Re: [PATCH 10/21] block: Add fops atomic write support

[Ming Lei <ming.lei@redhat.com>]

On Mon, Dec 04, 2023 at 01:13:55PM +0000, John Garry wrote:
> 
> > > 
> > > I added this here (as opposed to the caller), as I was not really worried
> > > about speeding up the failure path. Are you saying to call even earlier in
> > > submission path?
> > atomic_write_unit_min is one hardware property, and it should be checked
> > in blk_queue_atomic_write_unit_min_sectors() from beginning, then you
> > can avoid this check every other where.
> 
> ok, but we still need to ensure in the submission path that the block device
> actually supports atomic writes - this was the initial check.

Then you may add one helper bdev_support_atomic_write().

> 
> > 
> > > > > +	if (pos % atomic_write_unit_min_bytes)
> > > > > +		return false;
> > > > > +	if (iov_iter_count(iter) % atomic_write_unit_min_bytes)
> > > > > +		return false;
> > > > > +	if (!is_power_of_2(iov_iter_count(iter)))
> > > > > +		return false;
> > > > > +	if (iov_iter_count(iter) > atomic_write_unit_max_bytes)
> > > > > +		return false;
> > > > > +	if (pos % iov_iter_count(iter))
> > > > > +		return false;
> > > > I am a bit confused about relation between atomic_write_unit_max_bytes and
> > > > atomic_write_max_bytes.
> > > I think that naming could be improved. Or even just drop merging (and
> > > atomic_write_max_bytes concept) until we show it to improve performance.
> > > 
> > > So generally atomic_write_unit_max_bytes will be same as
> > > atomic_write_max_bytes, however it could be different if:
> > > a. request queue nr hw segments or other request queue limits needs to
> > > restrict atomic_write_unit_max_bytes
> > > b. atomic_write_unit_max_bytes does not need to be a power-of-2 and
> > > atomic_write_max_bytes does. So essentially:
> > > atomic_write_unit_max_bytes = rounddown_pow_of_2(atomic_write_max_bytes)
> > > 
> > plug merge often improves sequential IO perf, so if the hardware supports
> > this way, I think 'atomic_write_max_bytes' should be supported from the
> > beginning, such as:
> > 
> > - user space submits sequential N * (4k, 8k, 16k, ...) atomic writes, all can
> > be merged to single IO request, which is issued to driver.
> > 
> > Or
> > 
> > - user space submits sequential 4k, 4k, 8k, 16K, 32k, 64k atomic writes, all can
> > be merged to single IO request, which is issued to driver.
> 
> Right, we do expect userspace to use a fixed block size, but we give scope
> in the API to use variable size.

Maybe it is enough to just take atomic_write_unit_min_bytes
only, and allow length to be N * atomic_write_unit_min_bytes.

But it may violate atomic write boundary?

> 
> > 
> > The hardware should recognize unit size by start LBA, and check if length is
> > valid, so probably the interface might be relaxed to:
> > 
> > 1) start lba is unit aligned, and this unit is in the supported unit
> > range(power_2 in [unit_min, unit_max])
> > 
> > 2) length needs to be:
> > 
> > - N * this_unit_size
> > - <= atomic_write_max_bytes
> 
> Please note that we also need to consider:
> - any atomic write boundary (from NVMe)

Can you provide actual NVMe boundary value?

Firstly natural aligned write won't cross boundary, so boundary should
be >= write_unit_max, see blow code from patch 10/21:

+static bool bio_straddles_atomic_write_boundary(loff_t bi_sector,
+				unsigned int bi_size,
+				unsigned int boundary)
+{
+	loff_t start = bi_sector << SECTOR_SHIFT;
+	loff_t end = start + bi_size;
+	loff_t start_mod = start % boundary;
+	loff_t end_mod = end % boundary;
+
+	if (end - start > boundary)
+		return true;
+	if ((start_mod > end_mod) && (start_mod && end_mod))
+		return true;
+
+	return false;
+}
+

Then if the WRITE size is <= boundary, the above function should return
false, right? Looks like it is power_of(2) & aligned atomic_write_max_bytes?

> - virt boundary (from NVMe)

virt boundary is applied on bv_offset and bv_len, and NVMe's virt
bounary is (4k - 1), it shouldn't be one issue in reality.

> 
> And, as I mentioned elsewhere, I am still not 100% comfortable that we don't
> pay attention to regular max_sectors_kb...

max_sectors_kb should be bigger than atomic_write_max_bytes actually,
then what is your concern?



Thanks,
Ming