• Martin Schwidefsky's avatar
    s390/mm: implement software dirty bits · abf09bed
    Martin Schwidefsky authored
    The s390 architecture is unique in respect to dirty page detection,
    it uses the change bit in the per-page storage key to track page
    modifications. All other architectures track dirty bits by means
    of page table entries. This property of s390 has caused numerous
    problems in the past, e.g. see git commit ef5d437f
    "mm: fix XFS oops due to dirty pages without buffers on s390".
    To avoid future issues in regard to per-page dirty bits convert
    s390 to a fault based software dirty bit detection mechanism. All
    user page table entries which are marked as clean will be hardware
    read-only, even if the pte is supposed to be writable. A write by
    the user process will trigger a protection fault which will cause
    the user pte to be marked as dirty and the hardware read-only bit
    is removed.
    With this change the dirty bit in the storage key is irrelevant
    for Linux as a host, but the storage key is still required for
    KVM guests. The effect is that page_test_and_clear_dirty and the
    related code can be removed. The referenced bit in the storage
    key is still used by the page_test_and_clear_young primitive to
    provide page age information.
    For page cache pages of mappings with mapping_cap_account_dirty
    there will not be any change in behavior as the dirty bit tracking
    already uses read-only ptes to control the amount of dirty pages.
    Only for swap cache pages and pages of mappings without
    mapping_cap_account_dirty there can be additional protection faults.
    To avoid an excessive number of additional faults the mk_pte
    primitive checks for PageDirty if the pgprot value allows for writes
    and pre-dirties the pte. That avoids all additional faults for
    tmpfs and shmem pages until these pages are added to the swap cache.
    Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
setup.h 5.53 KB