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    SubjectRe: 2.4.0-test10-pre3:Oops in mm/filemap.c:filemap_write_pa

    On Mon, 23 Oct 2000, Linus Torvalds wrote:

    > On Mon, 23 Oct 2000, Alexander Viro wrote:
    > >
    > > Oh, crap... Who introduced ->i_mmap_shared/->i_mmap separation and what
    > > analysis had been done? Petr, can you reproduce the problem on -test7?
    > I don't think that is it - that code looks very straightforward (and is
    > needed on some silly architectures that cannot easily otherwise see if
    > they need to be coherent wrt user space - mainly sparc and virtual
    > caches).

    OK, I see where the race can happen. Yes, vmtruncate() tries to kill the
    mappings. Right. However, it does that _after_ truncate_inode_pages().
    And there is a window when the only lock we are holding is ->i_sem.
    sync_pte in that window ==> we are fucked.

    So the question being: WTF do we postpone zapping the page tables until
    after the truncate_inode_pages()? The following rules might make life
    simpler, AFAICS:
    * as soon as ->i_size is set, no new pagetable references to
    off-limits pages can appear.
    * as soon as we are don with vmtruncate_list() there is no
    pagetable references.
    * truncate_inode_pages() never has to deal with pages refered from
    * ->i_size can't increase until we return from vmtruncate().

    It's not the only problem, but I would feel _much_ safer if pagefault
    wouldn't rely on pagecache miss. Actually... Hey. Why don't we do the
    insertion into page tables _within_ ->nopage()? Look: let's take the tail
    of do_no_page() into helper function and just call it from the end of
    every bloody ->nopage() out there. It _is_ easy: we have only 9 instances
    in the tree not counting filemap_nopage(). Moreover,
    do_anonymous_page() will become symmetrical to the rest of the crowd.

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