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 at embeddededge.com> wrote: > > On Jul 13, 2005, at 1:10 PM, ming lei wrote: > > > Question: does the linux ever touch this section > of > > page table(virt address from PAGE_OFFSET to > > PAGE_OFFSET+A) again? like remove or modify one of > the > > entry? > > Not usually. We used to do it on 8xx for mapping > uncached pages for coherent DMA. > > > Question: For every physical page, it should > already > > be mapped in #1, but vmalloc maps it again to a > virt > > address higher. Does it mean in kernel page table, > we > > may have two PTE entries pointing to the same > physical > > page, even their virt addresses are different? > > Yep. > > > 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 > > __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com
