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Date:   Thu, 29 Sep 2022 11:13:26 +0800
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Subject: Re: [PATCH] btrfs:remove redundant index_rbio_pages in
 raid56_rmw_stripe
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From:   Qu Wenruo <wqu@suse.com>
To:     "Flint.Wang" <hmsjwzb@zoho.com>
Cc:     stringbox8@zoho.com, Chris Mason <clm@fb.com>,
        Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>, linux-btrfs@vger.kernel.org,
        linux-kernel@vger.kernel.org
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On 2022/9/29 11:08, Qu Wenruo wrote:
> 
> 
> On 2022/9/29 09:44, Flint.Wang wrote:
>>    The index_rbio_pages in raid56_rmw_stripe is redundant.
> 
> index_rbio_pages() is to populate the rbio->bio_sectors array.
> 
> In raid56_rmw_stripe() we later calls sector_in_rbio(), which will check 
> if a sector is belonging to bio_lists.
> 
> If not called, all sector will be returned using the sectors in 
> rbio->bio_sectors, not using the sectors in bio lists.
> 
> Have you tried your patch with fstests runs?

Well, for raid56_rmw_stripe() it's fine, as without the 
index_rbio_pages() call, we just read all the sectors from the disk.

This would include the new pages from bio lists.

It would only cause extra IO, but since they can all be merged into one 
64K stripe, it should not cause performance problem.

Furthermore it would read all old sectors from disk, allowing us later 
to do the verification before doing the writes.

But it should really contain a more detailed explanation.

Thanks,
Qu
> 
> IMHO it should fail a lot of very basic writes in RAID56.
> 
> Thanks,
> Qu
> 
>>    It is invoked in finish_rmw anyway.
>>
>> Signed-off-by: Flint.Wang <hmsjwzb@zoho.com>
>> ---
>>   fs/btrfs/raid56.c | 2 --
>>   1 file changed, 2 deletions(-)
>>
>> diff --git a/fs/btrfs/raid56.c b/fs/btrfs/raid56.c
>> index f6395e8288d69..44266b2c5b86e 100644
>> --- a/fs/btrfs/raid56.c
>> +++ b/fs/btrfs/raid56.c
>> @@ -1546,8 +1546,6 @@ static int raid56_rmw_stripe(struct 
>> btrfs_raid_bio *rbio)
>>       if (ret)
>>           goto cleanup;
>> -    index_rbio_pages(rbio);
>> -
>>       atomic_set(&rbio->error, 0);
>>       /* Build a list of bios to read all the missing data sectors. */
>>       for (total_sector_nr = 0; total_sector_nr < nr_data_sectors;