lkml.org 
[lkml]   [2019]   [Mar]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v4] kmemleak: survive in a low-memory situation
    On Wed 27-03-19 07:34:32, Qian Cai wrote:
    > 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.

    What? Normal spin lock implementation doesn't disable interrupts. So
    either I misunderstand what you are saying or you seem to be confused.
    the thing is that in_atomic relies on preempt_count to work properly and
    if you have CONFIG_PREEMPT_COUNT=n then you simply never know whether
    preemption is disabled so you do not know that a spin_lock is held.
    irqs_disabled on the other hand checks whether arch specific flag for
    IRQs handling is set (or cleared). So you would only catch irq safe spin
    locks with the above check.

    > 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);

    --
    Michal Hocko
    SUSE Labs

    \
     
     \ /
      Last update: 2019-03-27 12:47    [W:4.130 / U:0.180 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site