[lkml]   [2011]   [Mar]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    SubjectRe: "x86-64, mm: Put early page table high" causes crash on Xen
    On 03/01/2011 09:21 AM, Stefano Stabellini wrote:
    > Yinghai Lu,
    > while testing tip/x86/mm on Xen I found out that the commit "x86-64, mm:
    > Put early page table high" reliably crashes Linux at boot.
    > The reason is well explained by the commit message of
    > fef5ba797991f9335bcfc295942b684f9bf613a1:
    > "Xen requires that all pages containing pagetable entries to be mapped
    > read-only. If pages used for the initial pagetable are already mapped
    > then we can change the mapping to RO. However, if they are initially
    > unmapped, we need to make sure that when they are later mapped, they
    > are also mapped RO.
    > We do this by knowing that the kernel pagetable memory is pre-allocated
    > in the range e820_table_start - e820_table_end, so any pfn within this
    > range should be mapped read-only. However, the pagetable setup code
    > early_ioremaps the pages to write their entries, so we must make sure
    > that mappings created in the early_ioremap fixmap area are mapped RW.
    > (Those mappings are removed before the pages are presented to Xen
    > as pagetable pages.)"
    > In other words mask_rw_pte (called by xen_set_pte_init) should mark RO
    > the already existing pagetable pages (like the ones belonging to the
    > initial mappings), while it should mark RW the new pages not yet hooked
    > into the pagetable. This is what the following lines used to achieve,
    > but don't anymore:
    > /*
    > * If the new pfn is within the range of the newly allocated
    > * kernel pagetable, and it isn't being mapped into an
    > * early_ioremap fixmap slot, make sure it is RO.
    > */
    > if (!is_early_ioremap_ptep(ptep) &&
    > pfn >= pgt_buf_start && pfn < pgt_buf_end)
    > pte = pte_wrprotect(pte);
    > Unfortunately now we map the already existing initial pagetable pages a
    > second time and the new zeroed pages using map_low_page, so we are
    > unable to distinguish between the two.
    > Can we go back to the previous way of accessing pagetable pages from
    > kernel_physical_mapping_init, while keeping the new pagetable allocation
    > strategy? It seems to me that the introduction of map_low_page is not
    > actually required, is it? In that case we could just revert that bit...
    > (appended partial revert example).

    We do need map_low_page ( BTW, that name is totally misleading...)

    the reason is we put page_table high and at that time is not under max_pfn_mapped. (aka not mapped).

    So have to use
    adr = early_memremap(phys & PAGE_MASK, PAGE_SIZE);
    to early map it and Read/Write to it.



     \ /
      Last update: 2011-03-01 23:17    [W:0.027 / U:18.732 seconds]
    ©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site