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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>