Messages in this thread | | | Date | Mon, 27 Mar 2023 19:11:55 +0800 | Subject | Re: [PATCH 3/3] ext4: fix race between writepages and remount | From | Baokun Li <> |
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On 2023/3/27 17:35, Jan Kara wrote: > On Thu 23-03-23 22:18:53, Baokun Li wrote: >> On 2023/3/23 19:44, Jan Kara wrote: >>>> --- >>>> fs/ext4/ext4.h | 3 ++- >>>> fs/ext4/ext4_jbd2.h | 9 +++++---- >>>> fs/ext4/super.c | 14 ++++++++++++++ >>>> 3 files changed, 21 insertions(+), 5 deletions(-) >>>> >>>> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h >>>> index 08b29c289da4..f60967fa648f 100644 >>>> --- a/fs/ext4/ext4.h >>>> +++ b/fs/ext4/ext4.h >>>> @@ -1703,7 +1703,8 @@ struct ext4_sb_info { >>>> /* >>>> * Barrier between writepages ops and changing any inode's JOURNAL_DATA >>>> - * or EXTENTS flag. >>>> + * or EXTENTS flag or between changing SHOULD_DIOREAD_NOLOCK flag on >>>> + * remount and writepages ops. >>>> */ >>>> struct percpu_rw_semaphore s_writepages_rwsem; >>>> struct dax_device *s_daxdev; >>>> diff --git a/fs/ext4/ext4_jbd2.h b/fs/ext4/ext4_jbd2.h >>>> index 0c77697d5e90..d82bfcdd56e5 100644 >>>> --- a/fs/ext4/ext4_jbd2.h >>>> +++ b/fs/ext4/ext4_jbd2.h >>>> @@ -488,6 +488,9 @@ static inline int ext4_free_data_revoke_credits(struct inode *inode, int blocks) >>>> return blocks + 2*(EXT4_SB(inode->i_sb)->s_cluster_ratio - 1); >>>> } >>>> +/* delalloc is a temporary fix to prevent generic/422 test failures*/ >>>> +#define EXT4_MOUNT_SHOULD_DIOREAD_NOLOCK (EXT4_MOUNT_DIOREAD_NOLOCK | \ >>>> + EXT4_MOUNT_DELALLOC) >>>> /* >>>> * This function controls whether or not we should try to go down the >>>> * dioread_nolock code paths, which makes it safe to avoid taking >>>> @@ -499,7 +502,8 @@ static inline int ext4_free_data_revoke_credits(struct inode *inode, int blocks) >>>> */ >>>> static inline int ext4_should_dioread_nolock(struct inode *inode) >>>> { >>>> - if (!test_opt(inode->i_sb, DIOREAD_NOLOCK)) >>>> + if (test_opt(inode->i_sb, SHOULD_DIOREAD_NOLOCK) != >>>> + EXT4_MOUNT_SHOULD_DIOREAD_NOLOCK) >>>> return 0; >>>> if (!S_ISREG(inode->i_mode)) >>>> return 0; >>>> @@ -507,9 +511,6 @@ static inline int ext4_should_dioread_nolock(struct inode *inode) >>>> return 0; >>>> if (ext4_should_journal_data(inode)) >>>> return 0; >>>> - /* temporary fix to prevent generic/422 test failures */ >>>> - if (!test_opt(inode->i_sb, DELALLOC)) >>>> - return 0; >>>> return 1; >>>> } >>> Is there a need for this SHOULD_DIOREAD_NOLOCK? When called from writeback >>> we will be protected by s_writepages_rwsem anyway. When called from other >>> places, we either decide to do dioread_nolock or don't but the situation >>> can change at any instant so I don't see how unifying this check would >>> help. And the new SHOULD_DIOREAD_NOLOCK somewhat obfuscates what's going >>> on. >> We're thinking that the mount-related flags in >> ext4_should_dioread_nolock() might be modified, such as DELALLOC being >> removed because generic/422 test failures were fixed in some other way, >> resulting in some unnecessary locking during remount, or for whatever >> reason a mount-related flag was added to ext4_should_dioread_nolock(), >> and we didn't make a synchronization change in __ext4_remount() that >> would cause the problem to recur. So we added this flag to this function >> (instead of in ext4.h), so that when we change the mount option in >> ext4_should_dioread_nolock(), we directly change this flag, and we don't >> have to consider making synchronization changes in __ext4_remount(). >> >> We have checked where this function is called and there are two types of >> calls to this function: >> 1. One category is ext4_do_writepages() and mpage_map_one_extent(), which >> are protected by s_writepages_rwsem, the location of the problem; >> 2. The other type is in ext4_page_mkwrite(), >> ext4_convert_inline_data_to_extent(), ext4_write_begin() to determine >> whether to get the block using ext4_get_block_unwritten() or >> ext4_get_block(). >> >> 1) If we just started fetching written blocks, it looks like there is no >> problem; >> 2) If we start getting unwritten blocks, when DIOREAD_NOLOCK is cleared >> by remount, >> we will convert the blocks to written in ext4_map_blocks(). The >> data=ordered mode ensures that we don't see stale data. > Yes. So do you agree that EXT4_MOUNT_SHOULD_DIOREAD_NOLOCK is not really > needed? > > Honza Yes, I totally agree! If we unconditionally grabbed s_writepages_rwsem when remounting, there would be no mount option synchronization problem, and the flag would be completely unnecessary.
I will send a patch V2 with the changes suggested by you. -- With Best Regards, Baokun Li .
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