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    SubjectRe: [PATCH] mm, oom: distinguish blockable mode for mmu notifiers
    On Fri 24-08-18 15:28:33, Christian König wrote:
    > Am 24.08.2018 um 15:24 schrieb Michal Hocko:
    > > On Fri 24-08-18 15:10:08, Christian König wrote:
    > > > Am 24.08.2018 um 15:01 schrieb Michal Hocko:
    > > > > On Fri 24-08-18 14:52:26, Christian König wrote:
    > > > > > Am 24.08.2018 um 14:33 schrieb Michal Hocko:
    > > > > [...]
    > > > > > > Thiking about it some more, I can imagine that a notifier callback which
    > > > > > > performs an allocation might trigger a memory reclaim and that in turn
    > > > > > > might trigger a notifier to be invoked and recurse. But notifier
    > > > > > > shouldn't really allocate memory. They are called from deep MM code
    > > > > > > paths and this would be extremely deadlock prone. Maybe Jerome can come
    > > > > > > up some more realistic scenario. If not then I would propose to simplify
    > > > > > > the locking here. We have lockdep to catch self deadlocks and it is
    > > > > > > always better to handle a specific issue rather than having a code
    > > > > > > without a clear indication how it can recurse.
    > > > > > Well I agree that we should probably fix that, but I have some concerns to
    > > > > > remove the existing workaround.
    > > > > >
    > > > > > See we added that to get rid of a real problem in a customer environment and
    > > > > > I don't want to that to show up again.
    > > > > It would really help to know more about that case and fix it properly
    > > > > rather than workaround it like this. Anyway, let me think how to handle
    > > > > the non-blocking notifier invocation then. I was not able to come up
    > > > > with anything remotely sane yet.
    > > > With avoiding allocating memory in the write lock path I don't see an issue
    > > > any more with that.
    > > >
    > > > All what the write lock path does now is adding items to a linked lists,
    > > > arrays etc....
    > > Can we change it to non-sleepable lock then?
    >
    > No, the write side doesn't sleep any more, but the read side does.
    >
    > See amdgpu_mn_invalidate_node() and that is where you actually need to
    > handle the non-blocking flag correctly.

    Ohh, right you are. We already handle that by bailing out before calling
    amdgpu_mn_invalidate_node in !blockable mode. So does this looks good to
    you?

    diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
    index e55508b39496..48fa152231be 100644
    --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
    +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
    @@ -180,11 +180,15 @@ void amdgpu_mn_unlock(struct amdgpu_mn *mn)
    */
    static int amdgpu_mn_read_lock(struct amdgpu_mn *amn, bool blockable)
    {
    - if (blockable)
    - mutex_lock(&amn->read_lock);
    - else if (!mutex_trylock(&amn->read_lock))
    - return -EAGAIN;
    -
    + /*
    + * We can take sleepable lock even on !blockable mode because
    + * read_lock is only ever take from this path and the notifier
    + * lock never really sleeps. In fact the only reason why the
    + * later is sleepable is because the notifier itself might sleep
    + * in amdgpu_mn_invalidate_node but blockable mode is handled
    + * before calling into that path.
    + */
    + mutex_lock(&amn->read_lock);
    if (atomic_inc_return(&amn->recursion) == 1)
    down_read_non_owner(&amn->lock);
    mutex_unlock(&amn->read_lock);
    --
    Michal Hocko
    SUSE Labs
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