On 12.05.2015 15:05, Mark Williamson wrote:
Hi Konstantin, I hope you won't mind me thinking out loud here on the idea of adding a flag to the v2 pagemap fields... From a kernel PoV, I agree that this seems like the cleanest approach. However, with my application developer hat on: 1. I was hoping we'd be able to backport a compatible fix to older kernels that might adopt the pagemap permissions change. Using the V2 format flags rules out doing this for kernels that are too old to have soft-dirty, I think.
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2. From our software's PoV, I feel it's worth noting that it doesn't strictly fix ABI compatibility, though I realise that's probably not your primary concern here. We'll need to modify our code to write the clear_refs file but that change is OK for us if it's the preferred solution. In the patches I've been playing with, I was considering putting the Exclusive flag in the now-unused PFN field of the pagemap entries. Since we're specifically trying to work around for the lack of PFN information, would there be any appetite for mirroring this flag unconditionally into the now-empty PFN field (i.e. whether using v1 or v2 flags) when accessed by an unprivileged process? I realise it's ugly from a kernel PoV and I feel a little bad for suggesting it - but it would address points 1 and 2 for us (our existing code just looks for changes in the pagemap entry, so sticking the flag in there would cause it to do the right thing). I'm sorry to raise application-specific issues at this point; I appreciate that your primary concern is to improve the kernel and technically I like the approach that you've taken! I'll try and provide more code-oriented feedback once I've tried out the changes.
I prefer to backport v2 format (except soft-dirty bit and clear_refs) into older kernels. Page-shift bits are barely used so nobody will see the difference. -- Konstantin -- 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>