Re: [PATCH 2/8] vfs: Protect write paths by sb_start_write - sb_end_write

Jan Kara
2012-02-06T15:33:15Z

On Sun 05-02-12 00:13:38, Eric Sandeen wrote:
> On 1/20/12 2:34 PM, Jan Kara wrote:
> > There are three entry points which dirty pages in a filesystem. mmap (handled
> > by block_page_mkwrite()), buffered write (handled by
> > __generic_file_aio_write()), and truncate (it can dirty last partial page -
> > handled inside each filesystem separately). Protect these places with
> > sb_start_write() and sb_end_write().
>
> The protection for truncate got lost since the first patchset, was that
> on purpose?
It was not lost but it got moved down into the filesystem. I forgot to
update the changelog. But after Dave's comments I think it can go back into
VFS. Just lockdep complained about deadlocks in my first naive approach -
that's why I started doing weird things with XFS locks after all.

Anyway now I'm wiser regarding XFS locking and I also have better idea
how to achive proper lock ordering in VFS. Just we are finishing SLE11 SP2
so I didn't get to writing the patches last week... But I should get to it
maybe even today and if not then at least during this week ;)

Honza

> > Acked-by: "Theodore Ts'o" <tytso@mit.edu>
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> > fs/buffer.c | 22 ++++------------------
> > mm/filemap.c | 3 ++-
> > 2 files changed, 6 insertions(+), 19 deletions(-)
> >
> > diff --git a/fs/buffer.c b/fs/buffer.c
> > index 19d8eb7..550714d 100644
> > --- a/fs/buffer.c
> > +++ b/fs/buffer.c
> > @@ -2338,8 +2338,8 @@ EXPORT_SYMBOL(block_commit_write);
> > * beyond EOF, then the page is guaranteed safe against truncation until we
> > * unlock the page.
> > *
> > - * Direct callers of this function should call vfs_check_frozen() so that page
> > - * fault does not busyloop until the fs is thawed.
> > + * Direct callers of this function should protect against filesystem freezing
> > + * using sb_start_write() - sb_end_write() functions.
> > */
> > int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
> > get_block_t get_block)
> > @@ -2371,18 +2371,7 @@ int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
> >
> > if (unlikely(ret < 0))
> > goto out_unlock;
> > - /*
> > - * Freezing in progress? We check after the page is marked dirty and
> > - * with page lock held so if the test here fails, we are sure freezing
> > - * code will wait during syncing until the page fault is done - at that
> > - * point page will be dirty and unlocked so freezing code will write it
> > - * and writeprotect it again.
> > - */
> > set_page_dirty(page);
> > - if (inode->i_sb->s_frozen != SB_UNFROZEN) {
> > - ret = -EAGAIN;
> > - goto out_unlock;
> > - }
> > wait_on_page_writeback(page);
> > return 0;
> > out_unlock:
> > @@ -2397,12 +2386,9 @@ int block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
> > int ret;
> > struct super_block *sb = vma->vm_file->f_path.dentry->d_inode->i_sb;
> >
> > - /*
> > - * This check is racy but catches the common case. The check in
> > - * __block_page_mkwrite() is reliable.
> > - */
> > - vfs_check_frozen(sb, SB_FREEZE_WRITE);
> > + sb_start_write(sb, SB_FREEZE_WRITE);
> > ret = __block_page_mkwrite(vma, vmf, get_block);
> > + sb_end_write(sb, SB_FREEZE_WRITE);
> > return block_page_mkwrite_return(ret);
> > }
> > EXPORT_SYMBOL(block_page_mkwrite);
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index c0018f2..471b9ae 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -2529,7 +2529,7 @@ ssize_t __generic_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
> > count = ocount;
> > pos = *ppos;
> >
> > - vfs_check_frozen(inode->i_sb, SB_FREEZE_WRITE);
> > + sb_start_write(inode->i_sb, SB_FREEZE_WRITE);
> >
> > /* We can write back this queue in page reclaim */
> > current->backing_dev_info = mapping->backing_dev_info;
> > @@ -2601,6 +2601,7 @@ ssize_t __generic_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
> > pos, ppos, count, written);
> > }
> > out:
> > + sb_end_write(inode->i_sb, SB_FREEZE_WRITE);
> > current->backing_dev_info = NULL;
> > return written ? written : err;
> > }
>
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR