RECLAIM_ACCOUNT);
> - if (io_end_cachep == NULL) {
> - kmem_cache_destroy(io_page_cachep);
> + if (io_end_cachep == NULL)
> return -ENOMEM;
> - }
> return 0;
> }
>
> void ext4_exit_pageio(void)
> {
> kmem_cache_destroy(io_end_cachep);
> - kmem_cache_destroy(io_page_cachep);
> }
>
> void ext4_ioend_wait(struct inode *inode)
> @@ -57,15 +51,6 @@ void ext4_ioend_wait(struct inode *inode)
> wait_event(*wq, (atomic_read(&EXT4_I(inode)->i_ioend_count) == 0));
> }
>
> -static void put_io_page(struct ext4_io_page *io_page)
> -{
> - if (atomic_dec_and_test(&io_page->p_count)) {
> - end_page_writeback(io_page->p_page);
> - put_page(io_page->p_page);
> - kmem_cache_free(io_page_cachep, io_page);
> - }
> -}
> -
> void ext4_free_io_end(ext4_io_end_t *io)
> {
> int i;
> @@ -74,9 +59,6 @@ void ext4_free_io_end(ext4_io_end_t *io)
> BUG_ON(!list_empty(&io->list));
> BUG_ON(io->flag & EXT4_IO_END_UNWRITTEN);
>
> - for (i = 0; i < io->num_io_pages; i++)
> - put_io_page(io->pages[i]);
> - io->num_io_pages = 0;
> if (atomic_dec_and_test(&EXT4_I(io->inode)->i_ioend_count))
> wake_up_all(ext4_ioend_wq(io->inode));
> kmem_cache_free(io_end_cachep, io);
> @@ -233,45 +215,56 @@ static void ext4_end_bio(struct bio *bio, int error)
> ext4_io_end_t *io_end = bio->bi_private;
> struct inode *inode;
> int i;
> + int blocksize;
> sector_t bi_sector = bio->bi_sector;
>
> BUG_ON(!io_end);
> + inode = io_end->inode;
> + blocksize = 1 << inode->i_blkbits;
> bio->bi_private = NULL;
> bio->bi_end_io = NULL;
> if (test_bit(BIO_UPTODATE, &bio->bi_flags))
> error = 0;
> - bio_put(bio);
> -
> - for (i = 0; i < io_end->num_io_pages; i++) {
> - struct page *page = io_end->pages[i]->p_page;
> + for (i = 0; i < bio->bi_vcnt; i++) {
> + struct bio_vec *bvec = &bio->bi_io_vec[i];
> + struct page *page = bvec->bv_page;
> struct buffer_head *bh, *head;
> - loff_t offset;
> - loff_t io_end_offset;
> + unsigned bio_start = bvec->bv_offset;
> + unsigned bio_end = bio_start + bvec->bv_len;
> + unsigned under_io = 0;
> + unsigned long flags;
> +
> + if (!page)
> + continue;
>
> if (error) {
> SetPageError(page);
> set_bit(AS_EIO, &page->mapping->flags);
> - head = page_buffers(page);
> - BUG_ON(!head);
> -
> - io_end_offset = io_end->offset + io_end->size;
> -
> - offset = (sector_t) page->index << PAGE_CACHE_SHIFT;
> - bh = head;
> - do {
> - if ((offset >= io_end->offset) &&
> - (offset+bh->b_size <= io_end_offset))
> - buffer_io_error(bh);
> -
> - offset += bh->b_size;
> - bh = bh->b_this_page;
> - } while (bh != head);
> }
> -
> - put_io_page(io_end->pages[i]);
> + bh = head = page_buffers(page);
> + /*
> + * We check all buffers in the page under BH_Uptodate_Lock
> + * to avoid races with other end io clearing async_write flags
> + */
> + local_irq_save(flags);
> + bit_spin_lock(BH_Uptodate_Lock, &head->b_state);
> + do {
> + if (bh_offset(bh) < bio_start ||
> + bh_offset(bh) + blocksize > bio_end) {
> + if (buffer_async_write(bh))
> + under_io++;
> + continue;
> + }
> + clear_buffer_async_write(bh);
> + if (error)
> + buffer_io_error(bh);
> + } while ((bh = bh->b_this_page) != head);
> + bit_spin_unlock(BH_Uptodate_Lock, &head->b_state);
> + local_irq_restore(flags);
> + if (!under_io)
> + end_page_writeback(page);
> }
> - io_end->num_io_pages = 0;
> - inode = io_end->inode;
> + bio_put(bio);
>
> if (error) {
> io_end->flag |= EXT4_IO_END_ERROR;
> @@ -335,7 +328,6 @@ static int io_submit_init(struct ext4_io_submit *io,
> }
>
> static int io_submit_add_bh(struct ext4_io_submit *io,
> - struct ext4_io_page *io_page,
> struct inode *inode,
> struct writeback_control *wbc,
> struct buffer_head *bh)
> @@ -343,11 +335,6 @@ static int io_submit_add_bh(struct ext4_io_submit *io,
> ext4_io_end_t *io_end;
> int ret;
>
> - if (buffer_new(bh)) {
> - clear_buffer_new(bh);
> - unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
> - }
> -
> if (io->io_bio && bh->b_blocknr != io->io_next_block) {
> submit_and_retry:
> ext4_io_submit(io);
> @@ -358,9 +345,6 @@ submit_and_retry:
> return ret;
> }
> io_end = io->io_end;
> - if ((io_end->num_io_pages >= MAX_IO_PAGES) &&
> - (io_end->pages[io_end->num_io_pages-1] != io_page))
> - goto submit_and_retry;
> if (buffer_uninit(bh))
> ext4_set_io_unwritten_flag(inode, io_end);
> io->io_end->size += bh->b_size;
> @@ -368,11 +352,6 @@ submit_and_retry:
> ret = bio_add_page(io->io_bio, bh->b_page, bh->b_size, bh_offset(bh));
> if (ret != bh->b_size)
> goto submit_and_retry;
> - if ((io_end->num_io_pages == 0) ||
> - (io_end->pages[io_end->num_io_pages-1] != io_page)) {
> - io_end->pages[io_end->num_io_pages++] = io_page;
> - atomic_inc(&io_page->p_count);
> - }
> return 0;
> }
>
> @@ -382,33 +361,29 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
> struct writeback_control *wbc)
> {
> struct inode *inode = page->mapping->host;
> - unsigned block_start, block_end, blocksize;
> - struct ext4_io_page *io_page;
> + unsigned block_start, blocksize;
> struct buffer_head *bh, *head;
> int ret = 0;
> + int nr_submitted = 0;
>
> blocksize = 1 << inode->i_blkbits;
>
> BUG_ON(!PageLocked(page));
> BUG_ON(PageWriteback(page));
>
> - io_page = kmem_cache_alloc(io_page_cachep, GFP_NOFS);
> - if (!io_page) {
> - redirty_page_for_writepage(wbc, page);
> - unlock_page(page);
> - return -ENOMEM;
> - }
> - io_page->p_page = page;
> - atomic_set(&io_page->p_count, 1);
> - get_page(page);
> set_page_writeback(page);
> ClearPageError(page);
>
> - for (bh = head = page_buffers(page), block_start = 0;
> - bh != head || !block_start;
> - block_start = block_end, bh = bh->b_this_page) {
> -
> - block_end = block_start + blocksize;
> + /*
> + * In the first loop we prepare and mark buffers to submit. We have to
> + * mark all buffers in the page before submitting so that
> + * end_page_writeback() cannot be called from ext4_bio_end_io() when IO
> + * on the first buffer finishes and we are still working on submitting
> + * the second buffer.
> + */
> + bh = head = page_buffers(page);
> + do {
> + block_start = bh_offset(bh);
> if (block_start >= len) {
> /*
> * Comments copied from block_write_full_page_endio:
> @@ -421,7 +396,8 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
> * mapped, and writes to that region are not written
> * out to the file."
> */
> - zero_user_segment(page, block_start, block_end);
> + zero_user_segment(page, block_start,
> + block_start + blocksize);
> clear_buffer_dirty(bh);
> set_buffer_uptodate(bh);
> continue;
> @@ -435,7 +411,19 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
> ext4_io_submit(io);
> continue;
> }
> - ret = io_submit_add_bh(io, io_page, inode, wbc, bh);
> + if (buffer_new(bh)) {
> + clear_buffer_new(bh);
> + unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
> + }
> + set_buffer_async_write(bh);
> + } while ((bh = bh->b_this_page) != head);
> +
> + /* Now submit buffers to write */
> + bh = head = page_buffers(page);
> + do {
> + if (!buffer_async_write(bh))
> + continue;
> + ret = io_submit_add_bh(io, inode, wbc, bh);
> if (ret) {
> /*
> * We only get here on ENOMEM. Not much else
> @@ -445,17 +433,20 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
> redirty_page_for_writepage(wbc, page);
> break;
> }
> + nr_submitted++;
> clear_buffer_dirty(bh);
> + } while ((bh = bh->b_this_page) != head);
> +
> + /* Error stopped previous loop? Clean up buffers... */
> + if (ret) {
> + do {
> + clear_buffer_async_write(bh);
> + bh = bh->b_this_page;
> + } while (bh != head);
> }
> unlock_page(page);
> - /*
> - * If the page was truncated before we could do the writeback,
> - * or we had a memory allocation error while trying to write
> - * the first buffer head, we won't have submitted any pages for
> - * I/O. In that case we need to make sure we've cleared the
> - * PageWriteback bit from the page to prevent the system from
> - * wedging later on.
> - */
> - put_io_page(io_page);
> + /* Nothing submitted - we have to end page writeback */
> + if (!nr_submitted)
> + end_page_writeback(page);
> return ret;
> }
> --
> 1.7.1
>
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