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Date: Mon, 12 Apr 2010 15:36:12 +0200
From: Jan Kara <jack@...e.cz>
To: Eric Sandeen <sandeen@...hat.com>
Cc: ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 3/3] ext4: don't use quota reservation for speculative
metadata blocks
> Because we can badly over-reserve metadata when we
> calculate worst-case, it complicates things for quota, since
> we must reserve and then claim later, retry on EDQUOT, etc.
> Quota is also a generally smaller pool than fs free blocks,
> so this over-reservation hurts more, and more often.
>
> I'm of the opinion that it's not the worst thing to allow
> metadata to push a user slightly over quota. This simplifies
> the code and avoids the false quota rejections that result
> from worst-case speculation.
>
> This patch stops the speculative quota-charging for
> worst-case metadata requirements, and just charges quota
> when the blocks are allocated at writeout. It also is
> able to remove the try-again loop on EDQUOT.
>
> This patch has been tested indirectly by running the xfstests
> suite with a hack to mount & enable quota prior to the test.
>
> I also did a more specific test of fragmenting freespace
> and then doing a large delalloc write under quota; quota
> stopped me at the right amount of file IO, and then the
> writeout generated enough metadata (due to the fragmentation)
> that it put me slightly over quota, as expected.
>
> Signed-off-by: Eric Sandeen <sandeen@...hat.com>
This patch looks good as well. Acked-by: Jan Kara <jack@...e.cz>
Honza
> ---
>
> balloc.c | 5 ++--
> inode.c | 64 ++++++++++++++++++++-------------------------------------------
> 2 files changed, 24 insertions(+), 45 deletions(-)
>
>
> diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> index d2f37a5..95b7594 100644
> --- a/fs/ext4/balloc.c
> +++ b/fs/ext4/balloc.c
> @@ -591,14 +591,15 @@ ext4_fsblk_t ext4_new_meta_blocks(handle_t *handle, struct inode *inode,
> ret = ext4_mb_new_blocks(handle, &ar, errp);
> if (count)
> *count = ar.len;
> -
> /*
> - * Account for the allocated meta blocks
> + * Account for the allocated meta blocks. We will never
> + * fail EDQUOT for metdata, but we do account for it.
> */
> if (!(*errp) && EXT4_I(inode)->i_delalloc_reserved_flag) {
> spin_lock(&EXT4_I(inode)->i_block_reservation_lock);
> EXT4_I(inode)->i_allocated_meta_blocks += ar.len;
> spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
> + dquot_alloc_block_nofail(inode, ar.len);
> }
> return ret;
> }
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 5381802..dba674a 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -1076,7 +1076,6 @@ void ext4_da_update_reserve_space(struct inode *inode,
> {
> struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
> struct ext4_inode_info *ei = EXT4_I(inode);
> - int mdb_free = 0, allocated_meta_blocks = 0;
>
> spin_lock(&ei->i_block_reservation_lock);
> trace_ext4_da_update_reserve_space(inode, used);
> @@ -1091,11 +1090,10 @@ void ext4_da_update_reserve_space(struct inode *inode,
>
> /* Update per-inode reservations */
> ei->i_reserved_data_blocks -= used;
> - used += ei->i_allocated_meta_blocks;
> ei->i_reserved_meta_blocks -= ei->i_allocated_meta_blocks;
> - allocated_meta_blocks = ei->i_allocated_meta_blocks;
> + percpu_counter_sub(&sbi->s_dirtyblocks_counter,
> + used + ei->i_allocated_meta_blocks);
> ei->i_allocated_meta_blocks = 0;
> - percpu_counter_sub(&sbi->s_dirtyblocks_counter, used);
>
> if (ei->i_reserved_data_blocks == 0) {
> /*
> @@ -1103,30 +1101,23 @@ void ext4_da_update_reserve_space(struct inode *inode,
> * only when we have written all of the delayed
> * allocation blocks.
> */
> - mdb_free = ei->i_reserved_meta_blocks;
> + percpu_counter_sub(&sbi->s_dirtyblocks_counter,
> + ei->i_reserved_meta_blocks);
> ei->i_reserved_meta_blocks = 0;
> ei->i_da_metadata_calc_len = 0;
> - percpu_counter_sub(&sbi->s_dirtyblocks_counter, mdb_free);
> }
> spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
>
> - /* Update quota subsystem */
> + /* Update quota subsystem for data blocks */
> if (quota_claim) {
> dquot_claim_block(inode, used);
> - if (mdb_free)
> - dquot_release_reservation_block(inode, mdb_free);
> } else {
> /*
> * We did fallocate with an offset that is already delayed
> * allocated. So on delayed allocated writeback we should
> - * not update the quota for allocated blocks. But then
> - * converting an fallocate region to initialized region would
> - * have caused a metadata allocation. So claim quota for
> - * that
> + * not re-claim the quota for fallocated blocks.
> */
> - if (allocated_meta_blocks)
> - dquot_claim_block(inode, allocated_meta_blocks);
> - dquot_release_reservation_block(inode, mdb_free + used);
> + dquot_release_reservation_block(inode, used);
> }
>
> /*
> @@ -1860,7 +1851,7 @@ static int ext4_da_reserve_space(struct inode *inode, sector_t lblock)
> int retries = 0;
> struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
> struct ext4_inode_info *ei = EXT4_I(inode);
> - unsigned long md_needed, md_reserved;
> + unsigned long md_needed;
> int ret;
>
> /*
> @@ -1870,22 +1861,24 @@ static int ext4_da_reserve_space(struct inode *inode, sector_t lblock)
> */
> repeat:
> spin_lock(&ei->i_block_reservation_lock);
> - md_reserved = ei->i_reserved_meta_blocks;
> md_needed = ext4_calc_metadata_amount(inode, lblock);
> trace_ext4_da_reserve_space(inode, md_needed);
> spin_unlock(&ei->i_block_reservation_lock);
>
> /*
> - * Make quota reservation here to prevent quota overflow
> - * later. Real quota accounting is done at pages writeout
> - * time.
> + * We will charge metadata quota at writeout time; this saves
> + * us from metadata over-estimation, though we may go over by
> + * a small amount in the end. Here we just reserve for data.
> */
> - ret = dquot_reserve_block(inode, md_needed + 1);
> + ret = dquot_reserve_block(inode, 1);
> if (ret)
> return ret;
> -
> + /*
> + * We do still charge estimated metadata to the sb though;
> + * we cannot afford to run out of free blocks.
> + */
> if (ext4_claim_free_blocks(sbi, md_needed + 1)) {
> - dquot_release_reservation_block(inode, md_needed + 1);
> + dquot_release_reservation_block(inode, 1);
> if (ext4_should_retry_alloc(inode->i_sb, &retries)) {
> yield();
> goto repeat;
> @@ -1932,12 +1925,13 @@ static void ext4_da_release_space(struct inode *inode, int to_free)
> * only when we have written all of the delayed
> * allocation blocks.
> */
> - to_free += ei->i_reserved_meta_blocks;
> + percpu_counter_sub(&sbi->s_dirtyblocks_counter,
> + ei->i_reserved_meta_blocks);
> ei->i_reserved_meta_blocks = 0;
> ei->i_da_metadata_calc_len = 0;
> }
>
> - /* update fs dirty blocks counter */
> + /* update fs dirty data blocks counter */
> percpu_counter_sub(&sbi->s_dirtyblocks_counter, to_free);
>
> spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
> @@ -3076,7 +3070,7 @@ static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
> loff_t pos, unsigned len, unsigned flags,
> struct page **pagep, void **fsdata)
> {
> - int ret, retries = 0, quota_retries = 0;
> + int ret, retries = 0;
> struct page *page;
> pgoff_t index;
> unsigned from, to;
> @@ -3135,22 +3129,6 @@ retry:
>
> if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
> goto retry;
> -
> - if ((ret == -EDQUOT) &&
> - EXT4_I(inode)->i_reserved_meta_blocks &&
> - (quota_retries++ < 3)) {
> - /*
> - * Since we often over-estimate the number of meta
> - * data blocks required, we may sometimes get a
> - * spurios out of quota error even though there would
> - * be enough space once we write the data blocks and
> - * find out how many meta data blocks were _really_
> - * required. So try forcing the inode write to see if
> - * that helps.
> - */
> - write_inode_now(inode, (quota_retries == 3));
> - goto retry;
> - }
> out:
> return ret;
> }
>
> --
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--
Jan Kara <jack@...e.cz>
SuSE CR Labs
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