On 01/31/2014 02:02 PM, James Bottomley wrote:
It has been reported: http://marc.info/?t=139111447200006 That large block devices (specifically devices > 16TB) crash when mounted on 32 bit systems. The problem specifically is that although CONFIG_LBDAF extends the size of sector_t within the block and storage layers to 64 bits, the buffer cache isn't big enough. Specifically, buffers are mapped through a single page cache mapping on the backing device inode. The size of the allowed offset in the page cache radix tree is pgoff_t which is 32 bits, so once the size of device goes beyond 16TB, this offset wraps and all hell breaks loose. The problem is that although the current single drive limit is about 4TB, it will only be a couple of years before 16TB devices are available. By then, I bet that most arm (and other exotic CPU) Linux based personal file servers are still going to be 32 bit, so they're not going to be able to take this generation (or beyond) of drives. The thing I'd like to discuss is how to fix this. There are several options I see, but there might be others. 1. Try to pretend that CONFIG_LBDAF is supposed to cap out at 16TB and there's nothing we can do about it ... this won't be at all popular with arm based file server manufacturers. 2. Slyly make sure that the buffer cache won't go over 16TB by keeping filesystem metadata below that limit ... the horse has probably already bolted on this one. 3. Increase pgoff_t and the radix tree indexes to u64 for CONFIG_LBDAF. This will blow out the size of struct page on 32 bits by 4 bytes and may have other knock on effects, but at least it will be transparent. 4. add an additional radix tree lookup within the buffer cache, so instead of a single inode for the buffer cache, we have a radix tree of them which are added and removed at the granularity of 16TB offsets as entries are requested.
I started typing up that #3 is going to cause problems with RCU radix, but it looks ok. I think we'll find a really scarey number of places that interchange pgoff_t with unsigned long though.
I prefer #4, but it means each FS needs to add code too. We assume page_offset(page) maps to the disk in more than a few places.
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