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Re: [dm-devel] [PATCH] block devices: validate block device capacity




On Thu, 30 Jan 2014, James Bottomley wrote:

> On Thu, 2014-01-30 at 18:10 -0500, Mikulas Patocka wrote:
> > 
> > On Thu, 30 Jan 2014, James Bottomley wrote:
> > 
> > > Why is this?  the whole reason for CONFIG_LBDAF is supposed to be to
> > > allow 64 bit offsets for block devices on 32 bit.  It sounds like
> > > there's somewhere not using sector_t ... or using it wrongly which needs
> > > fixing.
> > 
> > The page cache uses unsigned long as a page index. Therefore, if unsigned 
> > long is 32-bit, the block device may have at most 2^32-1 pages.
> 
> Um, that's the index into the mapping, not the device; a device can have
> multiple mappings and each mapping has a radix tree of pages.  For most
> filesystems a mapping is equivalent to a file, so we can have large
> filesystems, but they can't have files over actually 4GB on 32 bits
> otherwise mmap fails.

A device may be accessed direcly (by opening /dev/sdX) and it creates a 
mapping too - thus, the size of a mapping limits the size of a block 
device.

The main problem is that pgoff_t has 4 bytes - chaning it to 8 bytes may 
fix it - but there may be some hidden places where pgoff is converted to 
unsigned long - who knows, if they exist or not?

> Are we running into a problems with struct address_space where we've
> assumed the inode belongs to the file and lvm is doing something where
> it's the whole device?

lvm creates a 64TiB device, udev runs blkid on that device and blkid opens 
the device and gets stuck because of unsigned long overflow.

> > > > On 32-bit architectures, we must limit block device size to
> > > > PAGE_SIZE*(2^32-1).
> > > 
> > > So you're saying CONFIG_LBDAF can never work, why?
> > > 
> > > James
> > 
> > CONFIG_LBDAF works, but it doesn't allow unlimited capacity: on x86, 
> > without CONFIG_LBDAF, the limit is 2TiB. With CONFIG_LBDAF, the limit is 
> > 16TiB (4096*2^32).
> 
> I don't think the people who did the large block device work expected to
> gain only 3 bits for all their pain.
> 
> James

One could change it to have three choices:
2TiB limit - 32-bit sector_t and 32-bit pgoff_t
16TiB limit - 64-bit sector_t and 32-bit pgoff_t
32PiB limit - 64-bit sector_t and 64-bit pgoff_t

Though, we need to know if the people who designed memory management agree 
with changing pgoff_t to 64 bits.

Mikulas


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