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Date: Wed, 27 Aug 2025 10:00:03 +0200
From: Christoph Hellwig <hch@lst.de>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, Damien Le Moal <dlemoal@kernel.org>,
	"Martin K . Petersen" <martin.petersen@oracle.com>
Subject: Re: [PATCH v2] block: Increase BLK_DEF_MAX_SECTORS_CAP
Message-ID: <20250827080003.GA26652@lst.de>
References: <20250618060045.37593-1-dlemoal@kernel.org> <175078375641.82625.9467584315092336312.b4-ty@kernel.dk> <20250827070705.4iWhHGPE@linutronix.de> <20250827073836.GA25169@lst.de> <20250827075221.6hTi-i7m@linutronix.de>
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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.