Re: double kernel page table entry for the same physical page?!

I am worried about memory corruption if there are two
PTE entries point to the same physical page.
Considering this case: vmalloc a page to a virt
address V1 and somehow other code uses V0(which is in
identy mapping) to modify the same physical page.

I checked i386 and looks like they have the same
problem.

Why linux kernel does such thing and no one consider
it's a problem? 

Thanks
Ming

--- Dan Malek <dan@embeddededge.com> wrote:

On Jul 13, 2005, at 1:10 PM, ming lei wrote:

quoted

Question: does the linux ever touch this section

of

quoted

page table(virt address from PAGE_OFFSET to
PAGE_OFFSET+A) again? like remove or modify one of

the

quoted

entry?

Not usually.  We used to do it on 8xx for mapping
uncached pages for coherent DMA.

quoted

Question: For every physical page, it should

already

quoted

be mapped in #1, but vmalloc maps it again to a

virt

quoted

address higher. Does it mean in kernel page table,

we

quoted

may have two PTE entries pointing to the same

physical

quoted

page, even their virt addresses are different?

Yep.

quoted

Last question:
If this is true, is this platform implemention
specific? Does i386 implemention have same

situation?

Other architectures will do the same, yes.  The
level to which they do this will vary depending upon
architectural features we have to hide/expose to
other functions.

One potential problem that arises is you have to be
careful about the virtual address access path to
these
pages.  All processors will behave errantly if you
access the same physical page, but with different
attributes, through the different VAs.  How they
behave
will depend upon the attributes.  If the PTEs have
different cache modes or page sizes, it's usually
a big problem.  If they just share different write
permissions, it's usually OK.  These problems
usually
arise when device driver writers either don't
understand
the mapping issues or try something that may work
on one architecture but not on another.


	-- Dan

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