Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap causes application-level breakage

From: Kirill A. Shutemov
Date: Wed Apr 29 2015 - 15:36:56 EST

On Wed, Apr 29, 2015 at 07:44:57PM +0100, Mark Williamson wrote:
> Hi all,
> We've been investigating further and found a snag with the PFN-hiding
> approach discussed last week - looks like it won't be enough on all
> the architectures we support. Our product runs on x86_32, x86_64 and
> ARM. For now, it looks like soft-dirty is only available on x86_64.
> A patch that simply zeros out the physical addresses in
> /proc/PID/pagemap will therefore help us on x86_64 but we'll still
> have problems on other platforms[1].
> For context, we were previously using pagemap as a cross-platform way
> to get soft-dirty-like functionality. Specifically, to ask "did a
> process write to any pages since fork()" by comparing addresses and
> deducing where CoW must have occurred. In the absence of soft-dirty
> and the physical addresses, it looks like we can't figure that out
> with the remaining information in pagemap.
> If the pagemap file included the "writeable" bit from the PTE, we
> think we'd have all the information required to deduce what we need
> (although I realise that's a bit of a nasty workaround). If I
> proposed including the PTE protection bits in pagemap, would that be
> controversial? I'm guessing yes but thought it was worth a shot ;-)
> Would anybody be able to suggest a more tasteful approach?

Emm.. I have hard time to understand how writable bit is enough to get
soft-dirty-alike functionality.

Let's say we have anon-mapping with COW setup after the fork(). It's not
writable PTEs to trigger COW on wp faults. But you can easily get to the
same non-writable PTE after breaking COW: fork() again or
mprotect(PROT_READ) and mprotect(PROT_READ|PROT_WRITE) back.


> Thanks,
> Mark
> [1] I'd note that using soft-dirty is clearly the right approach for
> us on x64, where available and that ideally we'd use it on other
> architectures - cross-arch support for soft-dirty is a slightly
> different discussion, which I hope to post another thread for.

Kirill A. Shutemov
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