Re: [PATCH] virtio_balloon: use non-blocking allocation

[Wei Wang]

On 01/02/2018 10:50 PM, Tetsuo Handa wrote:
> Commit c7cdff0e864713a0 ("virtio_balloon: fix deadlock on OOM") tried to
> avoid OOM lockup by moving memory allocations to outside of balloon_lock.
>
> Now, Wei is trying to allocate far more pages outside of balloon_lock and
> some more memory inside of balloon_lock in order to perform efficient
> communication between host and guest using scatter-gather API.
>
> Since pages allocated outside of balloon_lock are not visible to the OOM
> notifier path until fill_balloon() holds balloon_lock (and enqueues the
> pending pages), allocating more pages than now may lead to unacceptably
> premature OOM killer invocation.
>
> It would be possible to make the pending pages visible to the OOM notifier
> path. But there is no need to try to allocate memory so hard from the
> beginning. As of commit 18468d93e53b037e ("mm: introduce a common
> interface for balloon pages mobility"), it made sense to try allocation
> as hard as possible. But after commit 5a10b7dbf904bfe0 ("virtio_balloon:
> free some memory from balloon on OOM"), it no longer makes sense to try
> allocation as hard as possible, for fill_balloon() will after all have to
> release just allocated memory if some allocation request hits the OOM
> notifier path. Therefore, this patch disables __GFP_DIRECT_RECLAIM when
> allocating memory for inflating the balloon. Then, memory for inflating
> the balloon can be allocated inside balloon_lock, and we can release just
> allocated memory as needed.
>
> Also, this patch adds __GFP_NOWARN, for possibility of hitting memory
> allocation failure is increased by removing __GFP_DIRECT_RECLAIM, which
> might spam the kernel log buffer. At the same time, this patch moves
> "puff" messages to outside of balloon_lock, for it is not a good thing to
> block the OOM notifier path for 1/5 of a second. (Moreover, it is better
> to release the workqueue and allow processing other pending items. But
> that change is out of this patch's scope.)
>
> __GFP_NOMEMALLOC is currently not required because workqueue context
> which calls balloon_page_alloc() won't cause __gfp_pfmemalloc_flags()
> to return ALLOC_OOM. But since some process context might start calling
> balloon_page_alloc() in future, this patch does not remove
> __GFP_NOMEMALLOC.
>
> (Only compile tested. Please do runtime tests before committing.)
>
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Cc: Wei Wang <wei.w.wang@xxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> ---
>   drivers/virtio/virtio_balloon.c | 23 +++++++++++++----------
>   mm/balloon_compaction.c         |  5 +++--
>   2 files changed, 16 insertions(+), 12 deletions(-)

I think it is better to simply make the temporal "LIST_HEAD(pages)" to 
be visible to oom notify, e.g. make it "struct list_head 
vb->inflating_pages"

Then we can change virtioballoon_oom_notify():

static int oom_notify()
{
    ...
    if (*freed != oom_pages && !list_empty(&vb->inflating_pages))
               return NOTIFY_BAD;

    return NOTIFY_OK;
}


virtioballoon_oom_notify() {
    int ret;

    do {
        ret = oom_notify()
    } while (ret == NOTIFY_BAD);

    return ret;
}


I view the above as something "nice to have" (users also have an option 
to disable F_DEFLATE_ON_OOM, in which case inflated pages are also not 
released by oom).  I can help with this after the "virtio_balloon 
enhancement" series is done.


Best,
Wei