Re: [f2fs-dev] [PATCH] f2fs: synchronize atomic write aborts

From: Chao Yu
Date: Tue Jan 31 2023 - 06:38:02 EST

Hi Daeho,

On 2023/1/31 0:34, Daeho Jeong wrote:
Hi Chao,

I read your patch series now and I like it.

Thank you for checking the patches. :)

However, how about a race condition between start_atomic_write and
abort_atomic_write?

Yup, I noticed that issue, I guess we can avoid this race condition by
covering these two flows w/ i_atomic_sem.

abort_atomic_write is called without inode_lock in closing filp scenarios.
What do you think about this?

I'm fine w/ your change as it's more clean, but it's better to drop cow_inode's
page cache if atomic_write is committed or aborted to avoid caching obsolete page?

Thanks,


Thanks,


On Fri, Jan 27, 2023 at 6:07 PM Chao Yu <chao@xxxxxxxxxx> wrote:

Hi Daeho, Jaegeuk,

Please take a look at patchset in below link:

https://lore.kernel.org/linux-f2fs-devel/20230109034453.490176-1-chao@xxxxxxxxxx/T/#t

In PATCH 4/5, I'm trying to fix the same issue w/ alternative way, let me
know your preference. :)

One comment as below.

On 2023/1/13 8:49, Daeho Jeong wrote:
From: Daeho Jeong <daehojeong@xxxxxxxxxx>

To fix a race condition between atomic write aborts, I use the inode
lock and make COW inode to be re-usable thoroughout the whole
atomic file inode lifetime.

Reported-by: syzbot+823000d23b3400619f7c@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 3db1de0e582c ("f2fs: change the current atomic write way")
Signed-off-by: Daeho Jeong <daehojeong@xxxxxxxxxx>
---
fs/f2fs/file.c | 43 ++++++++++++++++++++++++++++---------------
fs/f2fs/inode.c | 11 +++++++++--
fs/f2fs/segment.c | 3 ---
fs/f2fs/super.c | 2 --
4 files changed, 37 insertions(+), 22 deletions(-)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index ecbc8c135b49..ff072a9ed258 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -1866,7 +1866,10 @@ static int f2fs_release_file(struct inode *inode, struct file *filp)
atomic_read(&inode->i_writecount) != 1)
return 0;

+ inode_lock(inode);
f2fs_abort_atomic_write(inode, true);
+ inode_unlock(inode);
+
return 0;
}

@@ -1880,8 +1883,11 @@ static int f2fs_file_flush(struct file *file, fl_owner_t id)
* until all the writers close its file. Since this should be done
* before dropping file lock, it needs to do in ->flush.
*/
- if (F2FS_I(inode)->atomic_write_task == current)
+ if (F2FS_I(inode)->atomic_write_task == current) {
+ inode_lock(inode);
f2fs_abort_atomic_write(inode, true);
+ inode_unlock(inode);
+ }
return 0;
}

@@ -2087,19 +2093,28 @@ static int f2fs_ioc_start_atomic_write(struct file *filp, bool truncate)
goto out;
}

- /* Create a COW inode for atomic write */
- pinode = f2fs_iget(inode->i_sb, fi->i_pino);
- if (IS_ERR(pinode)) {
- f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
- ret = PTR_ERR(pinode);
- goto out;
- }
+ /* Check if the inode already has a COW inode */
+ if (fi->cow_inode == NULL) {
+ /* Create a COW inode for atomic write */
+ pinode = f2fs_iget(inode->i_sb, fi->i_pino);
+ if (IS_ERR(pinode)) {
+ f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
+ ret = PTR_ERR(pinode);
+ goto out;
+ }

- ret = f2fs_get_tmpfile(mnt_userns, pinode, &fi->cow_inode);
- iput(pinode);
- if (ret) {
- f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
- goto out;
+ ret = f2fs_get_tmpfile(mnt_userns, pinode, &fi->cow_inode);
+ iput(pinode);
+ if (ret) {
+ f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
+ goto out;
+ }
+
+ set_inode_flag(fi->cow_inode, FI_COW_FILE);
+ clear_inode_flag(fi->cow_inode, FI_INLINE_DATA);
+ } else {
+ /* Reuse the already created COW inode */
+ f2fs_do_truncate_blocks(fi->cow_inode, 0, true);
}

f2fs_write_inode(inode, NULL);
@@ -2107,8 +2122,6 @@ static int f2fs_ioc_start_atomic_write(struct file *filp, bool truncate)
stat_inc_atomic_inode(inode);

set_inode_flag(inode, FI_ATOMIC_FILE);
- set_inode_flag(fi->cow_inode, FI_COW_FILE);
- clear_inode_flag(fi->cow_inode, FI_INLINE_DATA);

isize = i_size_read(inode);
fi->original_i_size = isize;
diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
index ff6cf66ed46b..4921f7209e28 100644
--- a/fs/f2fs/inode.c
+++ b/fs/f2fs/inode.c
@@ -766,11 +766,18 @@ int f2fs_write_inode(struct inode *inode, struct writeback_control *wbc)
void f2fs_evict_inode(struct inode *inode)
{
struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
- nid_t xnid = F2FS_I(inode)->i_xattr_nid;
+ struct f2fs_inode_info *fi = F2FS_I(inode);
+ nid_t xnid = fi->i_xattr_nid;
int err = 0;

f2fs_abort_atomic_write(inode, true);

+ if (fi->cow_inode) {
+ clear_inode_flag(fi->cow_inode, FI_COW_FILE);
+ iput(fi->cow_inode);
+ fi->cow_inode = NULL;
+ }
+
trace_f2fs_evict_inode(inode);
truncate_inode_pages_final(&inode->i_data);

@@ -857,7 +864,7 @@ void f2fs_evict_inode(struct inode *inode)
stat_dec_inline_inode(inode);
stat_dec_compr_inode(inode);
stat_sub_compr_blocks(inode,
- atomic_read(&F2FS_I(inode)->i_compr_blocks));
+ atomic_read(&fi->i_compr_blocks));

if (likely(!f2fs_cp_error(sbi) &&
!is_sbi_flag_set(sbi, SBI_CP_DISABLED)))
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index ae3c4e5474ef..536d7c674b04 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -192,9 +192,6 @@ void f2fs_abort_atomic_write(struct inode *inode, bool clean)
if (!f2fs_is_atomic_file(inode))
return;

- clear_inode_flag(fi->cow_inode, FI_COW_FILE);
- iput(fi->cow_inode);
- fi->cow_inode = NULL;
release_atomic_write_cnt(inode);
clear_inode_flag(inode, FI_ATOMIC_COMMITTED);
clear_inode_flag(inode, FI_ATOMIC_REPLACE);
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 1f812b9ce985..10463f084d30 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -1430,8 +1430,6 @@ static int f2fs_drop_inode(struct inode *inode)
atomic_inc(&inode->i_count);
spin_unlock(&inode->i_lock);

- f2fs_abort_atomic_write(inode, true);

In order to avoid caching obsolete page of cow_inode, how about truncating
them here?

if (f2fs_is_atomic_file() && cow_inode)
truncate_inode_pages_final(&cow_inode->i_data);

Thanks,

-
/* should remain fi->extent_tree for writepage */
f2fs_destroy_extent_node(inode);