Re: [PATCH] f2fs: avoid fi->i_gc_rwsem[WRITE] lock in f2fs_gc

From: Chao Yu
Date: Sun Jul 29 2018 - 23:37:27 EST

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On 2018/7/30 9:32, Jaegeuk Kim wrote:
> The f2fs_gc() called by f2fs_balance_fs() requires to be called outside of
> fi->i_gc_rwsem[WRITE], since f2fs_gc() can try to grab it in a loop.
>
> If it hits the miximum retrials in GC, let's give a chance to release
> gc_mutex for a short time in order not to go into live lock in the worst
> case.
>
> Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> ---
> fs/f2fs/f2fs.h | 1 +
> fs/f2fs/file.c | 62 ++++++++++++++++++++++-------------------------
> fs/f2fs/gc.c | 22 ++++++++++++-----
> fs/f2fs/segment.c | 5 +++-
> fs/f2fs/segment.h | 2 +-
> 5 files changed, 51 insertions(+), 41 deletions(-)
>
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index a9447c7d6570..50349780001b 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -1223,6 +1223,7 @@ struct f2fs_sb_info {
> unsigned int gc_mode; /* current GC state */
> /* for skip statistic */
> unsigned long long skipped_atomic_files[2]; /* FG_GC and BG_GC */
> + unsigned long long skipped_gc_rwsem; /* FG_GC only */
>
> /* threshold for gc trials on pinned files */
> u64 gc_pin_file_threshold;
> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> index 78c1bd6b8497..2b7d26ebb294 100644
> --- a/fs/f2fs/file.c
> +++ b/fs/f2fs/file.c
> @@ -1179,10 +1179,12 @@ static int __exchange_data_block(struct inode *src_inode,
> return ret;
> }
>
> -static int f2fs_do_collapse(struct inode *inode, pgoff_t start, pgoff_t end)
> +static int f2fs_do_collapse(struct inode *inode, loff_t offset, loff_t len)
> {
> struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
> pgoff_t nrpages = (i_size_read(inode) + PAGE_SIZE - 1) / PAGE_SIZE;
> + pgoff_t start = offset >> PAGE_SHIFT;
> + pgoff_t end = (offset + len) >> PAGE_SHIFT;
> int ret;
>
> f2fs_balance_fs(sbi, true);
> @@ -1190,14 +1192,18 @@ static int f2fs_do_collapse(struct inode *inode, pgoff_t start, pgoff_t end)
>
> f2fs_drop_extent_tree(inode);
>
> + /* avoid gc operation during block exchange */
> + down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> + truncate_pagecache(inode, offset);
> ret = __exchange_data_block(inode, inode, end, start, nrpages - end, true);
> + up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> +
> f2fs_unlock_op(sbi);
> return ret;
> }
>
> static int f2fs_collapse_range(struct inode *inode, loff_t offset, loff_t len)
> {
> - pgoff_t pg_start, pg_end;
> loff_t new_size;
> int ret;
>
> @@ -1212,21 +1218,13 @@ static int f2fs_collapse_range(struct inode *inode, loff_t offset, loff_t len)
> if (ret)
> return ret;
>
> - pg_start = offset >> PAGE_SHIFT;
> - pg_end = (offset + len) >> PAGE_SHIFT;
> -
> - /* avoid gc operation during block exchange */
> - down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> -
> down_write(&F2FS_I(inode)->i_mmap_sem);
> /* write out all dirty pages from offset */
> ret = filemap_write_and_wait_range(inode->i_mapping, offset, LLONG_MAX);
> if (ret)
> goto out_unlock;
>
> - truncate_pagecache(inode, offset);
> -
> - ret = f2fs_do_collapse(inode, pg_start, pg_end);
> + ret = f2fs_do_collapse(inode, offset, len);
> if (ret)
> goto out_unlock;
>
> @@ -1242,7 +1240,6 @@ static int f2fs_collapse_range(struct inode *inode, loff_t offset, loff_t len)
> f2fs_i_size_write(inode, new_size);
> out_unlock:
> up_write(&F2FS_I(inode)->i_mmap_sem);
> - up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> return ret;
> }
>
> @@ -1417,9 +1414,6 @@ static int f2fs_insert_range(struct inode *inode, loff_t offset, loff_t len)
>
> f2fs_balance_fs(sbi, true);
>
> - /* avoid gc operation during block exchange */
> - down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> -
> down_write(&F2FS_I(inode)->i_mmap_sem);
> ret = f2fs_truncate_blocks(inode, i_size_read(inode), true);
> if (ret)
> @@ -1430,13 +1424,15 @@ static int f2fs_insert_range(struct inode *inode, loff_t offset, loff_t len)
> if (ret)
> goto out;
>
> - truncate_pagecache(inode, offset);
> -
> pg_start = offset >> PAGE_SHIFT;
> pg_end = (offset + len) >> PAGE_SHIFT;
> delta = pg_end - pg_start;
> idx = (i_size_read(inode) + PAGE_SIZE - 1) / PAGE_SIZE;
>
> + /* avoid gc operation during block exchange */
> + down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> + truncate_pagecache(inode, offset);
> +
> while (!ret && idx > pg_start) {
> nr = idx - pg_start;
> if (nr > delta)
> @@ -1450,6 +1446,7 @@ static int f2fs_insert_range(struct inode *inode, loff_t offset, loff_t len)
> idx + delta, nr, false);
> f2fs_unlock_op(sbi);
> }
> + up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
>
> /* write out all moved pages, if possible */
> filemap_write_and_wait_range(inode->i_mapping, offset, LLONG_MAX);
> @@ -1459,7 +1456,6 @@ static int f2fs_insert_range(struct inode *inode, loff_t offset, loff_t len)
> f2fs_i_size_write(inode, new_size);
> out:
> up_write(&F2FS_I(inode)->i_mmap_sem);
> - up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> return ret;
> }
>
> @@ -1706,8 +1702,6 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
>
> inode_lock(inode);
>
> - down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);

After removing lock here, how can we handle below condition:

commit 27319ba4044c0c67d62ae39e53c0118c89f0a029
Author: Chao Yu <yuchao0@xxxxxxxxxx>
Date: Tue Apr 17 17:51:28 2018 +0800

f2fs: fix race in between GC and atomic open

Thread GC thread
- f2fs_ioc_start_atomic_write
- get_dirty_pages
- filemap_write_and_wait_range
- f2fs_gc
- do_garbage_collect
- gc_data_segment
- move_data_page
- f2fs_is_atomic_file
- set_page_dirty
- set_inode_flag(, FI_ATOMIC_FILE)

Dirty data page can still be generated by GC in race condition as
above call stack.

This patch adds fi->dio_rwsem[WRITE] in f2fs_ioc_start_atomic_write
to avoid such race.

Thanks,

> -
> if (f2fs_is_atomic_file(inode)) {
> if (is_inode_flag_set(inode, FI_ATOMIC_REVOKE_REQUEST))
> ret = -EINVAL;
> @@ -1736,7 +1730,6 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
> stat_inc_atomic_write(inode);
> stat_update_max_atomic_write(inode);
> out:
> - up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> inode_unlock(inode);
> mnt_drop_write_file(filp);
> return ret;
> @@ -1754,9 +1747,9 @@ static int f2fs_ioc_commit_atomic_write(struct file *filp)
> if (ret)
> return ret;
>
> - inode_lock(inode);
> + f2fs_balance_fs(F2FS_I_SB(inode), true);
>
> - down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> + inode_lock(inode);
>
> if (f2fs_is_volatile_file(inode)) {
> ret = -EINVAL;
> @@ -1782,7 +1775,6 @@ static int f2fs_ioc_commit_atomic_write(struct file *filp)
> clear_inode_flag(inode, FI_ATOMIC_REVOKE_REQUEST);
> ret = -EINVAL;
> }
> - up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> inode_unlock(inode);
> mnt_drop_write_file(filp);
> return ret;
> @@ -2378,15 +2370,10 @@ static int f2fs_move_file_range(struct file *file_in, loff_t pos_in,
> }
>
> inode_lock(src);
> - down_write(&F2FS_I(src)->i_gc_rwsem[WRITE]);
> if (src != dst) {
> ret = -EBUSY;
> if (!inode_trylock(dst))
> goto out;
> - if (!down_write_trylock(&F2FS_I(dst)->i_gc_rwsem[WRITE])) {
> - inode_unlock(dst);
> - goto out;
> - }
> }
>
> ret = -EINVAL;
> @@ -2432,6 +2419,14 @@ static int f2fs_move_file_range(struct file *file_in, loff_t pos_in,
>
> f2fs_balance_fs(sbi, true);
> f2fs_lock_op(sbi);
> +
> + down_write(&F2FS_I(src)->i_gc_rwsem[WRITE]);
> + if (src != dst) {
> + ret = -EBUSY;
> + if (!down_write_trylock(&F2FS_I(dst)->i_gc_rwsem[WRITE]))
> + goto out_src;
> + }
> +
> ret = __exchange_data_block(src, dst, pos_in >> F2FS_BLKSIZE_BITS,
> pos_out >> F2FS_BLKSIZE_BITS,
> len >> F2FS_BLKSIZE_BITS, false);
> @@ -2442,14 +2437,15 @@ static int f2fs_move_file_range(struct file *file_in, loff_t pos_in,
> else if (dst_osize != dst->i_size)
> f2fs_i_size_write(dst, dst_osize);
> }
> + if (src != dst)
> + up_write(&F2FS_I(dst)->i_gc_rwsem[WRITE]);
> +out_src:
> + up_write(&F2FS_I(src)->i_gc_rwsem[WRITE]);
> f2fs_unlock_op(sbi);
> out_unlock:
> - if (src != dst) {
> - up_write(&F2FS_I(dst)->i_gc_rwsem[WRITE]);
> + if (src != dst)
> inode_unlock(dst);
> - }
> out:
> - up_write(&F2FS_I(src)->i_gc_rwsem[WRITE]);
> inode_unlock(src);
> return ret;
> }
> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> index e352fbd33848..cac317e37306 100644
> --- a/fs/f2fs/gc.c
> +++ b/fs/f2fs/gc.c
> @@ -884,6 +884,7 @@ static void gc_data_segment(struct f2fs_sb_info *sbi, struct f2fs_summary *sum,
> if (!down_write_trylock(
> &F2FS_I(inode)->i_gc_rwsem[WRITE])) {
> iput(inode);
> + sbi->skipped_gc_rwsem++;
> continue;
> }
>
> @@ -913,6 +914,7 @@ static void gc_data_segment(struct f2fs_sb_info *sbi, struct f2fs_summary *sum,
> continue;
> if (!down_write_trylock(
> &fi->i_gc_rwsem[WRITE])) {
> + sbi->skipped_gc_rwsem++;
> up_write(&fi->i_gc_rwsem[READ]);
> continue;
> }
> @@ -1062,6 +1064,7 @@ int f2fs_gc(struct f2fs_sb_info *sbi, bool sync,
> prefree_segments(sbi));
>
> cpc.reason = __get_cp_reason(sbi);
> + sbi->skipped_gc_rwsem = 0;
> gc_more:
> if (unlikely(!(sbi->sb->s_flags & SB_ACTIVE))) {
> ret = -EINVAL;
> @@ -1103,7 +1106,8 @@ int f2fs_gc(struct f2fs_sb_info *sbi, bool sync,
> total_freed += seg_freed;
>
> if (gc_type == FG_GC) {
> - if (sbi->skipped_atomic_files[FG_GC] > last_skipped)
> + if (sbi->skipped_atomic_files[FG_GC] > last_skipped ||
> + sbi->skipped_gc_rwsem)
> skipped_round++;
> last_skipped = sbi->skipped_atomic_files[FG_GC];
> round++;
> @@ -1112,15 +1116,21 @@ int f2fs_gc(struct f2fs_sb_info *sbi, bool sync,
> if (gc_type == FG_GC)
> sbi->cur_victim_sec = NULL_SEGNO;
>
> - if (!sync) {
> - if (has_not_enough_free_secs(sbi, sec_freed, 0)) {
> - if (skipped_round > MAX_SKIP_ATOMIC_COUNT &&
> - skipped_round * 2 >= round)
> - f2fs_drop_inmem_pages_all(sbi, true);
> + if (sync)
> + goto stop;
> +
> + if (has_not_enough_free_secs(sbi, sec_freed, 0)) {
> + if (skipped_round <= MAX_SKIP_GC_COUNT ||
> + skipped_round * 2 < round) {
> segno = NULL_SEGNO;
> goto gc_more;
> }
>
> + if (sbi->skipped_atomic_files[FG_GC] == last_skipped) {
> + f2fs_drop_inmem_pages_all(sbi, true);
> + segno = NULL_SEGNO;
> + goto gc_more;
> + }
> if (gc_type == FG_GC)
> ret = f2fs_write_checkpoint(sbi, &cpc);
> }
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 3662e1f429b4..15b3b095fd58 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -444,10 +444,12 @@ int f2fs_commit_inmem_pages(struct inode *inode)
> struct f2fs_inode_info *fi = F2FS_I(inode);
> int err;
>
> - f2fs_balance_fs(sbi, true);
> + f2fs_balance_fs(F2FS_I_SB(inode), true);
> +
> f2fs_lock_op(sbi);
>
> set_inode_flag(inode, FI_ATOMIC_COMMIT);
> + down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
>
> mutex_lock(&fi->inmem_lock);
> err = __f2fs_commit_inmem_pages(inode);
> @@ -458,6 +460,7 @@ int f2fs_commit_inmem_pages(struct inode *inode)
> spin_unlock(&sbi->inode_lock[ATOMIC_FILE]);
> mutex_unlock(&fi->inmem_lock);
>
> + up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> clear_inode_flag(inode, FI_ATOMIC_COMMIT);
>
> f2fs_unlock_op(sbi);
> diff --git a/fs/f2fs/segment.h b/fs/f2fs/segment.h
> index 50495515f0a0..b3d9e317ff0c 100644
> --- a/fs/f2fs/segment.h
> +++ b/fs/f2fs/segment.h
> @@ -215,7 +215,7 @@ struct segment_allocation {
> #define IS_DUMMY_WRITTEN_PAGE(page) \
> (page_private(page) == (unsigned long)DUMMY_WRITTEN_PAGE)
>
> -#define MAX_SKIP_ATOMIC_COUNT 16
> +#define MAX_SKIP_GC_COUNT 16
>
> struct inmem_pages {
> struct list_head list;
>

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