[haiku-bugs] Re: [Haiku] #5328: Had reserved page but threre is none after latest slab/vm changes
- From: "bonefish" <trac@xxxxxxxxxxxx>
- Date: Thu, 28 Jan 2010 08:14:40 -0000
#5328: Had reserved page but threre is none after latest slab/vm changes
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Reporter: mmlr | Owner: bonefish
Type: bug | Status: new
Priority: normal | Milestone: R1
Component: System/Kernel | Version: R1/Development
Keywords: | Blockedby:
Platform: All | Blocking:
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Comment(by bonefish):
Replying to [comment:2 mmlr]:
> Replying to [comment:1 bonefish]:
> > free count + clear count >= unreserved free pages + system reserved
pages
>
> As far as I've seen we only panic in case we actually failed to allocate
a page, hiding cases where there is a violation of the rules but we just
don't happen to be as short on pages. Couldn't we introduce a paranoid
checking of the reserve on each allocation and then panic in case of
violation?
If you mean checking the invariant, then this is not really possible. The
variables on the right hand side are accessed atomically without holding
any lock. The queue counts are protected by a R/W lock + spinlock combo --
unless one holds the write lock, they can change individually. There's no
way one can get a snapshot of all four variables at the same time.
Introducing a mutex to make that possible would kill all concurrency and
thus probably make the bug disappear, if it is some kind of race condition
in vm_page.cpp.
If r35295 introduced the bug, I don't think a problem outside of
vm_page.cpp is likely. I just read through the patch again and it really
doesn't change anything related other than adding passing of the priority
argument in various functions and methods. No accidental replacement of
vm_page_reserve_pages() by vm_page_unreserve_pages() or changing of the
numbers passed to them.
--
Ticket URL: <http://dev.haiku-os.org/ticket/5328#comment:4>
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