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Re: SEEK_DATA/SEEK_HOLE support

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: SEEK_DATA/SEEK_HOLE support
From: Jeff Liu <jeff.liu@xxxxxxxxxx>
Date: Wed, 05 Oct 2011 21:56:08 +0800
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20111005092329.GP3159@dastard>
Organization: Oracle
References: <4E887D7F.2010306@xxxxxxxxxx> <20111002154259.GA14543@xxxxxxxxxxxxx> <4E888C0D.9060701@xxxxxxxxxx> <20111002175902.GA9420@xxxxxxxxxxxxx> <4E89343B.4030007@xxxxxxxxxx> <20111003234305.GM3159@dastard> <20111004130208.GA19263@xxxxxxxxxxxxx> <20111005043659.GO3159@dastard> <4E8BEBDB.8040207@xxxxxxxxxx> <20111005092329.GP3159@dastard>
Reply-to: jeff.liu@xxxxxxxxxx
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On 10/05/2011 05:23 PM, Dave Chinner wrote:

> On Tue, Oct 04, 2011 at 10:32:11PM -0700, Jeff Liu wrote:
>> On 10/05/2011 12:36 PM, Dave Chinner wrote:
>>
>>> On Tue, Oct 04, 2011 at 09:02:08AM -0400, Christoph Hellwig wrote:
>>>> On Tue, Oct 04, 2011 at 10:43:05AM +1100, Dave Chinner wrote:
>>>>> The lookup is pretty simple - if there's cached data over the
>>>>> unwritten range, then I'm considering it a data range. If there's no
>>>>> cached data over the unwritten extent, it's a hole. That makes the
>>>>> lookup simply a case of finding the first cached page in the
>>>>> unwritten extent.
>>>>>
>>>>> It'll end up reading something like this:
>>>>>
>>>>>   iomap = offset_to_extent(offset);
>>>>>   first_index = extent_to_page_index(iomap);
>>>>>
>>>>>   nr_found = pagevec_lookup(&pvec, inode->i_mapping, first_index, 1);
>>>>>   if (!nr_found)
>>>>>           break;
>>>>>
>>>>>   offset = page->index << PAGECACHE_SHIFT;
>>>>>   pagevec_release(&pvec);
>>>>>
>>>>>   /* If we fell off the end of the extent lookup next extent */
>>>>>   if (offset >= end_of_extent(iomap)) {
>>>>>           offset = end_of_extent(iomap);
>>>>>           goto next_extent;
>>>>>   }
>>>>>
>>>>> All the extent manipulations are pretty filesystem specific, so
>>>>> there's not much that can be extracted into generic helper, I
>>>>> think...
>>>>
>>>> Actually pretty similar code will work just fine if you passt the
>>>> start + len of the extents in (which we got from looking it up
>>>> fs-specificly):
>>>>
>>>> Note that we have to look for both dirty and writeback pages to
>>>> make it safe.
>>>
>>> That will only work if you can prevent concurrent unwritten extent
>>> conversion from happening while we do the separate tag lookups on
>>> the range because it requires two radix tree tag lookups rather than
>>> just one index lookup. i.e. miss the dirty page because it went
>>> dirty->writeback during the dirty tag search, and miss the same page
>>> when doing the writeback lookup because it went writeback->clean
>>> very quickly due to IO completion.
>>
>>>
>>> So to stop that from happening, it requires that filesystems can
>>> exclude unwritten extent conversion from happening while a
>>> SEEK_HOLE/SEEK_DATA operation is in progress, and that the
>>> filesystem can safely do mapping tree lookups while providing that
>>> extent tree exclusion.  I know that XFS has no problems here, but
>>> filesystems that use i_mutex for everything might be in trouble.
>>>
>>> Besides, if we just look for pages in the cache over unwritten
>>> extents (i.e. someone has treated it as data already), then it can
>>> be done locklessly without having to worry about page state changes
>>> occurring concurrently...
>>
>> From some backup utility's point of view,  AFAICS, the benefits is,
>> utilites does not need to perform read/write for an unwritten extents
>> without dirty or writeback pages, instead, just call lseek(2) to treat
>> it as hole. 
> 
> Yes, that's a use case for SEEK_HOLE/DATA, but that has nothing to
> do with the filesystem implementation issues involved with page
> state changes. What we nee dto avoid is a page state change from
> causing a SEEK_XXX from missing a page it should have stopped the
> seek at because it changed state concurrently. A page changing from
> dirty to writeback is still a page containing DATA, so if we miss it
> in both the scan for dirty pages and for writeback pages, then the
> backup application will seek over data it should have backed up....
> 
>> The issue is, is it worth to do that? i.e, exclude
>> unwritten extent conversions when SEEK_XXX is ongoing.
> 
> It's a moot point for XFS - we need to hold the ILOCK shared
> while doing the extent tree walk, so we are going to hold off
> unwritten extent conversion as that requires the ILOCK
> exclusively...

Thanks for your clarification!
Now I understood your option. Indeed, if the utility treat unwritten
extents as data, we do not need to worry about the pages state
conversion procedures. Otherwise, we need to hold off that until
SEEK_XXX iteration done to avoid missing a page copy.


Thanks,
-Jeff

> 
> Cheers,
> 
> Dave.
> 


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