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Date:	Wed, 21 Nov 2012 14:48:42 +0100 (CET)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Peng Tao <bergwolf@...il.com>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org,
	gnehzuil.liu@...il.com, tytso@....edu, dmonakhov@...nvz.org
Subject: Re: [PATCH 1/2] ext4: Prevent race while walking extent tree

On Wed, 21 Nov 2012, Peng Tao wrote:

> Date: Wed, 21 Nov 2012 18:46:56 +0800
> From: Peng Tao <bergwolf@...il.com>
> To: Lukas Czerner <lczerner@...hat.com>
> Cc: linux-ext4@...r.kernel.org, gnehzuil.liu@...il.com, tytso@....edu,
>     dmonakhov@...nvz.org
> Subject: Re: [PATCH 1/2] ext4: Prevent race while walking extent tree
> 
> On Tue, Nov 20, 2012 at 10:57 PM, Lukas Czerner <lczerner@...hat.com> wrote:
> > Currently ext4_ext_walk_space() only takes i_data_sem for read when
> > searching for the extent at given block with ext4_ext_find_extent().
> > Then it drops the lock and the extent tree can be changed at will.
> > However later on we're searching for the 'next' extent, but the extent
> > tree might already have changed, so the information might not be
> > accurate.
> >
> > In fact we can hit BUG_ON(end <= start) if the extent got inserted into
> > the tree after the one we found and before the block we were searching
> > for. This has been reproduced by running xfstests 225 in loop on s390x
> > architecture, but theoretically we could hit this on any other
> > architecture as well, but probably not as often.
> >
> > Moreover the extent currently in delayed allocation might be allocated
> > after we search the extent tree and before we search extent status tree
> > delayed buffers resulting in those delayed buffers being completely
> > missed, even though completely written and allocated.
> >
> > We fix all those problems in several steps:
> >
> >  1. remove unnecessary callback indirection
> >  2. rename functions
> >         ext4_ext_walk_space -> ext4_fill_fiemap_extents
> >         ext4_ext_fiemap_cb -> ext4_find_delayed_extent
> >  3. move fiemap_fill_next_extent() into ext4_fill_fiemap_extents()
> >  4. hold the i_data_sem for:
> >         ext4_ext_find_extent()
> >         ext4_ext_next_allocated_block()
> >         ext4_find_delayed_extent()
> >  5. call fiemap_fill_next_extent after releasing the i_data_sem
> >  6. move path reinitialization into the critical section.
> >
> The whole patch looks good to me. Just a few comments bellow.
> 
> > Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> > ---
> >  fs/ext4/ext4_extents.h |   14 ------
> >  fs/ext4/extents.c      |  121 ++++++++++++++++++++++++++----------------------
> >  2 files changed, 65 insertions(+), 70 deletions(-)
> >
> > diff --git a/fs/ext4/ext4_extents.h b/fs/ext4/ext4_extents.h
> > index 603bb11..173b6c5 100644
> > --- a/fs/ext4/ext4_extents.h
> > +++ b/fs/ext4/ext4_extents.h
> > @@ -144,20 +144,6 @@ struct ext4_ext_path {
> >   */
> >
> >  /*
> > - * to be called by ext4_ext_walk_space()
> > - * negative retcode - error
> > - * positive retcode - signal for ext4_ext_walk_space(), see below
> > - * callback must return valid extent (passed or newly created)
> > - */
> > -typedef int (*ext_prepare_callback)(struct inode *, ext4_lblk_t,
> > -                                       struct ext4_ext_cache *,
> > -                                       struct ext4_extent *, void *);
> > -
> > -#define EXT_CONTINUE   0
> > -#define EXT_BREAK      1
> > -#define EXT_REPEAT     2
> > -
> > -/*
> >   * Maximum number of logical blocks in a file; ext4_extent's ee_block is
> >   * __le32.
> >   */
> > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> > index d3dd618..c7e3b58 100644
> > --- a/fs/ext4/extents.c
> > +++ b/fs/ext4/extents.c
> > @@ -109,6 +109,9 @@ static int ext4_split_extent_at(handle_t *handle,
> >                              int split_flag,
> >                              int flags);
> >
> > +static int ext4_find_delayed_extent(struct inode *inode,
> > +                                   struct ext4_ext_cache *newex);
> > +
> >  static int ext4_ext_truncate_extend_restart(handle_t *handle,
> >                                             struct inode *inode,
> >                                             int needed)
> > @@ -1959,27 +1962,35 @@ cleanup:
> >         return err;
> >  }
> >
> > -static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
> > -                              ext4_lblk_t num, ext_prepare_callback func,
> > -                              void *cbdata)
> > +static int ext4_fill_fiemap_extents(struct inode *inode,
> > +                                   ext4_lblk_t block, ext4_lblk_t num,
> > +                                   struct fiemap_extent_info *fieinfo)
> >  {
> >         struct ext4_ext_path *path = NULL;
> >         struct ext4_ext_cache cbex;
> >         struct ext4_extent *ex;
> > -       ext4_lblk_t next, start = 0, end = 0;
> > +       ext4_lblk_t next, next_del, start = 0, end = 0;
> >         ext4_lblk_t last = block + num;
> > -       int depth, exists, err = 0;
> > +       int exists, depth = 0, err = 0;
> > +       unsigned int flags = 0;
> > +       unsigned char blksize_bits = inode->i_sb->s_blocksize_bits;
> >
> > -       BUG_ON(func == NULL);
> >         BUG_ON(inode == NULL);
> The BUG_ON() doesn't make any sense. The inode pointer is already used
> in ext4_fiemap() multiple times.

Fair enough, I'll remove it.

> 
> >
> >         while (block < last && block != EXT_MAX_BLOCKS) {
> >                 num = last - block;
> >                 /* find extent for this block */
> >                 down_read(&EXT4_I(inode)->i_data_sem);
> > +
> > +               if (path && ext_depth(inode) != depth) {
> > +                       /* depth was changed. we have to realloc path */
> > +                       kfree(path);
> > +                       path = NULL;
> > +               }
> > +
> >                 path = ext4_ext_find_extent(inode, block, path);
> > -               up_read(&EXT4_I(inode)->i_data_sem);
> >                 if (IS_ERR(path)) {
> > +                       up_read(&EXT4_I(inode)->i_data_sem);
> >                         err = PTR_ERR(path);
> >                         path = NULL;
> >                         break;
> > @@ -1987,13 +1998,16 @@ static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
> >
> >                 depth = ext_depth(inode);
> >                 if (unlikely(path[depth].p_hdr == NULL)) {
> > +                       up_read(&EXT4_I(inode)->i_data_sem);
> >                         EXT4_ERROR_INODE(inode, "path[%d].p_hdr == NULL", depth);
> >                         err = -EIO;
> >                         break;
> >                 }
> >                 ex = path[depth].p_ext;
> >                 next = ext4_ext_next_allocated_block(path);
> > +               ext4_ext_drop_refs(path);
> >
> > +               flags = 0;
> >                 exists = 0;
> >                 if (!ex) {
> >                         /* there is no extent yet, so try to allocate
> > @@ -2037,30 +2051,49 @@ static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
> >                         cbex.ec_block = le32_to_cpu(ex->ee_block);
> >                         cbex.ec_len = ext4_ext_get_actual_len(ex);
> >                         cbex.ec_start = ext4_ext_pblock(ex);
> > +                       if (ext4_ext_is_uninitialized(ex))
> > +                               flags |= FIEMAP_EXTENT_UNWRITTEN;
> >                 }
> >
> > +               next_del = ext4_find_delayed_extent(inode, &cbex);
> May be a line of comment is better here? I was wondering why
> ext4_find_delayed_extent() is called for (!exists) case as well. And
> suddenly I realized that it is to check for the last extent in file
> which will report to user.

Sure, I can add some comment there.

> 
> > +               if (next_del) {
> Need to check EXT_MAX_BLOCKS as well? It seems possible when
> cbex->ec_start is nonzero and no delayed extent found.

Nice catch. Otherwise we could mark extent as delalloc even though
it has already bean allocated. However checking for EXT_MAX_BLOCKS
is wrong since we could miss the last delayed extent that way.
This should be enough

	if (!exists && next_del)


> 
> > +                       exists = 1;
> > +                       flags |= FIEMAP_EXTENT_DELALLOC;
> > +               }
> > +               up_read(&EXT4_I(inode)->i_data_sem);
> > +
> >                 if (unlikely(cbex.ec_len == 0)) {
> >                         EXT4_ERROR_INODE(inode, "cbex.ec_len == 0");
> >                         err = -EIO;
> >                         break;
> >                 }
> > -               err = func(inode, next, &cbex, ex, cbdata);
> > -               ext4_ext_drop_refs(path);
> >
> > -               if (err < 0)
> > -                       break;
> > -
> > -               if (err == EXT_REPEAT)
> > -                       continue;
> > -               else if (err == EXT_BREAK) {
> > -                       err = 0;
> > -                       break;
> > +               /* This is possible iff next == next_del == EXT_MAX_BLOCKS */
> > +               if (next == next_del) {
> > +                       flags |= FIEMAP_EXTENT_LAST;
> > +                       if (unlikely(next_del != EXT_MAX_BLOCKS ||
> > +                                    next != EXT_MAX_BLOCKS)) {
> > +                               EXT4_ERROR_INODE(inode,
> > +                                                "next extent == %u, next "
> > +                                                "delalloc extent = %u",
> > +                                                next, next_del);
> > +                               err = -EIO;
> > +                               break;
> > +                       }
> >                 }
> >
> > -               if (ext_depth(inode) != depth) {
> > -                       /* depth was changed. we have to realloc path */
> > -                       kfree(path);
> > -                       path = NULL;
> > +               if (exists) {
> > +                       err = fiemap_fill_next_extent(fieinfo,
> > +                               (__u64)cbex.ec_block << blksize_bits,
> > +                               (__u64)cbex.ec_start << blksize_bits,
> > +                               (__u64)cbex.ec_len << blksize_bits,
> > +                               flags);
> > +                       if (err < 0)
> > +                               break;
> > +                       if (err == 1) {
> > +                               err = 0;
> > +                               break;
> > +                       }
> >                 }
> >
> >                 block = cbex.ec_block + cbex.ec_len;
> > @@ -4493,26 +4526,19 @@ int ext4_convert_unwritten_extents(struct inode *inode, loff_t offset,
> >  }
> >
> >  /*
> > - * Callback function called for each extent to gather FIEMAP information.
> > + * Find delayed extent at newex->ec_start update newex accordingly and
> > + * return start of next delayed extent, or return zero if no delayed
> > + * extent found.
> + return EXT_MAX_BLOCKS when newex->ec_start is 0 and no delayed extent found.

I am going to rewrite the comment entirely. Hopefully this makes
more sense.

/*
 * If newex is not existing extent (newex->ec_start equals zero) find
 * delayed extent at start of newex and update newex accordingly and
 * return start of the next delayed extent.
 *
 * If newex is existing extent (newex->ec_start is not equal zero)
 * return start of next delayed extent or EXT_MAX_BLOCKS if no delayed
 * extent found. Leave newex unmodified.
 */


> 
> >   */
> > -static int ext4_ext_fiemap_cb(struct inode *inode, ext4_lblk_t next,
> > -                      struct ext4_ext_cache *newex, struct ext4_extent *ex,
> > -                      void *data)
> > +static int ext4_find_delayed_extent(struct inode *inode,
> > +                                   struct ext4_ext_cache *newex)
> >  {
> >         struct extent_status es;
> > -       __u64   logical;
> > -       __u64   physical;
> > -       __u64   length;
> > -       __u32   flags = 0;
> >         ext4_lblk_t next_del;
> > -       int             ret = 0;
> > -       struct fiemap_extent_info *fieinfo = data;
> > -       unsigned char blksize_bits;
> >
> >         es.start = newex->ec_block;
> >         next_del = ext4_es_find_extent(inode, &es);
> Theoritically it is impossible, but given that this can be called when
> extent is already found, it feels safer to add a check
> if (next_del != EXT_MAX_BLOCKS && nexex->ec_start != 0)

This is perfectly valid case isn't it ? We can find delayed extent
after the allocated one.

We would actually need to check whether the two extents (the
original newex and es) overlaps. And if it does, then we have a
problem. But I do not think this is the right place to do such
checks given that it is called only from fiemap.

The least we can do, is to check whether the extent status tree and
extent tree is not pointing to the same "next" extent. This is much
simpler and should be checked only once in fiemap, given that
everything is well. Such check is already in
ext4_fill_fiemap_extents().

> report errors and return 0 here, to detect finding the same range in
> both extent tree and extent status tree. What do you think? If it is
> already done somewhere in the extent status code, pls just ignore me
> :)
> 
> Thanks,
> Tao

Thanks for review! I'll resend the patches.
-Lukas

> 
> >
> > -       next = min(next_del, next);
> >         if (newex->ec_start == 0) {
> >                 /*
> >                  * No extent in extent-tree contains block @newex->ec_start,
> > @@ -4520,37 +4546,19 @@ static int ext4_ext_fiemap_cb(struct inode *inode, ext4_lblk_t next,
> >                  */
> >                 if (es.len == 0)
> >                         /* A hole found. */
> > -                       return EXT_CONTINUE;
> > +                       return 0;
> >
> >                 if (es.start > newex->ec_block) {
> >                         /* A hole found. */
> >                         newex->ec_len = min(es.start - newex->ec_block,
> >                                             newex->ec_len);
> > -                       return EXT_CONTINUE;
> > +                       return 0;
> >                 }
> >
> > -               flags |= FIEMAP_EXTENT_DELALLOC;
> >                 newex->ec_len = es.start + es.len - newex->ec_block;
> >         }
> >
> > -       if (ex && ext4_ext_is_uninitialized(ex))
> > -               flags |= FIEMAP_EXTENT_UNWRITTEN;
> > -
> > -       if (next == EXT_MAX_BLOCKS)
> > -               flags |= FIEMAP_EXTENT_LAST;
> > -
> > -       blksize_bits = inode->i_sb->s_blocksize_bits;
> > -       logical = (__u64)newex->ec_block << blksize_bits;
> > -       physical = (__u64)newex->ec_start << blksize_bits;
> > -       length =   (__u64)newex->ec_len << blksize_bits;
> > -
> > -       ret = fiemap_fill_next_extent(fieinfo, logical, physical,
> > -                                       length, flags);
> > -       if (ret < 0)
> > -               return ret;
> > -       if (ret == 1)
> > -               return EXT_BREAK;
> > -       return EXT_CONTINUE;
> > +       return next_del;
> >  }
> >  /* fiemap flags we can handle specified here */
> >  #define EXT4_FIEMAP_FLAGS      (FIEMAP_FLAG_SYNC|FIEMAP_FLAG_XATTR)
> > @@ -4772,6 +4780,7 @@ out_mutex:
> >         mutex_unlock(&inode->i_mutex);
> >         return err;
> >  }
> > +
> >  int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
> >                 __u64 start, __u64 len)
> >  {
> > @@ -4802,8 +4811,8 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
> >                  * Walk the extent tree gathering extent information.
> >                  * ext4_ext_fiemap_cb will push extents back to user.
> >                  */
> > -               error = ext4_ext_walk_space(inode, start_blk, len_blks,
> > -                                         ext4_ext_fiemap_cb, fieinfo);
> > +               error = ext4_fill_fiemap_extents(inode, start_blk,
> > +                                                len_blks, fieinfo);
> >         }
> >
> >         return error;
> > --
> > 1.7.7.6
> >
> 
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