Re: Delayed allocation and page_lock vs transaction start ordering

Badari Pulavarty
2008-04-16T19:55:19Z

On Wed, 2008-04-16 at 11:24 -0700, Mingming Cao wrote:
> On Wed, 2008-04-16 at 12:35 +0200, Jan Kara wrote:
> > On Tue 15-04-08 16:33:17, Mingming Cao wrote:
> > > On Tue, 2008-04-15 at 16:28 -0700, Mingming Cao wrote:
> > > > On Tue, 2008-04-15 at 11:08 -0700, Mingming Cao wrote:
> > > > > On Tue, 2008-04-15 at 18:14 +0200, Jan Kara wrote:
> > > > > > Hi,
> > > > > >
> > > > > > I've ported my patch inversing locking ordering of page_lock and
> > > > > > transaction start to ext4 (on top of ext4 patch queue). Everything except
> > > > > > delayed allocation is converted (the patch is below for interested
> > > > > > readers). The question is how to proceed with delayed allocation. Its
> > > > > > current implementation in VFS is designed to work well with the old
> > > > > > ordering (page lock first, then start a transaction). We could bend it to
> > > > > > work with the new locking ordering but I really see no point since ext4 is
> > > > > > the only user.
> > > > >
> > > > > I think the plan is port the changes to ext2/3/JFS and support delayed
> > > > > allocation on those filesystems.
> > > > >
> > > > > > Also XFS has AFAIK ordering first start transaction, then
> > > > > > lock pages so if we should ever merge delayed alloc implementations the new
> > > > > > ordering would make it easier.
> > > > > > So what do people think here? Do you agree with reimplementing current
> > > > > > mpage_da_... functions?
> > > > >
> > > > > It worth a try, but I could not see how to bend delayed allocation to
> > > > > work the new ordering:( With delayed allocation Ext4 gets into
> > > > > writepage() directly with page locked, but we need to start transaction
> > > > > to do block allocation...:(
> > > >
> > > > Looked again it seems possible to reservse the order with delayed
> > > > allocation. with ext3_da_writepgaes() we could start the journal before
> > > > calling mpage_da_writepages()(which will lock the pages), instead of
> > > > start the journal inside ext4_da_get_block_write(). So that we could get
> > > > the locking order right. Just need to taking care of the estimated
> > > > credits right.
> > > >
> > > > How about this? (untested, just throw out for comment)
> > >
> > > Seems sent out an old version, this version compiles
> > Thanks for the patch. Some comments are below.
> >
> > > ---
> > > fs/ext4/inode.c | 53 ++++++++++++++++++++++++++++++++++++++++-------------
> > > 1 file changed, 40 insertions(+), 13 deletions(-)
> > >
> > > Index: linux-2.6.25-rc9/fs/ext4/inode.c
> > > ===================================================================
> > > --- linux-2.6.25-rc9.orig/fs/ext4/inode.c 2008-04-15 15:40:33.000000000 -0700
> > > +++ linux-2.6.25-rc9/fs/ext4/inode.c 2008-04-15 16:32:10.000000000 -0700
> > > @@ -1437,18 +1437,12 @@ static int ext4_da_get_block_prep(struct
> > > static int ext4_da_get_block_write(struct inode *inode, sector_t iblock,
> > > struct buffer_head *bh_result, int create)
> > > {
> > > - int ret, needed_blocks = ext4_writepage_trans_blocks(inode);
> > > + int ret;
> > > unsigned max_blocks = bh_result->b_size >> inode->i_blkbits;
> > > loff_t disksize = EXT4_I(inode)->i_disksize;
> > > handle_t *handle = NULL;
> > >
> > > - if (create) {
> > > - handle = ext4_journal_start(inode, needed_blocks);
> > > - if (IS_ERR(handle)) {
> > > - ret = PTR_ERR(handle);
> > > - goto out;
> > > - }
> > > - }
> > > + handle = ext4_journal_current_handle();
> > Maybe we could assert that handle != NULL? When using delayed allocation,
> > a transaction should always be started.
> >
> Agreed.
>
> > > ret = ext4_get_blocks_wrap(handle, inode, iblock, max_blocks,
> > > bh_result, create, 0);
> > > @@ -1483,17 +1477,51 @@ static int ext4_da_get_block_write(struc
> > > ret = 0;
> > > }
> > >
> > > -out:
> > > - if (handle && !IS_ERR(handle))
> > > - ext4_journal_stop(handle);
> > > -
> > > return ret;
> > > }
> > >
> > > +/*
> > > + * For now just follow the DIO way to estimate the max credits
> > > + * needed to write out EXT4_MAX_BUF_BLOCKS pages.
> > > + * todo: need to calculate the max credits need for
> > > + * extent based files, currently the DIO credits is based on
> > > + * indirect-blocks mapping way.
> > > + *
> > > + * Probably should have a generic way to calculate credits
> > > + * for DIO, writepages, and truncate
> > > + */
> > > +#define EXT4_MAX_BUF_BLOCKS DIO_MAX_BLOCKS
> > > +#define EXT4_MAX_BUF_CREDITS DIO_CREDITS
> > > +
> > > static int ext4_da_writepages(struct address_space *mapping,
> > > struct writeback_control *wbc)
> > > {
> > > - return mpage_da_writepages(mapping, wbc, ext4_da_get_block_write);
> > > + struct inode *inode = mapping->host;
> > > + handle_t *handle = NULL;
> > > + int needed_blocks;
> > > + int ret;
> > > +
> > > + /*
> > > + * Estimate the worse case needed credits to write out
> > > + * EXT4_MAX_BUF_BLOCKS pages
> > > + */
> > > + needed_blocks = EXT4_MAX_BUF_CREDITS;
> > > +
> > > + /* start the transaction with credits*/
> > > + handle = ext4_journal_start(inode, needed_blocks);
> > > + if (IS_ERR(handle)) {
> > > + ret = PTR_ERR(handle);
> > > + return ret;
> > > + }
> > > +
> > > + /* set the max pages could be write-out at a time */
> > > + wbc->range_end = wbc->range_start +
> > > + EXT4_MAX_BUF_BLOCKS << PAGE_CACHE_SHIFT - 1;
> > I think limiting mpage_da_writepages through nr_to_write is better than
> > through range_end. That way you don't count clean pages...
> >
>
> You are right.
>
> > > +
> > > + ret = mpage_da_writepages(mapping, wbc, ext4_da_get_block_write);
> > > + ext4_journal_stop(handle);
> > But here we can't just stop. We have to write everything original caller
> > has asked about (at least in WB_SYNC_ALL mode). But the question is where
> > to resume because scanning the whole range again is kind-of excessive and
> > prone do livelock with other process dirtying the file via mmap. Maybe if
> > we slightly modified write_cache_pages() to always store in writeback_index
> > where they finished, we could use this value.
>
> Thanks for pointing this out.
> How about this?
> ---
> fs/ext4/inode.c | 70 ++++++++++++++++++++++++++++++++++++++++++----------
> mm/page-writeback.c | 2 -
> 2 files changed, 58 insertions(+), 14 deletions(-)
>
> Index: linux-2.6.25-rc9/fs/ext4/inode.c
> ===================================================================
> --- linux-2.6.25-rc9.orig/fs/ext4/inode.c 2008-04-16 09:59:00.000000000 -0700
> +++ linux-2.6.25-rc9/fs/ext4/inode.c 2008-04-16 11:23:12.000000000 -0700
> @@ -1437,18 +1437,13 @@ static int ext4_da_get_block_prep(struct
> static int ext4_da_get_block_write(struct inode *inode, sector_t iblock,
> struct buffer_head *bh_result, int create)
> {
> - int ret, needed_blocks = ext4_writepage_trans_blocks(inode);
> + int ret;
> unsigned max_blocks = bh_result->b_size >> inode->i_blkbits;
> loff_t disksize = EXT4_I(inode)->i_disksize;
> handle_t *handle = NULL;
>
> - if (create) {
> - handle = ext4_journal_start(inode, needed_blocks);
> - if (IS_ERR(handle)) {
> - ret = PTR_ERR(handle);
> - goto out;
> - }
> - }
> + J_ASSERT(handle != NULL || create == 0);
> + handle = ext4_journal_current_handle();
>
> ret = ext4_get_blocks_wrap(handle, inode, iblock, max_blocks,
> bh_result, create, 0);
> @@ -1483,17 +1478,66 @@ static int ext4_da_get_block_write(struc
> ret = 0;
> }
>
> -out:
> - if (handle && !IS_ERR(handle))
> - ext4_journal_stop(handle);
> -
> return ret;
> }
>
> +/*
> + * For now just follow the DIO way to estimate the max credits
> + * needed to write out EXT4_MAX_WRITEBACK_PAGES.
> + * todo: need to calculate the max credits need for
> + * extent based files, currently the DIO credits is based on
> + * indirect-blocks mapping way.
> + *
> + * Probably should have a generic way to calculate credits
> + * for DIO, writepages, and truncate
> + */
> +#define EXT4_MAX_WRITEBACK_PAGES DIO_MAX_BLOCKS
> +#define EXT4_MAX_WRITEBACK_CREDITS DIO_CREDITS
> +
> static int ext4_da_writepages(struct address_space *mapping,
> struct writeback_control *wbc)
> {
> - return mpage_da_writepages(mapping, wbc, ext4_da_get_block_write);
> + struct inode *inode = mapping->host;
> + handle_t *handle = NULL;
> + int needed_blocks;
> + int ret = 0;
> + unsigned range_cyclic;
> + long to_write;
> +
> + /*
> + * Estimate the worse case needed credits to write out
> + * EXT4_MAX_BUF_BLOCKS pages
> + */
> + needed_blocks = EXT4_MAX_WRITEBACK_CREDITS;
> +
> + to_write = wbc->nr_to_write;
> + range_cyclic = wbc->range_cyclic;
> + wbc->range_cyclic = 1;
> +
> + while (!ret && to_write) {
> + /* start a new transaction*/
> + handle = ext4_journal_start(inode, needed_blocks);
> + if (IS_ERR(handle)) {
> + ret = PTR_ERR(handle);
> + goto out_writepages;
> + }
> + /*
> + * set the max dirty pages could be write at a time
> + * to fit into the reserved transaction credits
> + */
> + if (wbc->nr_to_write > EXT4_MAX_WRITEBACK_PAGES)
> + wbc->nr_to_write = EXT4_MAX_WRITEBACK_PAGES;
> + to_write -= wbc->nr_to_write;
> +
> + ret = mpage_da_writepages(mapping, wbc, ext4_da_get_block_write);
> + ext4_journal_stop(handle);
> + to_write +=wbc->nr_to_write;
> + }

You need to set wbc->nr_to_write in the loop before calling
mpage_da_write_page() (for the next iteration).

> +
> +out_writepages:
> + wbc->nr_to_write = to_write;
> + wbc->range_cyclic = range_cyclic;
> + return ret;
> }
>
> static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
> Index: linux-2.6.25-rc9/mm/page-writeback.c
> ===================================================================
> --- linux-2.6.25-rc9.orig/mm/page-writeback.c 2008-04-16 11:00:20.000000000 -0700
> +++ linux-2.6.25-rc9/mm/page-writeback.c 2008-04-16 11:07:59.000000000 -0700
> @@ -816,7 +816,7 @@ int write_cache_pages(struct address_spa
> pagevec_init(&pvec, 0);
> if (wbc->range_cyclic) {
> index = mapping->writeback_index; /* Start from prev offset */
> - end = -1;
> + end = wbc->range_end >> PAGE_CACHE_SHIFT;

Hmm. There are other callers to write_cache_pages() using
"range_cyclic" . Did you check them to make sure, they set range_end
correctly ?

Thanks,
Badari