Re: [PATCH] shmem: call set_page_dirty() with locked page

[Wu Fengguang]

On Wed, Jul 15, 2009 at 10:22:46PM +0800, Hugh Dickins wrote:
> On Wed, 15 Jul 2009, Wu Fengguang wrote:
> > shmem: call set_page_dirty() with locked page
> > 
> > The dirtying of page and set_page_dirty() can be moved into the page lock.
> > 
> > - In shmem_write_end(), the page was dirtied while the page lock was held,
> >   but it's being marked dirty just after dropping the page lock.
> > - In shmem_symlink(), both dirtying and marking can be moved into page lock.
> > 
> > It's valuable for the hwpoison code to know whether one bad page can be dropped
> > without losing data. It mainly judges by testing the PG_dirty bit after taking
> > the page lock. So it becomes important that the dirtying of page and the
> > marking of dirtiness are both done inside the page lock. Which is a common
> > practice, but sadly not a rule.
> > 
> > The noticeable exceptions are
> > - mapped pages
> > - pages with buffer_heads
> > The above pages could go dirty at any time. Fortunately the hwpoison will
> > unmap the page and release the buffer_heads beforehand anyway.
> > 
> > Many other types of pages (eg. metadata pages) can also be dirtied at will by
> > their owners, the hwpoison code cannot do meaningful things to them anyway. 
> > Only the dirtiness of pagecache pages owned by regular files are interested.
> > 
> > CC: Hugh Dickins <hugh@xxxxxxxxxxx>
> 
> Ah, those were the days... ;)

Ah yeah :)

> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> 
> Acked-by: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx>

Thanks for you kind review!

---
shmem: call set_page_dirty() with locked page

The dirtying of page and set_page_dirty() can be moved into the page lock.

- In shmem_write_end(), the page was dirtied while the page lock was held,
  but it's being marked dirty just after dropping the page lock.
- In shmem_symlink(), both dirtying and marking can be moved into page lock.

It's valuable for the hwpoison code to know whether one bad page can be dropped
without losing data. It mainly judges by testing the PG_dirty bit after taking
the page lock. So it becomes important that the dirtying of page and the
marking of dirtiness are both done inside the page lock. Which is a common
practice, but sadly not a rule.

The noticeable exceptions are
- mapped pages
- pages with buffer_heads
The above pages could go dirty at any time. Fortunately the hwpoison will
unmap the page and release the buffer_heads beforehand anyway.

Many other types of pages (eg. metadata pages) can also be dirtied at will by
their owners, the hwpoison code cannot do meaningful things to them anyway. 
Only the dirtiness of pagecache pages owned by regular files are interested.

Acked-by: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 mm/shmem.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- linux.orig/mm/shmem.c
+++ linux/mm/shmem.c
@@ -1630,8 +1630,8 @@ shmem_write_end(struct file *file, struc
 	if (pos + copied > inode->i_size)
 		i_size_write(inode, pos + copied);
 
-	unlock_page(page);
 	set_page_dirty(page);
+	unlock_page(page);
 	page_cache_release(page);
 
 	return copied;
@@ -1968,13 +1968,13 @@ static int shmem_symlink(struct inode *d
 			iput(inode);
 			return error;
 		}
-		unlock_page(page);
 		inode->i_mapping->a_ops = &shmem_aops;
 		inode->i_op = &shmem_symlink_inode_operations;
 		kaddr = kmap_atomic(page, KM_USER0);
 		memcpy(kaddr, symname, len);
 		kunmap_atomic(kaddr, KM_USER0);
 		set_page_dirty(page);
+		unlock_page(page);
 		page_cache_release(page);
 	}
 	if (dir->i_mode & S_ISGID)
--
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