Messages in this thread | | | Date | Wed, 10 Apr 2024 08:20:37 +0200 | Subject | Re: [PATCH v6 00/10] block atomic writes | From | Hannes Reinecke <> |
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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 SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich
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