Re: [PATCH 2/2] mm/kmemleak: No need to check kmemleak_initialized in set_track_prepare()

[wang xiaolei]

On 8/10/23 9:16 PM, Vlastimil Babka wrote:
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> On 8/10/23 12:03, Vlastimil Babka wrote:
> > On 8/10/23 09:47, Xiaolei Wang wrote:
> > > The kmemleak_late_init() is defined as a late_initcall. The current
> > > implementation of set_track_prepare() depends on the kmemleak init.
> > > That also means there is no call trace for the memory leak which object
> > > is created before the kmemleak_late_init().
> > So if I understand correctly, we have the following sequence of events durin
> > boot
> >
> > ...
> > A: stack_depot is initialized
> > ...
> > B: kmemleak is initialized
> > ...
> >
> > before this patchset, we can miss allocations before B, aftewards only
> > before A (which can't be helped), so we now have between A and B.
> >
> > That's nice, but it's weird that can record kmemleak when
> > !kmemleak_initialized. Why can't it be initialized sooner in that case?
> Looking closer, I think what you want could be achieved by kmemleak_init()
> setting a variable that is checked in kmemleak_initialized() instead of the
> kmemleak_initialized that's set too late.
>
> I think this should work because:
> - I assume kmemleak can't record anything before kmemleak_init()
> - stack depot early init is requested one way or the other
> - mm_core_init() calls stack_depot_early_init() before kmemleak_init()
>
> But I also wonder how kmemleak can even reach set_track_prepare() before
> kmemleak_init(), maybe that's the issue?

Before kmemleak_init, many places also need to allocate kmemleak_object,

and also need to save stack in advance, but kmemleak_object is allocated

in the form of an array, after kmemleak_init 'object_cache = 
KMEM_CACHE(kmemleak_object, SLAB_NOLEAKTRACE);'


I think there is still some memory not recorded on the backtrace before

stack_depot_early_init(), does anyone have a better suggestion?


thanks

xiaolei

> > > In a previous patch, we have fixed a bug in stack_depot_save() so that
> > > it can be invoked even before stack depot is initialized. So there is
> > > no reason to check the kmemleak_initialized in set_track_prepare().
> > > So delete the kmemleak_initialized judgment in set_track_prepare()
> > >
> > > unreferenced object 0xc674ca80 (size 64):
> > >    comm "swapper/0", pid 1, jiffies 4294938337 (age 204.880s)
> > >    hex dump (first 32 bytes):
> > >      80 55 75 c6 80 54 75 c6 00 55 75 c6 80 52 75 c6 .Uu..Tu..Uu..Ru.
> > >      00 53 75 c6 00 00 00 00 00 00 00 00 00 00 00 00 .Su..........
> > >
> > > Fixes: 56a61617dd22 ("mm: use stack_depot for recording kmemleak's backtrace")
> > > Signed-off-by: Xiaolei Wang <xiaolei.wang@xxxxxxxxxxxxx>
> > > ---
> > >   mm/kmemleak.c | 2 --
> > >   1 file changed, 2 deletions(-)
> > >
> > > diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> > > index a2d34226e3c8..c9f2f816db19 100644
> > > --- a/mm/kmemleak.c
> > > +++ b/mm/kmemleak.c
> > > @@ -610,8 +610,6 @@ static noinline depot_stack_handle_t set_track_prepare(void)
> > >       unsigned long entries[MAX_TRACE];
> > >       unsigned int nr_entries;
> > >
> > > -    if (!kmemleak_initialized)
> > > -            return 0;
> > >       nr_entries = stack_trace_save(entries, ARRAY_SIZE(entries), 3);
> > >       trace_handle = stack_depot_save(entries, nr_entries, GFP_NOWAIT);