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

    -Eric

    > 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;
    > }



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