Re: [PATCH 1/2] mm: Protect operations adding pages to page cache with i_mapping_lock

Dave Chinner
2021-02-09T01:14:00Z

On Mon, Feb 08, 2021 at 05:39:17PM +0100, Jan Kara wrote:
> Currently, serializing operations such as page fault, read, or readahead
> against hole punching is rather difficult. The basic race scheme is
> like:
>
> fallocate(FALLOC_FL_PUNCH_HOLE) read / fault / ..
> truncate_inode_pages_range()
> <create pages in page
> cache here>
> <update fs block mapping and free blocks>
>
> Now the problem is in this way read / page fault / readahead can
> instantiate pages in page cache with potentially stale data (if blocks
> get quickly reused). Avoiding this race is not simple - page locks do
> not work because we want to make sure there are *no* pages in given
> range. inode->i_rwsem does not work because page fault happens under
> mmap_sem which ranks below inode->i_rwsem. Also using it for reads makes
> the performance for mixed read-write workloads suffer.
>
> So create a new rw_semaphore in the inode - i_mapping_sem - that
> protects adding of pages to page cache for page faults / reads /
> readahead.
>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
> fs/inode.c | 3 +++
> include/linux/fs.h | 2 ++
> mm/filemap.c | 45 +++++++++++++++++++++++++++++++++++++++++++--
> mm/readahead.c | 2 ++
> 4 files changed, 50 insertions(+), 2 deletions(-)
>
> diff --git a/fs/inode.c b/fs/inode.c
> index 6442d97d9a4a..8df49d98e1cd 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -174,6 +174,9 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
>
> init_rwsem(&inode->i_rwsem);
> lockdep_set_class(&inode->i_rwsem, &sb->s_type->i_mutex_key);
> + init_rwsem(&inode->i_mapping_sem);
> + lockdep_set_class(&inode->i_mapping_sem,
> + &sb->s_type->i_mapping_sem_key);
>
> atomic_set(&inode->i_dio_count, 0);
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index b20ddd8a6e62..248609bc61a2 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -658,6 +658,7 @@ struct inode {
> /* Misc */
> unsigned long i_state;
> struct rw_semaphore i_rwsem;
> + struct rw_semaphore i_mapping_sem;
>
> unsigned long dirtied_when; /* jiffies of first dirtying */
> unsigned long dirtied_time_when;
> @@ -2249,6 +2250,7 @@ struct file_system_type {
>
> struct lock_class_key i_lock_key;
> struct lock_class_key i_mutex_key;
> + struct lock_class_key i_mapping_sem_key;
> struct lock_class_key i_mutex_dir_key;
> };
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 16a3bf693d4a..02f778ff02e0 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2257,16 +2257,28 @@ static int filemap_update_page(struct kiocb *iocb,
> {
> int error;
>
> + if (iocb->ki_flags & IOCB_NOWAIT) {
> + if (!down_read_trylock(&mapping->host->i_mapping_sem))
> + return -EAGAIN;
> + } else {
> + down_read(&mapping->host->i_mapping_sem);
> + }
> +
> if (!trylock_page(page)) {
> - if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_NOIO))
> + if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_NOIO)) {
> + up_read(&mapping->host->i_mapping_sem);
> return -EAGAIN;
> + }
> if (!(iocb->ki_flags & IOCB_WAITQ)) {
> + up_read(&mapping->host->i_mapping_sem);
> put_and_wait_on_page_locked(page, TASK_KILLABLE);
> return AOP_TRUNCATED_PAGE;
> }
> error = __lock_page_async(page, iocb->ki_waitq);
> - if (error)
> + if (error) {
> + up_read(&mapping->host->i_mapping_sem);
> return error;
> + }
> }

What tree is this against? I don't see filemap_update_page() in a
5.11-rc7 tree...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com