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    SubjectRe: [PATCH v4] kmemleak: survive in a low-memory situation
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    On 3/27/19 4:44 AM, Michal Hocko wrote:
    >> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
    >> index a2d894d3de07..7f4545ab1f84 100644
    >> --- a/mm/kmemleak.c
    >> +++ b/mm/kmemleak.c
    >> @@ -580,7 +580,16 @@ static struct kmemleak_object *create_object(unsigned long ptr, size_t size,
    >> struct rb_node **link, *rb_parent;
    >> unsigned long untagged_ptr;
    >>
    >> - object = kmem_cache_alloc(object_cache, gfp_kmemleak_mask(gfp));
    >> + /*
    >> + * The tracked memory was allocated successful, if the kmemleak object
    >> + * failed to allocate for some reasons, it ends up with the whole
    >> + * kmemleak disabled, so try it harder.
    >> + */
    >> + gfp = (in_atomic() || irqs_disabled()) ?
    >> + gfp_kmemleak_mask(gfp) | GFP_ATOMIC :
    >> + gfp_kmemleak_mask(gfp) | __GFP_DIRECT_RECLAIM;
    >
    >
    > The comment for in_atomic says:
    > * Are we running in atomic context? WARNING: this macro cannot
    > * always detect atomic context; in particular, it cannot know about
    > * held spinlocks in non-preemptible kernels. Thus it should not be
    > * used in the general case to determine whether sleeping is possible.
    > * Do not use in_atomic() in driver code.

    That is why it needs both in_atomic() and irqs_disabled(), so irqs_disabled()
    can detect kernel functions held spinlocks even in non-preemptible kernels.

    According to [1],

    "This [2] is useful if you know that the data in question is only ever
    manipulated from a "process context", ie no interrupts involved."

    Since kmemleak only deal with kernel context, if a spinlock was held, it always
    has local interrupt disabled.

    ftrace is in the same boat where this commit was merged a while back that has
    the same check.

    ef99b88b16be
    tracing: Handle ftrace_dump() atomic context in graph_trace_open()

    [1] https://www.kernel.org/doc/Documentation/locking/spinlocks.txt
    [2]
    spin_lock(&lock);
    ...
    spin_unlock(&lock);

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