Re: Introduce SEEK_DATA/SEEK_HOLE to XFS V5

Mark Tinguely
2012-01-12T15:01:49Z

On 01/12/12 07:52, Jeff Liu wrote:
> Hi Mark,
>
> On 01/12/2012 05:12 AM, Mark Tinguely wrote:
>
>> xfs_has_unwritten_buffer() always returns the offset of the first
>> dirty unwritten page. This can cause xfs_seek_data() and xfs_seek_hole()
>> to give the wrong results in certain circumstances.
>
> Sorry, am was well understood your opinions in this point for now.
> IMHO, we can only find and return the data buffer offset at a dirty or
> unwritten page once the first page was probed.
>

From my tests, xfs_bmapi_read() can only find holes if they cross or
start on a 64KB boundary. It would be nice if unwritten extents were at
least that good at finding holes.

In xfs_has_unwritten_buffer(), could you start searching from the seek
offset? The variable *offset could pass in that seek address and us that
offset as the starting "index" rather than the beginning of the extent?

You start:

index = XFS_FSB_TO_B(mp, map->br_startoff) >> PAGE_CACHE_SHIFT;

Could we do?:

index = XFS_FSB_TO_B(mp, *offset) >> PAGE_CACHE_SHIFT;

And before calling xfs_has_unwritten_buffer():
offset = seekoff;

Also, my idea to find the next data/hole requires that
xfs_has_unwritten_buffer() finds the smallest PAGECACHE_TAG_DIRTY or
PAGECACHE_TAG_WRITEBACK page if any starting at the seek offset.

>>
>> In xfs_seek_data(), every page past first dirty/unwritten page in the
>> unwritten extent will be reported as data.
>
> Hmm, consider the user level utility that make use of SEEK_XXX stuff to
> copy data from an offset in source file:
>
> Generally, it will call xfs_seek_data() firstly,
> if we read an unwritten extent and there is data buffer was probed in
> xfs_seek_data(), it only means we can read file data starting from the
> returned offset of xfs_has_unwritten_buffer().
>
> Then it will call xfs_seek_hole() to calculate this extent length.
> next, a couple of read()/write() will be called in a loop depending on
> the extent length.
>
> [ page 1 ] | [ page 2 ] | [ page 3 ] | .... [ page N ]
> |data offset at page 2|
>
> If we got the data offset from page2, and there is no data at page 3,
> the user utility call read(2) will returns ZERO, and it will break
> immediately.
>

Something like:
loop
s = lseek(fd, off, SEEK_DATA);
if (s == -1)
if we errno == ENXIO
return done /* eof */
else
return errno

e = lseek(fd, s, SEEK_HOLE);
if (e == -1)
return errno

dest = copy from s to e
off = e
end loop (if not eof or other condition)

You will seek for next hole at the found data position. Even if
xfs_has_unwritten_buffer() does the right thing and returns the
dirty/unwritten page starting from seekoff, we need go a page past the
current page (which has data) to look for the next hole.

Something like (again psuedo-code)
loop
offset1 = offset2 = seekoff
xfs_has_unwritten_buffer(seekoff, &offset1, DIRTY)
xfs_has_unwritten_buffer(seekoff, &offset2, WRITEBACK)
d = min(offset1, offset2)

if (d > seekoff OR d == NULL)
return found a hole at seekoff

if (d == seekoff) /* standard case assuming how we
* use SEEK_DATA/SEEK_HOLE
* This is the step your code
* does not perform. It jumps
* to the next extent
*/
seekoff += page size of dirty/writeback **
end while the seekoff < extent size

** here we could jump to the next 64KB boundary and be as accurate as
xfs_bmapi_read().

Good job. This is an important feature.

--Mark Tinguely.