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Date: Mon, 5 Jan 2015 15:52:10 +0000 From: One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk> To: Leon Pollak <leon.pollak@...il.com> Cc: linux-kernel@...r.kernel.org Subject: Re: Old O_DIRECT story > I tried in all ways to implement this with mmap(), but it does not success, > because I did not find a way to mmap() file as O_WRONLY. Mapping as O_RDWR > makes kernel to pre-fill mapped memory with partition data. So, kernel and > DMA actually compete on the RAM area to fill it - one with garbage, one > with actual data. Kernel wins. > > So, how to implement Linus's advice? Use O_DIRECT. There are lots of problems with the mmap() model, in particular with how mmu table changes scale to large numbers of CPU threads (ie they don't). You would need to modify the kernel to add an madvise type of soemthing like ZEROFILL (you can't do WRONLY because the x86 CPU can't really do write only) but then you'd be stuck with only running on the latest and greatest kernel. A ZEROFILL madvise would at least mean each time you touched a new page it took a fault and did a 4K clear not a read. mmap works well if you don't need to change the page permissions or map new pages all the time. If you have to keep touching the MMU then it gets ugly because you tend to need to synchronize btween cores. Alan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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