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    SubjectRe: [patch 9/9] mm: fix pagecache write deadlocks
    On Sun, 4 Feb 2007 11:15:29 +0100 Nick Piggin <npiggin@suse.de> wrote:

    > On Sun, Feb 04, 2007 at 01:44:45AM -0800, Andrew Morton wrote:
    > > On Sun, 4 Feb 2007 09:51:07 +0100 (CET) Nick Piggin <npiggin@suse.de> wrote:
    > >
    > > > 2. If we find the destination page is non uptodate, unlock it (this could be
    > > > made slightly more optimal), then find and pin the source page with
    > > > get_user_pages. Relock the destination page and continue with the copy.
    > > > However, instead of a usercopy (which might take a fault), copy the data
    > > > via the kernel address space.
    > >
    > > argh. We just can't go adding all this gunk into the write() path.
    > >
    > > mmap_sem, a full pte-walk, taking of pte-page locks, etc. For every page.
    > > Even single-process write() will suffer, let along multithreaded stuff,
    > > where mmap_sem contention may be the bigger problem.
    >
    > The write path is broken. I prefer my kernels slow, than buggy.

    That won't fly.

    > > There's a build error in filemap_xip.c btw.

    ?

    > >
    > > We need to think different.
    > >
    > > What happened to the idea of doing an atomic copy into the non-uptodate
    > > page and handling it somehow?
    >
    > That was my second idea.

    Coulda sworn it was mine ;) I thought you ended up deciding it wasn't
    practical because of the games we needed to play with ->commit_write.

    > I didn't get any feedback on that patchset
    > except to try this method, so I assume everyone hated it.
    >
    > I actually liked it, because it didn't have to do the writev
    > segment-at-a-time for !uptodate pages like this one does. Considering
    > this code gets called from mm-less contexts, maybe I'll have to go back
    > to this approach.

    OK.

    > > Another option might be to effectively pin the whole mm during the copy:
    > >
    > > down_read(&current->mm->unpaging_lock);
    > > get_user(addr); /* Fault the page in */
    > > ...
    > > copy_from_user()
    > > up_read(&current->mm->unpaging_lock);
    > >
    > > then, anyone who wants to unmap pages from this mm requires
    > > write_lock(unpaging_lock). So we know the results of that get_user()
    > > cannot be undone.
    >
    > Fugly.

    I invited you to think different - don't just fixate on one random
    tossed-out-there suggestion.

    > but you introduce the theoretical memory deadlock
    > where a task cannot reclaim its own memory.

    Nah, that'll never happen - both pages are already allocated.

    It's better than taking mmap_sem and walking pagetables...
    -
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