Re: [PATCH v2] block: Increase BLK_DEF_MAX_SECTORS_CAP

[Christoph Hellwig <hch@lst.de>]

On Wed, Aug 27, 2025 at 09:52:21AM +0200, Sebastian Andrzej Siewior wrote:
> > this for you, you could also reproduce it before by say doing a large
> > direct I/O read.
> 
> On a kernel without that commit in question? Booting Debian's current
> v6.12 and
> |  dd if=vmlinux.o of=/dev/null bs=1G count=1 iflag=direct
> 
> works like a charm. According to strace it does
> | openat(AT_FDCWD, "vmlinux.o", O_RDONLY|O_DIRECT) = 3
> | dup2(3, 0)                              = 0
> | lseek(0, 0, SEEK_CUR)                   = 0
> | read(0, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\1\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 1073741824) = 841980992
> 
> so it should be what you asked for. Asked for 1G, got ~800M.

This is probably splitting thing up into multiple bios because your
output memory is fragmented.  You'd have to do it into hugetlbfs or
vma otherwise backed by very larger folios.