Re: [PATCH] ext4:Fix a bug in ext4_ext_fiemap_cb().

[Yongqiang Yang <xiaoqiangnk@gmail.com>]

On Thu, Feb 24, 2011 at 8:40 AM, Yongqiang Yang <xiaoqiangnk@gmail.com> wrote:
> On Thu, Feb 24, 2011 at 12:41 AM, Eric Sandeen <sandeen@redhat.com> wrote:
>> On 2/23/11 9:59 AM, Yongqiang Yang wrote:
>>> 1] Delayed extents after a hole are neglected.
>>>
>>>    By using find_get_pages() instead of find_get_page() to
>>>    lookup pagecache, delayed extents can be found, because
>>>    find_get_pages() with nr_pages=1 will return the next page
>>>    in pagecache.
>>>
>>> 2] Extents after a delayed extent or a hole are neglected as well.
>>>
>>>    Fix it by accurating the request range by the result of
>>>    ext4_ext_next_allocated_block().
>>>
>>> Reported by Chris Mason <chris.mason@oracle.com>:
>>> We've had reports on btrfs that cp is giving us files full of zeros
>>> instead of actually copying them.  It was tracked down to a bug with
>>> the btrfs fiemap implementation where it was returning holes for
>>> delalloc ranges.
>>>
>>> Newer versions of cp are trusting fiemap to tell it where the holes
>>> are, which does seem like a pretty neat trick.
>>>
>>> I decided to give xfs and ext4 a shot with a few tests cases too, xfs
>>> passed with all the ones btrfs was getting wrong, and ext4 got the basic
>>> delalloc case right.
>>> $ mkfs.ext4 /dev/xxx
>>> $ mount /dev/xxx /mnt
>>> $ dd if=/dev/zero of=/mnt/foo bs=1M count=1
>>> $ fiemap-test foo
>>> ext:   0 logical: [       0..     255] phys:        0..     255
>>> flags: 0x007 tot: 256
>>>
>>> Horray!  But once we throw a hole in, things go bad:
>>> $ mkfs.ext4 /dev/xxx
>>> $ mount /dev/xxx /mnt
>>> $ dd if=/dev/zero of=/mnt/foo bs=1M count=1 seek=1
>>> $ fiemap-test foo
>>> < no output >
>>>
>>> We've got a delalloc extent after the hole and ext4 fiemap didn't find
>>> it.  If I run sync to kick the delalloc out:
>>> $sync
>>> $ fiemap-test foo
>>> ext:   0 logical: [     256..     511] phys:    34048..   34303
>>> flags: 0x001 tot: 256
>>>
>>> fiemap-test is sitting in my /usr/local/bin, and I have no idea how it
>>> got there.  It's full of pretty comments so I know it isn't mine, but
>>> you can grab it here:
>>>
>>> http://oss.oracle.com/~mason/fiemap-test.c
>>>
>>> xfsqa has a fiemap program too.
>>>
>>> After Fix, test results are as follows:
>>> ext:   0 logical: [     256..     511] phys:        0..     255
>>> flags: 0x007 tot: 256
>>> ext:   0 logical: [     256..     511] phys:    33280..   33535
>>> flags: 0x001 tot: 256
>>>
>>> Signe-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
>>> ---
>>>  fs/ext4/extents.c |   26 +++++++++++++++++++++++---
>>>  mm/filemap.c      |    1 +
>>>  2 files changed, 24 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
>>> index ccce8a7..ad455a0 100644
>>> --- a/fs/ext4/extents.c
>>> +++ b/fs/ext4/extents.c
>>> @@ -3788,17 +3788,27 @@ static int ext4_ext_fiemap_cb(struct inode *inode, struct ext4_ext_path *path,
>>>       __u64   physical;
>>>       __u64   length;
>>>       __u32   flags = 0;
>>> +     ext4_lblk_t end;
>>>       int     error;
>>>
>>>       logical =  (__u64)newex->ec_block << blksize_bits;
>>>
>>> -     if (newex->ec_start == 0) {
>>> +     if (!newex->ec_start) {
>>> +             /*
>>> +              * There is no extent contains @newex->ec_block block.
>>> +              * It implies that @newex->ec_block block lies 1)a hole
>>> +              * or 2)delayed-allocated blocks that has not been
>>> +              * allocated, so pagecache is needed to lookup.
>>> +              *
>>> +              * And if it is case 2, @newex->ec_len needs to be corrected.
>>> +              *
>>> +              */
>>>               pgoff_t offset;
>>>               struct page *page;
>>>               struct buffer_head *bh = NULL;
>>>
>>>               offset = logical >> PAGE_SHIFT;
>>> -             page = find_get_page(inode->i_mapping, offset);
>>> +             (void)find_get_pages(inode->i_mapping, offset, 1, &page);
>>>               if (!page || !page_has_buffers(page))
>>>                       return EXT_CONTINUE;
>>>
>>> @@ -3807,8 +3817,13 @@ static int ext4_ext_fiemap_cb(struct inode *inode, struct ext4_ext_path *path,
>>>               if (!bh)
>>>                       return EXT_CONTINUE;
>>>
>>> +             /* Assume block-size equals page-size. */
>>>               if (buffer_delay(bh)) {
>>>                       flags |= FIEMAP_EXTENT_DELALLOC;
>>> +                     if (page->index > offset) {
>>> +                             logical =  ((__u64)page->index << PAGE_SHIFT);
>>> +                             newex->ec_block = logical >> blksize_bits;
>>> +                     }
>>>                       page_cache_release(page);
>>>               } else {
>>>                       page_cache_release(page);
>>> @@ -3830,7 +3845,8 @@ static int ext4_ext_fiemap_cb(struct inode *inode, struct ext4_ext_path *path,
>>>        *
>>>        * XXX this might miss a single-block extent at EXT_MAX_BLOCK
>>>        */
>>> -     if (ext4_ext_next_allocated_block(path) == EXT_MAX_BLOCK ||
>>> +     end = ext4_ext_next_allocated_block(path);
>>
>> I think this will fall down if you have:
>>
>> [ HOLE ][ DELALLOC ][ HOLE ][ ALLOCATED ] won't it?
>>
>> i.e. your "end" will be the first block of "allocated" right?
> We use pagevec_lookup_tag() instead of find_get_page() and check
> BH_Delay of contiguous pages. Then, we can deal this model.
>
> How do you think?

If we have a function which can get contiguous pages with specified
tag, it will be greater!  I am not sure if adding this function is
allowed.
>>
>> -Eric
>>
>>> +     if (end == EXT_MAX_BLOCK ||
>>>           newex->ec_block + newex->ec_len - 1 == EXT_MAX_BLOCK) {
>>>               loff_t size = i_size_read(inode);
>>>               loff_t bs = EXT4_BLOCK_SIZE(inode->i_sb);
>>> @@ -3839,8 +3855,12 @@ static int ext4_ext_fiemap_cb(struct inode *inode, struct ext4_ext_path *path,
>>>               if ((flags & FIEMAP_EXTENT_DELALLOC) &&
>>>                   logical+length > size)
>>>                       length = (size - logical + bs - 1) & ~(bs-1);
>>> +     } else {
>>> +             newex->ec_len = end - newex->ec_block;
>>> +             length = (__u64)newex->ec_len << blksize_bits;
>>>       }
>>>
>>> +
>>>       error = fiemap_fill_next_extent(fieinfo, logical, physical,
>>>                                       length, flags);
>>>       if (error < 0)
>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>> index 83a45d3..1c01ffc 100644
>>> --- a/mm/filemap.c
>>> +++ b/mm/filemap.c
>>> @@ -803,6 +803,7 @@ repeat:
>>>       rcu_read_unlock();
>>>       return ret;
>>>  }
>>> +EXPORT_SYMBOL(find_get_pages);
>>>
>>>  /**
>>>   * find_get_pages_contig - gang contiguous pagecache lookup
>>
>>
>
>
>
> --
> Best Wishes
> Yongqiang Yang
>



-- 
Best Wishes
Yongqiang Yang
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