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    SubjectRe: [patch 9/9] mm: fix pagecache write deadlocks
    On Sun, Feb 04, 2007 at 03:10:39AM -0800, Andrew Morton wrote:
    > On Sun, 4 Feb 2007 10:59:58 +0000 (GMT) Anton Altaparmakov <aia21@cam.ac.uk> wrote:
    > >
    > > How about leaving the existing code with the following minor
    > > modifications:
    > >
    > > Instead of calling filemap_copy_from_user{,_iovec}() do only the atomic
    > > bit with pagefaults disabled, i.e. instead of filemap_copy_from_user() we
    > > would do (could of course move into a helper function of course):
    > >
    > > pagefault_disable()
    > > kaddr = kmap_atomic(page, KM_USER0);
    > > left = __copy_from_user_inatomic_nocache(kaddr + offset, buf, bytes);
    > > kunmap_atomic(kaddr, KM_USER0);
    > > pagefault_enable()
    > >
    > > if (unlikely(left)) {
    > > /* The user space page got unmapped before we got to copy it. */
    > > ...
    > > }
    > >
    > > Thus the 99.999% (or more!) of the time the code would just work as it
    > > always has and there is no bug and no speed impact. Only in the very rare
    > > and hard to trigger race condition that the user space page after being
    > > faulted in got thrown out again before we did the atomic memory copy do we
    > > run into the above "..." code path.
    >
    > Right. And what I wanted to do here is to zero out the uncopied part of
    > the page (if it wasn't uptodate), then run commit_write(), then retry the
    > whole thing.
    >
    > iirc, we ruled that out because those temporary zeroes are exposed to
    > userspace. But the kernel used to do that anyway for a long time (years)
    > until someone noticed, and we'll only do it in your 0.0001% case anyway.

    Serious? I'd rather leave the deadlock in there than introduce a
    very hard to reproduce data corruption bug to fix it. At least the
    deadlock is fail-stop and you can tell exactly what happened when
    you hit it (assuming you can get a trace).

    Then again, we've got lots more similar little correctness corner
    cases like this that most people don't notice most of the time. Am
    I aiming too high?

    > (Actually, perhaps we can prevent it by not marking the page uptodate in
    > this case. But that'll cause a read()er to try to bring it uptodate...)

    We have to write something back to the filesystem because it may have
    allocated blocks at this point.

    > > I would propose to call out a different function altogether which could do
    > > a multitude of things including drop the lock on the destination page
    > > (maintaining a reference on the page!), allocate a temporary page, copy
    > > from the user space page into the temporary page, then lock the
    > > destination page again, and copy from the temporary page into the
    > > destination page.
    >
    > The problem with all these things is that as soon as we unlock the page,
    > it's visible to read(). And in fact, as soon as we mark it uptodate it's
    > visible to mmap, even if it's still locked.
    >
    > > This would be slow but who cares given it would only happen incredibly
    > > rarely and on majority of machines it would never happen at all.
    > >
    > > The only potential problem I can see is that the destination page could be
    > > truncated whilst it is unlocked. I can see two possible solutions to
    > > this:
    >
    > truncate's OK: we're holding i_mutex.

    Not all truncates hold i_mutex. Neither do all invalidates, for that
    matter.
    -
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