linux-kernel - Re: [PATCH 2/2] fuse: wait for writeback in fuse_file

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Date:	Tue, 13 Aug 2013 09:23:30 -0400
From:	Brian Foster <bfoster@...hat.com>
To:	Maxim Patlasov <mpatlasov@...allels.com>
CC:	miklos@...redi.hu, fuse-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, devel@...nvz.org, xemul@...allels.com
Subject: Re: [PATCH 2/2] fuse: wait for writeback in fuse_file_fallocate()

On 08/13/2013 08:56 AM, Maxim Patlasov wrote:
> Hi,
> 
> 08/13/2013 04:05 PM, Brian Foster пишет:
>> ...
>> @@ -2478,8 +2516,11 @@ static long fuse_file_fallocate(struct file
>> *file, int mode, loff_t offset,
>>         if (lock_inode) {
>>           mutex_lock(&inode->i_mutex);
>> -        if (mode & FALLOC_FL_PUNCH_HOLE)
>> -            fuse_set_nowrite(inode);
>> +        if (mode & FALLOC_FL_PUNCH_HOLE) {
>> +            truncate_pagecache_range(inode, offset,
>> +                         offset + length - 1);
>> +            fuse_wait_on_writeback(inode, offset, length);
>> +        }
>> If this happens to be the first attempt on an fs that doesn't support
>> fallocate, we'll return -EOPNOTSUPP after having already punched out the
>> data in the pagecache.
> 
> Yes, this is unpleasant, but it's not critical, imo. We're returning an
> error code (even though equal to -EOPNOTSUPP) and a sane application
> should not make any assumption about current state of the punched
> region. Also, the application intended to discard given region of the
> file, so why should it pay care for its content afterwards?
> 

I agree, though most users probably wouldn't expect that a blatant error
like EOPNOTSUPP leave the range in a weird state. What's more, it only
does so if it's the first attempt and behaves more appropriately after
that.

>> What about replacing the nowrite logic with a
>> flush (and still followed by your new writeback wait logic) rather than
>> moving the pagecache truncate?
> 
> The "flush" you mentioned should firstly flush page cache.
> invalidate_inode_pages2_range() seems to be a candidate. We definitely
> cannot ignore error code from it because it can be fuse_launder_page()
> who got -ENOMEM from fuse_writepage_locked(). In case of err == -ENOMEM,
> we could safely fail fallocate, but what should we do if it's -EBUSY?
> Any ideas?
> 

I was referring to something like filemap_write_and_wait_range(), for
example. Then continue to use truncate_pagecache_range() as we do today.
Thoughts?

Brian

> Thanks,
> Maxim

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