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

[wangxiaolei]

On 8/15/23 12:20 AM, Catalin Marinas wrote:
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> On Fri, Aug 11, 2023 at 10:09:08AM +0200, Vlastimil Babka wrote:
> > On 8/11/23 04:03, wang xiaolei wrote:
> > > On 8/10/23 9:16 PM, Vlastimil Babka wrote:
> > > > 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);'
> > Hm I see, kmemleak has this static mempool so it really can record some
> > objects very early.
> Indeed, otherwise we'd get a lot of false positives.
>
> > > I think there is still some memory not recorded on the backtrace before
> > >
> > > stack_depot_early_init(), does anyone have a better suggestion?
> > No we can't record the backtrace earlier. But I don't think it's a problem
> > in practice. AFAIU kmemleak needs to record these very early allocations so
> > if they point to further objects, those are not suspected as orphans. But
> > the early allocations themselves also are very unlikely to be leaks, so does
> > it really matter that we don't have a backtrace for their allocation?
> > Because the backtrace is the only thing that's missing - the object is
> > otherwise recorded even if set_track_prepare() returns 0.
> It's not a functional problem, just a reporting one. There are
> rare early leaks (usually false positives) so identifying them would
> help. That said, I think set_track_prepare() is too conservative in
> waiting for kmemleak_initialized to be set in kmemleak_late_init().
> That's a late_initcall() meant for the scanning thread etc. not the core
> kmemleak functionality (which is on from early boot).
>
> We can instead use a different variable to check in set_track_prepare(),
> e.g. object_cache. stack_depot_early_init() is called prior to
> kmemleak_init(), so it should be fine.
>
> If "kmemleak_initialized" is confusing, we could rename it to
> "kmemleak_late_initialized" or "kmemleak_fully_initialized". I'm not too
> fussed about this as long as we add some comment on why we check
> object_cache instead of kmemleak_initialized.

Ok, I will send v2 version, use object_cache instead of kmemleak_initialized

to check in set_track_prepare, and update kmemleak_initialized to 
kmemleak_late_initialized

thanks

xiaolei

> --
> Catalin