On Thu, Sep 03, 2020 at 01:40:32PM -0300, Jason Gunthorpe wrote: > However if the sizeof(*pXX) is 8 on a 32 bit platform then load > tearing is a problem. At lest the various pXX_*() test functions > operate on a single 32 bit word so don't tear, but to to convert the > *pXX to a lower level page table pointer a coherent, untorn, read is > required. > > So, looking again, I remember now, I could never quite figure out why > gup_pmd_range() was safe to do: > > pmd_t pmd = READ_ONCE(*pmdp); > [..] > } else if (!gup_pte_range(pmd, addr, next, flags, pages, nr)) > [..] > ptem = ptep = pte_offset_map(&pmd, addr); > > As I don't see what prevents load tearing a 64 bit pmd.. Eg no > pmd_trans_unstable() or equivalent here. I don't think there are any 32-bit page tables which support a PUD-sized page. Pretty sure x86 doesn't until you get to 4- or 5- level page tables (which need you to be running in 64-bit mode). There's not much utility in having 1GB of your 3GB process address space taken up by a single page. I'm OK if there are some oddball architectures which support it, but Linux doesn't.
