Messages in this thread | | | Subject | Re: Access to non-RAM pages | From | Benjamin Herrenschmidt <> | Date | Mon, 03 Sep 2018 12:25:10 +1000 |
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On Sun, 2018-09-02 at 19:10 -0700, Linus Torvalds wrote: > On Sun, Sep 2, 2018 at 7:01 PM Benjamin Herrenschmidt > <benh@kernel.crashing.org> wrote: > > > > Still, I can potentially see an issue with DEBUG_PAGEALLOC > > An unmapped page isn't a problem. That's what the whole > load_unaligned_zeropad() is about: it's ok to take a fault on the part > that crosses a page, and we'll just fill the value with zeroes (that's > the "zeropad" part).
Ah, my bad reading, I was looking at read_word_at_a_time() instead of load_unaligned_zeropad(). I'm not familiar enough with the dentry qstr stuff, I assume this is safe ?
> So as long as it's rare (and it is), it's all fine. > > That said, I think we turn off for DEBUG_PAGEALLOC simply because it's > not rare _enough_. > > And vmalloc() should actually be safe too, simply because I think we > strive for a guard page between vmalloc areas. > > So only a *mapped* page after the page that matters, and only if it's > something you can't read without side effects. > > Which basically doesn't happen on x86 in reality. BIOSes just don't > put MMIO right after the last page of RAM. I think this is why it only > triggered on Xen, due to some crazy "Xen reacts badly" case where we > do the speculation into a balloon address. > > So _practically_ this is just a Xen bug, nothing more. > > But since in _theory_ you could have MMIO abut regular RAM directly, > it's worth maybe making sure it's purely theory.
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