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SubjectRe: [PATCH mmotm] nilfs2: insert checks and hole block allocation in page_mkwrite
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On Wed, 07 Jan 2009 09:43:02 -0500, Chris Mason wrote:
> On Wed, 2009-01-07 at 22:06 +0900, Ryusuke Konishi wrote:
> > Chris Mason told me that page_mkwrite() has some works to do.
> >
> > On Wed, 17 Dec 2008 21:52:55 -0500, Chris Mason wrote:
> > > nilfs_page_mkwrite doesn't seem to dirty the page? block_page_mkwrite
> > > does more, including checks against i_size and others.
> >
> > This will insert i_size check, and hole block allocation code in the
> > nilfs_page_mkwrite.
> >
> > Previously, the hole block allocation was delayed until just before
> > writing for mmapped pages. This accompanies removal of the delayed
> > allocation code.
<snip>
> > static int nilfs_page_mkwrite(struct vm_area_struct *vma, struct page *page)
> > {
> > - if (!(vma->vm_flags & (VM_WRITE | VM_MAYWRITE)))
> > - return -EPERM;
> > + struct inode *inode = vma->vm_file->f_dentry->d_inode;
> > + struct address_space *mapping = inode->i_mapping;
> > + struct nilfs_transaction_info ti;
> > + struct buffer_head *bh, *head;
> > + int fully_mapped = 1;
> > + int ret = -EINVAL;
> > +
> > + /*
> > + * use i_alloc_sem to stop truncate operations on the inode
> > + */
> > + down_read(&inode->i_alloc_sem);
>
> I'm not 100% sure this is safe. It seems likely the direct io paths
> could trigger a mkwrite with the i_alloc_sem already held?

vmtruncate() can change i_size outside the page lock, and it even
calls unmap_mapping_range(). Does it have any problems?

> If I'm reading the code correctly you're:
>
> * grab i_alloc sem to protect against truncate
> * check to see if the page is mapped already (no holes)
>
> * if there are holes, lock the page, start transaction and do the full
> block_page_mkwrite
>
> * drop i_alloc_sem

Yes, it is.

> Would this work instead?:
>
> * lock the page, check to see if it is mapped
>
> * if there are holes, drop the lock, start transaction and do the full
> block_page_mkwrite
>
> -chris

If we don't need care for vmtruncate(), I think it's better, too.

Regards,
Ryusuke


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