Messages in this thread | | | Date | Fri, 18 Nov 2005 19:58:17 +0000 (GMT) | From | Hugh Dickins <> | Subject | Re: [PATCH 09/11] unpaged: ZERO_PAGE in VM_UNPAGED |
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On Thu, 17 Nov 2005, Ingo Oeser wrote: > > We do we refcount ZERO_PAGE at all?
We never used to. They were, and for the moment still are, marked PageReserved. Prior to 2.6.15-rc we didn't refcount reserved pages, but now we're trying to move away from PageReserved (some differences of opinion how far to go), so refcounting them.
We're currently refcounting the ZERO_PAGE(s) simply because the common case is refcounted: it would just be extra tests and code NOT to refcount the ZERO_PAGE(s). If they were a commoner case, then it would indeed be worth avoiding refcounting them, but it currently doesn't appear to be worth the effort.
But it is still up in the air: there is or may be an issue with refcounts overflowing, and if it's clear that the ZERO_PAGE is the only one vulnerable on any architecture, then I'm sure we'd deal with it by not refcounting them. However, I believe the issue extends to mapped file pages too: though you need a huge amount of RAM to reach overflow, so it's not something we need to resolve this week.
> Ok, there may be multiple, but they exist always and always at > the same physical addresses, right?
Right.
> So why do we care at all? > Memory hotplug? > Doesn't it suffice there, that they are reverse mappable?
Actually, they're not reverse mappable: we tend to find there's not much gained by swapping out the ZERO_PAGE ;)
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