Re: [PATCH v6 00/10] block atomic writes

[Hannes Reinecke <hare@suse.de>]

On 4/10/24 06:05, Matthew Wilcox wrote:
> On Mon, Apr 08, 2024 at 10:50:47AM -0700, Luis Chamberlain wrote:
>> On Fri, Apr 05, 2024 at 11:06:00AM +0100, John Garry wrote:
>>> On 04/04/2024 17:48, Matthew Wilcox wrote:
>>>>>> The thing is that there's no requirement for an interface as complex as
>>>>>> the one you're proposing here.  I've talked to a few database people
>>>>>> and all they want is to increase the untorn write boundary from "one
>>>>>> disc block" to one database block, typically 8kB or 16kB.
>>>>>>
>>>>>> So they would be quite happy with a much simpler interface where they
>>>>>> set the inode block size at inode creation time,
>>>>> We want to support untorn writes for bdev file operations - how can we set
>>>>> the inode block size there? Currently it is based on logical block size.
>>>> ioctl(BLKBSZSET), I guess?  That currently limits to PAGE_SIZE, but I
>>>> think we can remove that limitation with the bs>PS patches.
>>
>> I can say a bit more on this, as I explored that. Essentially Matthew,
>> yes, I got that to work but it requires a set of different patches. We have
>> what we tried and then based on feedback from Chinner we have a
>> direction on what to try next. The last effort on that front was having the
>> iomap aops for bdev be used and lifting the PAGE_SIZE limit up to the
>> page cache limits. The crux on that front was that we end requiring
>> disabling BUFFER_HEAD and that is pretty limitting, so my old
>> implementation had dynamic aops so to let us use the buffer-head aops
>> only when using filesystems which require it and use iomap aops
>> otherwise. But as Chinner noted we learned through the DAX experience
>> that's not a route we want to again try, so the real solution is to
>> extend iomap bdev aops code with buffer-head compatibility.
> 
> Have you tried just using the buffer_head code?  I think you heard bad
> advice at last LSFMM.  Since then I've landed a bunch of patches which
> remove PAGE_SIZE assumptions throughout the buffer_head code, and while
> I haven't tried it, it might work.  And it might be easier to make work
> than adding more BH hacks to the iomap code.
> 
> A quick audit for problems ...
> 
> __getblk_slow:
>         if (unlikely(size & (bdev_logical_block_size(bdev)-1) ||
>                          (size < 512 || size > PAGE_SIZE))) {
> 
> cont_expand_zero (not used by bdev code)
> cont_write_begin (ditto)
> 
> That's all I spot from a quick grep for PAGE, offset_in_page() and kmap.
> 
> You can't do a lot of buffer_heads per folio, because you'll overrun
>          struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
> in block_read_full_folio(), but you can certainly do _one_ buffer_head
> per folio, and that's all you need for bs>PS.
> 
Indeed; I got a patch here to just restart the submission loop if one
reaches the end of the array. But maybe submitting one bh at a time and
using plugging should achieve that same thing. Let's see.

>> I suspect this is a use case where perhaps the max folio order could be
>> set for the bdev in the future, the logical block size the min order,
>> and max order the large atomic.
> 
> No, that's not what we want to do at all!  Minimum writeback size needs
> to be the atomic size, otherwise we have to keep track of which writes
> are atomic and which ones aren't.  So, just set the logical block size
> to the atomic size, and we're done.
> 
+1. My thoughts all along.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
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