• Peter Zijlstra's avatar
    [PATCH] mm: tracking shared dirty pages · d08b3851
    Peter Zijlstra authored
    Tracking of dirty pages in shared writeable mmap()s.
    
    The idea is simple: write protect clean shared writeable pages, catch the
    write-fault, make writeable and set dirty.  On page write-back clean all the
    PTE dirty bits and write protect them once again.
    
    The implementation is a tad harder, mainly because the default
    backing_dev_info capabilities were too loosely maintained.  Hence it is not
    enough to test the backing_dev_info for cap_account_dirty.
    
    The current heuristic is as follows, a VMA is eligible when:
     - its shared writeable
        (vm_flags & (VM_WRITE|VM_SHARED)) == (VM_WRITE|VM_SHARED)
     - it is not a 'special' mapping
        (vm_flags & (VM_PFNMAP|VM_INSERTPAGE)) == 0
     - the backing_dev_info is cap_account_dirty
        mapping_cap_account_dirty(vma->vm_file->f_mapping)
     - f_op->mmap() didn't change the default page protection
    
    Page from remap_pfn_range() are explicitly excluded because their COW
    semantics are already horrid enough (see vm_normal_page() in do_wp_page()) and
    because they don't have a backing store anyway.
    
    mprotect() is taught about the new behaviour as well.  However it overrides
    the last condition.
    
    Cleaning the pages on write-back is done with page_mkclean() a new rmap call.
    It can be called on any page, but is currently only implemented for mapped
    pages, if the page is found the be of a VMA that accounts dirty pages it will
    also wrprotect the PTE.
    
    Finally, in fs/buffers.c:try_to_free_buffers(); remove clear_page_dirty() from
    under ->private_lock.  This seems to be safe, since ->private_lock is used to
    serialize access to the buffers, not the page itself.  This is needed because
    clear_page_dirty() will call into page_mkclean() and would thereby violate
    locking order.
    
    [dhowells@redhat.com: Provide a page_mkclean() implementation for NOMMU]
    Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Hugh Dickins <hugh@veritas.com>
    Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
    d08b3851
memory.c 67.2 KB