[PATCH 0/2] Fix data corruption when blocksize < pagesize

[PATCH 0/2] Fix data corruption when blocksize < pagesize

[Jan Kara]

  Hello,

  these two patches fix the data corruption triggered by xfstests
generic/030 test for ext4. I believe XFS can use the same function to
deal with the problem... Dave can you verify? If the function is indeed
usable for XFS as well, through which tree are we going to merge it?
ext4 or xfs?

								Honza

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[PATCH 1/2] vfs: Fix data corruption when blocksize < pagesize for mmaped data

[Jan Kara]

->page_mkwrite() is used by filesystems to allocate blocks under a page
which is becoming writeably mmapped in some process' address space. This
allows a filesystem to return a page fault if there is not enough space
available, user exceeds quota or similar problem happens, rather than
silently discarding data later when writepage is called.

However VFS fails to call ->page_mkwrite() in all the cases where
filesystems need it when blocksize < pagesize. For example when
blocksize = 1024, pagesize = 4096 the following is problematic:
  ftruncate(fd, 0);
  pwrite(fd, buf, 1024, 0);
  map = mmap(NULL, 1024, PROT_WRITE, MAP_SHARED, fd, 0);
  map[0] = 'a';       ----> page_mkwrite() for index 0 is called
  ftruncate(fd, 10000); /* or even pwrite(fd, buf, 1, 10000) */
  mremap(map, 1024, 10000, 0);
  map[4095] = 'a';    ----> no page_mkwrite() called

At the moment ->page_mkwrite() is called, filesystem can allocate only
one block for the page because i_size == 1024. Otherwise it would create
blocks beyond i_size which is generally undesirable. But later at
->writepage() time, we also need to store data at offset 4095 but we
don't have block allocated for it.

This patch introduces a helper function filesystems can use to have
->page_mkwrite() called at all the necessary moments.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/buffer.c                 | 57 +++++++++++++++++++++++++++++++++++++++++++++
 include/linux/buffer_head.h |  7 ++++++
 2 files changed, 64 insertions(+)

diff --git a/fs/buffer.c b/fs/buffer.c
index 8f05111bbb8b..2e3a1190dd0a 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -41,6 +41,7 @@
 #include <linux/bitops.h>
 #include <linux/mpage.h>
 #include <linux/bit_spinlock.h>
+#include <linux/rmap.h>
 #include <trace/events/block.h>
 
 static int fsync_buffers_list(spinlock_t *lock, struct list_head *list);
@@ -2010,6 +2011,59 @@ static int __block_commit_write(struct inode *inode, struct page *page,
 	return 0;
 }
 
+#ifdef CONFIG_MMU
+/**
+ * block_create_hole - handle creation of a hole in a file
+ * @inode:	inode where the hole is created
+ * @from:	offset in bytes where the hole starts
+ * @to:		offset in bytes where the hole ends.
+ *
+ * Handle creation of a hole in a file either caused by extending truncate or
+ * by write starting after current i_size. We mark the page straddling @from RO
+ * so that page_mkwrite() is called on the nearest write access to the page.
+ * This way filesystem can be sure that page_mkwrite() is called on the page
+ * before user writes to the page via mmap after the i_size has been changed.
+ *
+ * This function must be called after i_size is updated so that page_mkwrite()
+ * happenning immediately after we unlock the page initializes it correctly.
+ * Also the function must be called while we still hold i_mutex - this not only
+ * makes sure i_size is stable but also that userspace cannot observe new
+ * i_size value before we are prepared to store mmap writes at new inode size.
+ */
+void block_create_hole(struct inode *inode, loff_t from, loff_t to)
+{
+	int bsize = 1 << inode->i_blkbits;
+	loff_t rounded_from;
+	struct page *page;
+	pgoff_t index;
+
+	WARN_ON(!mutex_is_locked(&inode->i_mutex));
+	WARN_ON(to > inode->i_size);
+
+	if (from >= to || bsize == PAGE_CACHE_SIZE)
+		return;
+	/* Currently last page will not have any hole block created? */
+	rounded_from = ALIGN(from, bsize);
+	if (to <= rounded_from || !(rounded_from & (PAGE_CACHE_SIZE - 1)))
+		return;
+
+	index = from >> PAGE_CACHE_SHIFT;
+	page = find_lock_page(inode->i_mapping, index);
+	/* Page not cached? Nothing to do */
+	if (!page)
+		return;
+	/*
+	 * See clear_page_dirty_for_io() for details why set_page_dirty()
+	 * is needed.
+	 */
+	if (page_mkclean(page))
+		set_page_dirty(page);
+	unlock_page(page);
+	page_cache_release(page);
+}
+EXPORT_SYMBOL(block_create_hole);
+#endif
+
 /*
  * block_write_begin takes care of the basic task of block allocation and
  * bringing partial write blocks uptodate first.
@@ -2080,6 +2134,7 @@ int generic_write_end(struct file *file, struct address_space *mapping,
 			struct page *page, void *fsdata)
 {
 	struct inode *inode = mapping->host;
+	loff_t old_size = inode->i_size;
 	int i_size_changed = 0;
 
 	copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);
@@ -2099,6 +2154,8 @@ int generic_write_end(struct file *file, struct address_space *mapping,
 	unlock_page(page);
 	page_cache_release(page);
 
+	if (old_size < pos)
+		block_create_hole(inode, old_size, pos);
 	/*
 	 * Don't mark the inode dirty under page lock. First, it unnecessarily
 	 * makes the holding time of page lock longer. Second, it forces lock
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 324329ceea1e..b4f79eeca7c5 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -244,6 +244,13 @@ static inline int block_page_mkwrite_return(int err)
 	/* -ENOSPC, -EDQUOT, -EIO ... */
 	return VM_FAULT_SIGBUS;
 }
+#ifdef CONFIG_MMU
+void block_create_hole(struct inode *inode, loff_t from, loff_t to);
+#else
+static inline void block_create_hole(struct inode *inode, loff_t from, loff_t to)
+{
+}
+#endif
 sector_t generic_block_bmap(struct address_space *, sector_t, get_block_t *);
 int block_truncate_page(struct address_space *, loff_t, get_block_t *);
 int nobh_write_begin(struct address_space *, loff_t, unsigned, unsigned,
-- 
1.8.1.4

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Re: [PATCH 1/2] vfs: Fix data corruption when blocksize < pagesize for mmaped data

[Dave Chinner]

On Tue, Sep 23, 2014 at 05:03:22PM +0200, Jan Kara wrote:
> ->page_mkwrite() is used by filesystems to allocate blocks under a page
> which is becoming writeably mmapped in some process' address space. This
> allows a filesystem to return a page fault if there is not enough space
> available, user exceeds quota or similar problem happens, rather than
> silently discarding data later when writepage is called.
> 
> However VFS fails to call ->page_mkwrite() in all the cases where
> filesystems need it when blocksize < pagesize. For example when
> blocksize = 1024, pagesize = 4096 the following is problematic:
>   ftruncate(fd, 0);
>   pwrite(fd, buf, 1024, 0);
>   map = mmap(NULL, 1024, PROT_WRITE, MAP_SHARED, fd, 0);
>   map[0] = 'a';       ----> page_mkwrite() for index 0 is called
>   ftruncate(fd, 10000); /* or even pwrite(fd, buf, 1, 10000) */
>   mremap(map, 1024, 10000, 0);
>   map[4095] = 'a';    ----> no page_mkwrite() called
> 
> At the moment ->page_mkwrite() is called, filesystem can allocate only
> one block for the page because i_size == 1024. Otherwise it would create
> blocks beyond i_size which is generally undesirable. But later at
> ->writepage() time, we also need to store data at offset 4095 but we
> don't have block allocated for it.
...
>  
> +#ifdef CONFIG_MMU
> +/**
> + * block_create_hole - handle creation of a hole in a file
> + * @inode:	inode where the hole is created
> + * @from:	offset in bytes where the hole starts
> + * @to:		offset in bytes where the hole ends.

This function doesn't create holes.  It also manipulates page state,
not block state.  Probably could do with a better name, but I'm not
sure what a better name is - something like
pagecache_extend_isize(old_eof, new_eof)?


> +void block_create_hole(struct inode *inode, loff_t from, loff_t to)
> +{
> +	int bsize = 1 << inode->i_blkbits;
> +	loff_t rounded_from;
> +	struct page *page;
> +	pgoff_t index;
> +
> +	WARN_ON(!mutex_is_locked(&inode->i_mutex));
> +	WARN_ON(to > inode->i_size);

We've already changed i_size, so shouldn't that be:

	WARN_ON(to != inode->i_size);

> +
> +	if (from >= to || bsize == PAGE_CACHE_SIZE)
> +		return;
> +	/* Currently last page will not have any hole block created? */
> +	rounded_from = ALIGN(from, bsize);

That rounds down? or up? round_down/round_up are much better than
ALIGN() because they tell you exactly what rounding was intended...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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Re: [PATCH 1/2] vfs: Fix data corruption when blocksize < pagesize for mmaped data

[Jan Kara]

On Thu 25-09-14 11:32:16, Dave Chinner wrote:
> On Tue, Sep 23, 2014 at 05:03:22PM +0200, Jan Kara wrote:
> > ->page_mkwrite() is used by filesystems to allocate blocks under a page
> > which is becoming writeably mmapped in some process' address space. This
> > allows a filesystem to return a page fault if there is not enough space
> > available, user exceeds quota or similar problem happens, rather than
> > silently discarding data later when writepage is called.
> > 
> > However VFS fails to call ->page_mkwrite() in all the cases where
> > filesystems need it when blocksize < pagesize. For example when
> > blocksize = 1024, pagesize = 4096 the following is problematic:
> >   ftruncate(fd, 0);
> >   pwrite(fd, buf, 1024, 0);
> >   map = mmap(NULL, 1024, PROT_WRITE, MAP_SHARED, fd, 0);
> >   map[0] = 'a';       ----> page_mkwrite() for index 0 is called
> >   ftruncate(fd, 10000); /* or even pwrite(fd, buf, 1, 10000) */
> >   mremap(map, 1024, 10000, 0);
> >   map[4095] = 'a';    ----> no page_mkwrite() called
> > 
> > At the moment ->page_mkwrite() is called, filesystem can allocate only
> > one block for the page because i_size == 1024. Otherwise it would create
> > blocks beyond i_size which is generally undesirable. But later at
> > ->writepage() time, we also need to store data at offset 4095 but we
> > don't have block allocated for it.
> ...
> >  
> > +#ifdef CONFIG_MMU
> > +/**
> > + * block_create_hole - handle creation of a hole in a file
> > + * @inode:	inode where the hole is created
> > + * @from:	offset in bytes where the hole starts
> > + * @to:		offset in bytes where the hole ends.
> 
> This function doesn't create holes.  It also manipulates page state,
> not block state.  Probably could do with a better name, but I'm not
> sure what a better name is - something like
> pagecache_extend_isize(old_eof, new_eof)?
  Yeah, you are right. I should be actually better off moving that function
to mm/truncate.c. Regarding the name I agree block_create_hole() isn't
very accurate but I don't like pagecache_extend_isize() too much either -
see below for reason.

> > +void block_create_hole(struct inode *inode, loff_t from, loff_t to)
> > +{
> > +	int bsize = 1 << inode->i_blkbits;
> > +	loff_t rounded_from;
> > +	struct page *page;
> > +	pgoff_t index;
> > +
> > +	WARN_ON(!mutex_is_locked(&inode->i_mutex));
> > +	WARN_ON(to > inode->i_size);
> 
> We've already changed i_size, so shouldn't that be:
  Not quite. When you have 1k blocksize, filesize == 512 and you do
pwrite(file, buf, 1024, 8192);
  Then you want the function to be called for range 512 - 8192, however
i_size is already 9216. So the assertion is correct as is. This is also
the reason why I don't like pagecache_extend_isize() because it suggests
'to' is the final i_size but it's not.

Maybe the most simple interface would be to really call the function
pagecache_extend_isize(), let it take just 'to' and it will write the new
i_size and handle pagecache tricks. The extending writes will then be
handled by first calling pagecache_extend_isize() to extend to 'pos' and
then just i_size_write() the final size...

								Honza
> 	WARN_ON(to != inode->i_size);
> 
> > +
> > +	if (from >= to || bsize == PAGE_CACHE_SIZE)
> > +		return;
> > +	/* Currently last page will not have any hole block created? */
> > +	rounded_from = ALIGN(from, bsize);
> 
> That rounds down? or up? round_down/round_up are much better than
> ALIGN() because they tell you exactly what rounding was intended...
  Good point. I'll use round_up().

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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[PATCH 2/2] ext4: Fix mmap data corruption when blocksize < pagesize

[Jan Kara]

Use block_create_hole() when hole is being created in a file so that
->page_mkwrite() will get called for the partial tail page if it is
mmaped (see the first patch in the series for details).

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/ext4/inode.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 3aa26e9117c4..fdcb007c2c9e 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4536,8 +4536,12 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
 				ext4_orphan_del(NULL, inode);
 				goto err_out;
 			}
-		} else
+		} else {
+			loff_t old_size = inode->i_size;
+
 			i_size_write(inode, attr->ia_size);
+			block_create_hole(inode, old_size, inode->i_size);
+		}
 
 		/*
 		 * Blocks are going to be removed from the inode. Wait
-- 
1.8.1.4

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Re: [PATCH 2/2] ext4: Fix mmap data corruption when blocksize < pagesize

[Jan Kara]

On Tue 23-09-14 17:03:23, Jan Kara wrote:
> Use block_create_hole() when hole is being created in a file so that
> ->page_mkwrite() will get called for the partial tail page if it is
> mmaped (see the first patch in the series for details).
  Just out of curiosity I did a change similar to this one for ext4 to XFS
and indeed it fixed generic/030 test failures for XFS with blocksize 1k.

								Honza

PS: I forgot to CC xfs list in the original posting. You can find the VFS
    patch e.g. at http://www.spinics.net/lists/linux-mm/msg78976.html

> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/ext4/inode.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 3aa26e9117c4..fdcb007c2c9e 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -4536,8 +4536,12 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
>  				ext4_orphan_del(NULL, inode);
>  				goto err_out;
>  			}
> -		} else
> +		} else {
> +			loff_t old_size = inode->i_size;
> +
>  			i_size_write(inode, attr->ia_size);
> +			block_create_hole(inode, old_size, inode->i_size);
> +		}
>  
>  		/*
>  		 * Blocks are going to be removed from the inode. Wait
> -- 
> 1.8.1.4
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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Re: [PATCH 2/2] ext4: Fix mmap data corruption when blocksize < pagesize

[Jan Kara]

[-- Attachment #1: Type: text/plain, Size: 1624 bytes --]

On Wed 24-09-14 10:45:19, Jan Kara wrote:
> On Tue 23-09-14 17:03:23, Jan Kara wrote:
> > Use block_create_hole() when hole is being created in a file so that
> > ->page_mkwrite() will get called for the partial tail page if it is
> > mmaped (see the first patch in the series for details).
>   Just out of curiosity I did a change similar to this one for ext4 to XFS
> and indeed it fixed generic/030 test failures for XFS with blocksize 1k.
  Just for reference attached the patch I was testing - I can resend with
proper changelog etc. if people are fine with this approach.

								Honza

> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  fs/ext4/inode.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 3aa26e9117c4..fdcb007c2c9e 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -4536,8 +4536,12 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
> >  				ext4_orphan_del(NULL, inode);
> >  				goto err_out;
> >  			}
> > -		} else
> > +		} else {
> > +			loff_t old_size = inode->i_size;
> > +
> >  			i_size_write(inode, attr->ia_size);
> > +			block_create_hole(inode, old_size, inode->i_size);
> > +		}
> >  
> >  		/*
> >  		 * Blocks are going to be removed from the inode. Wait
> > -- 
> > 1.8.1.4
> > 
> -- 
> Jan Kara <jack@suse.cz>
> SUSE Labs, CR
> --
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-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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