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    SubjectRe: "x86-64, mm: Put early page table high" causes crash on Xen
    On Mon, 7 Mar 2011, Yinghai Lu wrote:
    > On Mon, Mar 7, 2011 at 7:47 AM, Stefano Stabellini
    > <stefano.stabellini@eu.citrix.com> wrote:
    > > On Tue, 1 Mar 2011, Yinghai Lu wrote:
    > >> 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
    > >> > (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
    > >> >      */
    > >> >         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.
    > >
    > > As you might already know from my other email few days ago, I have found
    > > a solution to this problem modifying only Xen specific code.
    > > However I went back to this because I want to be sure that the patch is
    > > the correct solution rather than a workaround.
    > >
    > > It seems to me that only pagetable pages freshly allocated with
    > > alloc_low_page needs to be mapped because initial pagetable pages are
    > > under max_pfn_mapped.
    > > Considering that kernel_physical_mapping_init doesn't go through the
    > > same pagetable page twice, it is never the case that we map_low_page
    > > anything but initial pagetable pages that are under max_pfn_mapped
    > > anyway.
    > > For example this is what happens on my machine:
    > >
    > >
    > > [    0.000000] initial memory mapped : 0 - 024be000
    > > [    0.000000] init_memory_mapping: 0000000000000000-00000000cffc2000
    > > [    0.000000]  0000000000 - 00cffc2000 page 4k
    > > [    0.000000] kernel direct mapping tables up to cffc2000 @ cf93d000-cffc2000
    > > [    0.000000] DEBUG map low page(2004000, 00001000)
    > > [    0.000000] DEBUG map low page(2008000, 00001000)
    > > [    0.000000] DEBUG map low page(2c62000, 00001000)
    > > [    0.000000] DEBUG map low page(2c65000, 00001000)
    > > [    0.000000] DEBUG map low page(2c66000, 00001000)
    > > [    0.000000] DEBUG map low page(2c67000, 00001000)
    > > [    0.000000] DEBUG map low page(2c68000, 00001000)
    > > [    0.000000] DEBUG map low page(2c69000, 00001000)
    > > [    0.000000] DEBUG map low page(2c6a000, 00001000)
    > > [    0.000000] DEBUG map low page(2c6b000, 00001000)
    > > [    0.000000] DEBUG map low page(2c6c000, 00001000)
    > > [    0.000000] DEBUG map low page(2c6d000, 00001000)
    > > [    0.000000] DEBUG map low page(2c6e000, 00001000)
    > > [    0.000000] DEBUG map low page(2c6f000, 00001000)
    > > [    0.000000] DEBUG map low page(2c70000, 00001000)
    > > [    0.000000] DEBUG map low page(2c71000, 00001000)
    > > [    0.000000] DEBUG map low page(2c72000, 00001000)
    > > [    0.000000] DEBUG map low page(2c73000, 00001000)
    > > [    0.000000] DEBUG map low page(2c74000, 00001000)
    > > [    0.000000] DEBUG map low page(2c75000, 00001000)
    > > [    0.000000] DEBUG map low page(2c76000, 00001000)
    > > [    0.000000] DEBUG map low page(2c77000, 00001000)
    > > [    0.000000] DEBUG map low page(2c78000, 00001000)
    > > [    0.000000] DEBUG map low page(2c79000, 00001000)
    > > [    0.000000] DEBUG map low page(2c7a000, 00001000)
    > > [    0.000000] DEBUG map low page(2c7b000, 00001000)
    > > [    0.000000] DEBUG map low page(239a000, 00001000)
    > > [    0.000000] DEBUG map low page(239b000, 00001000)
    > > [    0.000000] DEBUG map low page(239c000, 00001000)
    > > [    0.000000] DEBUG map low page(239d000, 00001000)
    >
    > these already mapped.

    Right. So why are we mapping them again using map_low_page?
    Maybe we can just access them directly like we did before?
    In my tests going back to using phys_pud_update does not cause any
    regressions.


    > > [    0.000000] DEBUG alloc low page(cf93d000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf93e000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf93f000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf940000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf941000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf942000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf943000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf944000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf945000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf946000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf947000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf948000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf949000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf94a000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf94b000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf94c000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf94d000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf94e000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf94f000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf950000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf951000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf952000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf953000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf954000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf955000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf956000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf957000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf958000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf959000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf95a000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf95b000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf95c000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf95d000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf95e000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf95f000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf960000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf961000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf962000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf963000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf964000, 00001000)
    > > [    0.000000] DEBUG alloc low page(cf965000, 00001000)
    >
    > these are not mapped yet, and need to use early_ioremap.

    In fact we are mapping them inside alloc_low_page. No need for
    map_low_page, right?


    > > if this is the case the introduction of map_low_page is unnecessary and
    > > actually makes the kernel map pages that are already mapped anyway.
    > > Am I correct?
    >
    > initial mapping only to 512M, and new page for page tables could above that.

    Sure, but the new pages are mapped from within alloc_low_page, so it
    looks like the call to map_low_page is not useful and we could just go
    back to access already existing pagetable pages directly.
    For example:


    @@ -554,6 +554,17 @@ phys_pud_init(pud_t *pud_page, unsigned long addr, unsigned long end,
    return last_map_addr;
    }

    +static unsigned long __meminit
    +phys_pud_update(pgd_t *pgd, unsigned long addr, unsigned long end,
    + unsigned long page_size_mask)
    +{
    + pud_t *pud;
    +
    + pud = (pud_t *)pgd_page_vaddr(*pgd);
    +
    + return phys_pud_init(pud, addr, end, page_size_mask);
    +}
    +
    unsigned long __meminit
    kernel_physical_mapping_init(unsigned long start,
    unsigned long end,
    @@ -577,10 +588,8 @@ kernel_physical_mapping_init(unsigned long start,
    next = end;

    if (pgd_val(*pgd)) {
    - pud = map_low_page((pud_t *)pgd_page_vaddr(*pgd));
    - last_map_addr = phys_pud_init(pud, __pa(start),
    + last_map_addr = phys_pud_update(pgd, __pa(start),
    __pa(end), page_size_mask);
    - unmap_low_page(pud);
    continue;
    }
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      Last update: 2011-03-08 13:17    [W:0.044 / U:0.604 seconds]
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