• Aneesh Kumar K.V's avatar
    powerpc/mm/radix: Fix PTE/PMD fragment count for early page table mappings · 645d5ce2
    Aneesh Kumar K.V authored
    We can hit the following BUG_ON during memory unplug:
    
    kernel BUG at arch/powerpc/mm/book3s64/pgtable.c:342!
    Oops: Exception in kernel mode, sig: 5 [#1]
    LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA pSeries
    NIP [c000000000093308] pmd_fragment_free+0x48/0xc0
    LR [c00000000147bfec] remove_pagetable+0x578/0x60c
    Call Trace:
    0xc000008050000000 (unreliable)
    remove_pagetable+0x384/0x60c
    radix__remove_section_mapping+0x18/0x2c
    remove_section_mapping+0x1c/0x3c
    arch_remove_memory+0x11c/0x180
    try_remove_memory+0x120/0x1b0
    __remove_memory+0x20/0x40
    dlpar_remove_lmb+0xc0/0x114
    dlpar_memory+0x8b0/0xb20
    handle_dlpar_errorlog+0xc0/0x190
    pseries_hp_work_fn+0x2c/0x60
    process_one_work+0x30c/0x810
    worker_thread+0x98/0x540
    kthread+0x1c4/0x1d0
    ret_from_kernel_thread+0x5c/0x74
    
    This occurs when unplug is attempted for such memory which has
    been mapped using memblock pages as part of early kernel page
    table setup. We wouldn't have initialized the PMD or PTE fragment
    count for those PMD or PTE pages.
    
    This can be fixed by allocating memory in PAGE_SIZE granularity
    during early page table allocation. This makes sure a specific
    page is not shared for another memblock allocation and we can
    free them correctly on removing page-table pages.
    
    Since we now do PAGE_SIZE allocations for both PUD table and
    PMD table (Note that PTE table allocation is already of PAGE_SIZE),
    we end up allocating more memory for the same amount of system RAM.
    Here is a comparision of how much more we need for a 64T and 2G
    system after this patch:
    
    1. 64T system
    -------------
    64T RAM would need 64G for vmemmap with struct page size being 64B.
    
    128 PUD tables for 64T memory (1G mappings)
    1 PUD table and 64 PMD tables for 64G vmemmap (2M mappings)
    
    With default PUD[PMD]_TABLE_SIZE(4K), (128+1+64)*4K=772K
    With PAGE_SIZE(64K) table allocations, (128+1+64)*64K=12352K
    
    2. 2G system
    ------------
    2G RAM would need 2M for vmemmap with struct page size being 64B.
    
    1 PUD table for 2G memory (1G mapping)
    1 PUD table and 1 PMD table for 2M vmemmap (2M mappings)
    
    With default PUD[PMD]_TABLE_SIZE(4K), (1+1+1)*4K=12K
    With new PAGE_SIZE(64K) table allocations, (1+1+1)*64K=192K
    Signed-off-by: default avatarBharata B Rao <bharata@linux.ibm.com>
    Signed-off-by: default avatarAneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    Link: https://lore.kernel.org/r/20200709131925.922266-2-aneesh.kumar@linux.ibm.com
    645d5ce2
pgtable.c 13.7 KB