• Will Deacon's avatar
    arm64: mm: fix pmd_write CoW brokenness · ceb21835
    Will Deacon authored
    Commit 9c7e535f ("arm64: mm: Route pmd thp functions through pte
    equivalents") changed the pmd manipulator and accessor functions to
    convert the target pmd to a pte, process it with the pte functions, then
    convert it back. Along the way, we gained support for PTE_WRITE, however
    this is completely ignored by set_pmd_at, and so we fail to set the
    PMD_SECT_RDONLY for PMDs, resulting in all sorts of lovely failures (like
    CoW not working).
    
    Partially reverting the offending commit (by making use of
    PMD_SECT_RDONLY explicitly for pmd_{write,wrprotect,mkwrite} functions)
    leads to further issues because pmd_write can then return potentially
    incorrect values for page table entries marked as RDONLY, leading to
    BUG_ON(pmd_write(entry)) tripping under some THP workloads.
    
    This patch fixes the issue by routing set_pmd_at through set_pte_at,
    which correctly takes the PTE_WRITE flag into account. Given that
    THP mappings are always anonymous, the additional cache-flushing code
    in __sync_icache_dcache won't impose any significant overhead as the
    flush will be skipped.
    
    Cc: Catalin Marinas <catalin.marinas@arm.com>
    Acked-by: default avatarSteve Capper <steve.capper@arm.com>
    Tested-by: default avatarMarc Zyngier <marc.zyngier@arm.com>
    Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
    ceb21835
pgtable.h 13.2 KB