Date:   Mon, 31 Jul 2017 13:25:32 +0100
From:   Steven Whitehouse <swhiteho@...hat.com>
To:     Jeff Layton <jlayton@...hat.com>, Jan Kara <jack@...e.cz>,
        Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Matthew Wilcox <willy@...radead.org>,
        Bob Peterson <rpeterso@...hat.com>, cluster-devel@...hat.com
Subject: Re: [PATCH v2 2/4] mm: add file_fdatawait_range and
 file_write_and_wait

Hi,


On 31/07/17 13:22, Jeff Layton wrote:
> On Mon, 2017-07-31 at 13:05 +0100, Steven Whitehouse wrote:
>> Hi,
>>
>>
>> On 31/07/17 12:44, Jeff Layton wrote:
>>> On Mon, 2017-07-31 at 12:32 +0100, Steven Whitehouse wrote:
>>>> Hi,
>>>>
>>>>
>>>> On 31/07/17 12:27, Jeff Layton wrote:
>>>>> On Thu, 2017-07-27 at 08:48 -0400, Jeff Layton wrote:
>>>>>> On Thu, 2017-07-27 at 10:49 +0200, Jan Kara wrote:
>>>>>>> On Wed 26-07-17 13:55:36, Jeff Layton wrote:
>>>>>>>> +int file_write_and_wait(struct file *file)
>>>>>>>> +{
>>>>>>>> +	int err = 0, err2;
>>>>>>>> +	struct address_space *mapping = file->f_mapping;
>>>>>>>> +
>>>>>>>> +	if ((!dax_mapping(mapping) && mapping->nrpages) ||
>>>>>>>> +	    (dax_mapping(mapping) && mapping->nrexceptional)) {
>>>>>>>> +		err = filemap_fdatawrite(mapping);
>>>>>>>> +		/* See comment of filemap_write_and_wait() */
>>>>>>>> +		if (err != -EIO) {
>>>>>>>> +			loff_t i_size = i_size_read(mapping->host);
>>>>>>>> +
>>>>>>>> +			if (i_size != 0)
>>>>>>>> +				__filemap_fdatawait_range(mapping, 0,
>>>>>>>> +							  i_size - 1);
>>>>>>>> +		}
>>>>>>>> +	}
>>>>>>> Err, what's the i_size check doing here? I'd just pass ~0 as the end of the
>>>>>>> range and ignore i_size. It is much easier than trying to wrap your head
>>>>>>> around possible races with file operations modifying i_size.
>>>>>>>
>>>>>>> 								Honza
>>>>>> I'm basically emulating _exactly_ what filemap_write_and_wait does here,
>>>>>> as I'm leery of making subtle behavior changes in the actual writeback
>>>>>> behavior. For example:
>>>>>>
>>>>>> -----------------8<----------------
>>>>>> static inline int __filemap_fdatawrite(struct address_space *mapping,
>>>>>>            int sync_mode)
>>>>>> {
>>>>>>            return __filemap_fdatawrite_range(mapping, 0, LLONG_MAX, sync_mode);
>>>>>> }
>>>>>>
>>>>>> int filemap_fdatawrite(struct address_space *mapping)
>>>>>> {
>>>>>>            return __filemap_fdatawrite(mapping, WB_SYNC_ALL);
>>>>>> }
>>>>>> EXPORT_SYMBOL(filemap_fdatawrite);
>>>>>> -----------------8<----------------
>>>>>>
>>>>>> ...which then sets up the wbc with the right ranges and sync mode and
>>>>>> kicks off writepages. But then, it does the i_size_read to figure out
>>>>>> what range it should wait on (with the shortcut for the size == 0 case).
>>>>>>
>>>>>> My assumption was that it was intentionally designed that way, but I'm
>>>>>> guessing from your comments that it wasn't? If so, then we can turn
>>>>>> file_write_and_wait a static inline wrapper around
>>>>>> file_write_and_wait_range.
>>>>> FWIW, I did a bit of archaeology in the linux-history tree and found
>>>>> this patch from Marcelo in 2004. Is this optimization still helpful? If
>>>>> not, then that does simplify the code a bit.
>>>>>
>>>>> -------------------8<--------------------
>>>>>
>>>>> [PATCH] small wait_on_page_writeback_range() optimization
>>>>>
>>>>> filemap_fdatawait() calls wait_on_page_writeback_range() with -1 as "end"
>>>>> parameter.  This is not needed since we know the EOF from the inode.  Use
>>>>> that instead.
>>>>>
>>>>> Signed-off-by: Marcelo Tosatti <marcelo.tosatti@...lades.com>
>>>>> Signed-off-by: Andrew Morton <akpm@...l.org>
>>>>> Signed-off-by: Linus Torvalds <torvalds@...l.org>
>>>>> ---
>>>>>     mm/filemap.c | 8 +++++++-
>>>>>     1 file changed, 7 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>>>> index 78e18b7639b6..55fb7b4141e4 100644
>>>>> --- a/mm/filemap.c
>>>>> +++ b/mm/filemap.c
>>>>> @@ -287,7 +287,13 @@ EXPORT_SYMBOL(sync_page_range);
>>>>>      */
>>>>>     int filemap_fdatawait(struct address_space *mapping)
>>>>>     {
>>>>> -	return wait_on_page_writeback_range(mapping, 0, -1);
>>>>> +	loff_t i_size = i_size_read(mapping->host);
>>>>> +
>>>>> +	if (i_size == 0)
>>>>> +		return 0;
>>>>> +
>>>>> +	return wait_on_page_writeback_range(mapping, 0,
>>>>> +				(i_size - 1) >> PAGE_CACHE_SHIFT);
>>>>>     }
>>>>>     EXPORT_SYMBOL(filemap_fdatawait);
>>>>>
>>>> Does this ever get called in cases where we would not hold fs locks? In
>>>> that case we definitely don't want to be relying on i_size,
>>>>
>>>> Steve.
>>>>
>>> Yes. We can initiate and wait on writeback from any context where you
>>> can sleep, really.
>>>
>>> We're just waiting on whole file writeback here, so I don't think
>>> there's anything wrong. As long as the i_size was valid at some point in
>>> time prior to waiting then you're ok.
>>>
>>> The question I have is more whether this optimization is still useful.
>>>
>>> What we do now is just walk the radix tree and wait_on_page_writeback
>>> for each page. Do we gain anything by avoiding ranges beyond the current
>>> EOF with the pagecache infrastructure of 2017?
>>>
>> If this can be called from anywhere without fs locks, then i_size is not
>> known. That has been a problem in the past since i_size may have changed
>> on another node. We avoid that in this case due to only changing i_size
>> under an exclusive lock, and also only having dirty pages when we have
>> an exclusive lock. There is another case though, if the inode is a block
>> device, i_size will be zero. That is the case for the address space that
>> looks after rgrps for GFS2. We do (luckily!) call
>> filemap_fdatawait_range() directly in that case. For "normal" inodes
>> though, the address space for metadata is backed by the block device
>> inode, so that looks like it might be an issue, since
>> fs/gfs2/glops.c:inode_go_sync() calls filemap_fdatawait() on the
>> metamapping. It might potentially be an issue in other cases too,
>>
>> Steve.
>>
> Some of those do sound problematic.
>
> Again though, we're only waiting on writeback here, and I assume with
> gfs2 that would only be pages that were written on the local node.
Yes
>
> Is it possible to have pages under writeback and in still in the tree,
> but that are beyond the current i_size? It seems like that's the main
> worrisome case.
>
Thats what I was wondering too. I'm not 100% sure without some more 
detailed investigation. Either way the block device case also seems 
problematic, although not impossible to special case I suppose. The real 
question is what do we get from this optmisation? Is the pain of 
checking correctness worth it for the benefits gained,

Steve.

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