Christophe Leroy <christophe.le...@csgroup.eu> writes: > Le 28/03/2022 à 12:37, Michael Ellerman a écrit : >> Kefeng Wang <wangkefeng.w...@huawei.com> writes: >>> Hi maintainers, >>> >>> I saw the patches has been reviewed[1], could they be merged? >> >> Maybe I'm just misreading the change log, but it seems wrong that we >> need to add extra checks. pfn_valid() shouldn't return true for vmalloc >> addresses in the first place, shouldn't we fix that instead? Who knows >> what else that might be broken because of that. > > pfn_valid() doesn't take an address but a PFN
Yeah sorry that was unclear wording on my part. What I mean is that pfn_valid(virt_to_pfn(some_vmalloc_addr)) should be false, because virt_to_pfn(vmalloc_addr) should fail. The right way to convert a vmalloc address to a pfn is with vmalloc_to_pfn(), which walks the page tables to find the actual pfn backing that vmalloc addr. > If you have 1Gbyte of memory you have 256k PFNs. > > In a generic config the kernel will map 768 Mbytes of mémory (From > 0xc0000000 to 0xe0000000) and will use 0xf0000000-0xffffffff for > everything else including vmalloc. > > If you take a page above that 768 Mbytes limit, and tries to linarly > convert it's PFN to a va, you'll hip vmalloc space. Anyway that PFN is > valid. That's true, but it's just some random page in vmalloc space, there's no guarantee that it's the same page as the PFN you started with. Note it's not true on 64-bit Book3S. There if you take a valid PFN (ie. backed by RAM) and convert it to a virtual address (with __va()), you will never get a vmalloc address. > So the check really needs to be done in virt_addr_valid(). I don't think it has to, but with the way our virt_to_pfn()/__pa() works I guess for now it's the easiest solution. > There is another thing however that would be worth fixing (in another > patch): that's the virt_to_pfn() in PPC64: > > #define virt_to_pfn(kaddr) (__pa(kaddr) >> PAGE_SHIFT) > > #define __pa(x) > \ > ({ \ > VIRTUAL_BUG_ON((unsigned long)(x) < PAGE_OFFSET); \ > (unsigned long)(x) & 0x0fffffffffffffffUL; \ > }) > > > So 0xc000000000000000 and 0xd000000000000000 have the same PFN. That's > wrong. Yes it was wrong. But we don't use 0xd000000000000000 anymore, so it shouldn't be an issue in practice. See 0034d395f89d ("powerpc/mm/hash64: Map all the kernel regions in the same 0xc range"). I guess it is still a problem for 64-bit Book3E, because they use 0xc and 0x8. I looked at fixing __pa()/__va() to use +/- a few years back, but from memory it still didn't work and/or generated bad code. There's a comment about it working around a GCC miscompile. The other thing that makes me reluctant to change it is that I worry we have places that inadvertantly use __pa() on addresses that are already physical addresses. If we changed __pa() to use subtraction that would break, ie. we'd end up with a negative address. See eg. a6e2c226c3d5 ("powerpc: Fix kernel crash in show_instructions() w/DEBUG_VIRTUAL") So to fix it we'd have to 1) verify that the compiler bug is no longer an issue and 2) audit uses of __pa()/__va(). cheers