Re: [PATCH 1/4] loop: Enable correct physical blocksize

[Hannes Reinecke]

On 11/13/2015 09:57 PM, Jeff Moyer wrote:
> Hi Hannes,
>
> Hannes Reinecke <hare@xxxxxxx> writes:
>
> > When running on files the physical blocksize is actually 4k,
>
> How did you come to that conclusion?  Are you basing it on the file
> system block size?  If so, that's configurable at mkfs time and can be
> anything from 512 bytes to 64k on current in-tree file systems that I
> know of (depending on platform, of course).
loop.c does this (in do_loop_switch()):

	mapping_set_gfp_mask(old_file->f_mapping, lo->old_gfp_mask);
	lo->lo_backing_file = file;
	lo->lo_blocksize = S_ISBLK(mapping->host->i_mode) ?
		mapping->host->i_bdev->bd_block_size : PAGE_SIZE;
	lo->old_gfp_mask = mapping_gfp_mask(mapping);

So either it's a block device, then we're taking the blocksize of 
the underlying device, or we're using PAGE_SIZE.
Which is architecture dependent, of course.

> The main use for physical block size, as I understand it, is to allow
> partitioning utilities to place partitions on physical block boundaries
> of the underlying storage.  The benefit of that is to avoid
> read-modify-writes for I/O which is naturally sized and aligned.  If we
> carry that forward to loop, then I think it does makes sense to key off
> of the file system block size, but the fact remains that 4k is not
> universal.
The main point here is that some utilities (eg bootloaders) need to 
know the _physical_ location of a particular blob, for which it 
needs to know the physical blocksize.

> So, I think the idea is sound, but you should be setting the physical
> block size to sb->s_blocksize.  And I don't see any reason why we
> wouldn't do this by default, do you?
Neither do I. But the code doesn't treat it that way, so I elected
to stay with the current version.

> If you end up reposting this patch, would you mind including more of
> this rationale in your commit message?
Sure.

Cheers,

Hannes
--
Dr. Hannes Reinecke		               zSeries & Storage
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