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Re: Introduce SEEK_DATA/SEEK_HOLE to XFS V5

To: Mark Tinguely <tinguely@xxxxxxx>
Subject: Re: Introduce SEEK_DATA/SEEK_HOLE to XFS V5
From: Jeff Liu <jeff.liu@xxxxxxxxxx>
Date: Fri, 13 Jan 2012 10:41:52 +0800
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, Ben Myers <bpm@xxxxxxx>, Chris Mason <chris.mason@xxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <4F0EF5DC.1070207@xxxxxxx>
Organization: Oracle
References: <4F06F71A.2010301@xxxxxxxxxx> <20120110171855.GX6390@xxxxxxx> <4F0D21E5.7010908@xxxxxxxxxx> <4F0DFB20.7030704@xxxxxxx> <4F0EE5B6.2040408@xxxxxxxxxx> <4F0EF5DC.1070207@xxxxxxx>
Reply-to: jeff.liu@xxxxxxxxxx
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On 01/12/2012 11:01 PM, Mark Tinguely wrote:

> 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;

Good catch!
Looks we need to examine the max value between seekoff and
map->br_startoff, before passing it to xfs_has_unwritten_buffer().

For SEEK_DATA, if the seekoff is less than map->br_startoff, IMHO, we
need to pass the map->br_startoff to it.

> 
> 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.

By combining with all your comments below, now I feel a bits clear about
your opinions. :)
I think it is definitely needed if we continue to use the current idea,
i.e, probing the unwritten extent twice(DIRTY, WRITEBACK).

Thanks,
-Jeff

> 
> 
>>>
>>> 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.
> 
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