Re: [PATCH 1/1] mm: kfence: skip kmemleak alloc in kfence_pool

[Marco Elver]

On Tue, 28 Jun 2022 at 08:41, Yee Lee <yee.lee@xxxxxxxxxxxx> wrote:
>
> On Fri, 2022-06-24 at 10:28 +0200, Marco Elver wrote:
>
> On Fri, 24 Jun 2022 at 10:20, 'Yee Lee' via kasan-dev
> <kasan-dev@xxxxxxxxxxxxxxxx> wrote:
>
>
> On Thu, 2022-06-23 at 13:59 +0200, Marco Elver wrote:
>
> On Thu, 23 Jun 2022 at 13:20, yee.lee via kasan-dev
> <kasan-dev@xxxxxxxxxxxxxxxx> wrote:
>
>
> From: Yee Lee <yee.lee@xxxxxxxxxxxx>
>
> Use MEMBLOCK_ALLOC_NOLEAKTRACE to skip kmemleak registration when
> the kfence pool is allocated from memblock. And the kmemleak_free
> later can be removed too.
>
>
> Is this purely meant to be a cleanup and non-functional change?
>
> Signed-off-by: Yee Lee <yee.lee@xxxxxxxxxxxx>
>
> ---
>  mm/kfence/core.c | 18 ++++++++----------
>  1 file changed, 8 insertions(+), 10 deletions(-)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index 4e7cd4c8e687..0d33d83f5244 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -600,14 +600,6 @@ static unsigned long kfence_init_pool(void)
>                 addr += 2 * PAGE_SIZE;
>         }
>
> -       /*
> -        * The pool is live and will never be deallocated from this
> point on.
> -        * Remove the pool object from the kmemleak object tree, as
> it would
> -        * otherwise overlap with allocations returned by
> kfence_alloc(), which
> -        * are registered with kmemleak through the slab post-alloc
> hook.
> -        */
> -       kmemleak_free(__kfence_pool);
>
>
> This appears to only be a non-functional change if the pool is
> allocated early. If the pool is allocated late using page-alloc, then
> there'll not be a kmemleak_free() on that memory and we'll have the
> same problem.
>
>
> Do you mean the kzalloc(slab_is_available) in memblock_allc()? That
> implies that MEMBLOCK_ALLOC_NOLEAKTRACE has no guarantee skipping
> kmemleak_alloc from this. (Maybe add it?)
>
>
> No, if KFENCE is initialized through kfence_init_late() ->
> kfence_init_pool_late() -> kfence_init_pool().
>
> Thanks for the information.
>
> But as I known, page-alloc does not request kmemleak areas.
> So the current kfence_pool_init_late() would cause another kmemleak warning on unknown freeing.
>
> Reproducing test: (kfence late enable + kmemleak debug on)
>
> / # echo 500 > /sys/module/kfence/parameters/sample_interval
> [  153.433518] kmemleak: Freeing unknown object at 0xffff0000c0600000
> [  153.433804] CPU: 0 PID: 100 Comm: sh Not tainted 5.19.0-rc3-74069-gde5c208d533a-dirty #1
> [  153.434027] Hardware name: linux,dummy-virt (DT)
> [  153.434265] Call trace:
> [  153.434331]  dump_backtrace+0xdc/0xfc
> [  153.434962]  show_stack+0x18/0x24
> [  153.435106]  dump_stack_lvl+0x64/0x7c
> [  153.435232]  dump_stack+0x18/0x38
> [  153.435347]  kmemleak_free+0x184/0x1c8
> [  153.435462]  kfence_init_pool+0x16c/0x194
> [  153.435587]  param_set_sample_interval+0xe0/0x1c4
> [  153.435694]  param_attr_store+0x98/0xf4
> [  153.435804]  module_attr_store+0x24/0x3c
> [  153.435910]  sysfs_kf_write+0x3c/0x50
> ...(skip)
> [  153.444496] kfence: initialized - using 524288 bytes for 63 objects at 0x00000000a3236b01-0x00000000901655d3
> / #
>
> Hence, now there are two issues to solve.
> (1) (The original)To prevent the undesired kmemleak scanning on the kfence pool. As Cataline's suggestion, we can just apply kmemleak_ignore_phys instead of free it at all.
> ref: https://lore.kernel.org/linux-mm/YrWPg3xIHbm9bFxP@xxxxxxx/
>
> (2) The late-allocated kfence pool doesn't need to go through kmemleak_free. We can relocate the opeartion to kfence_init_pool_early() to seperate them.
> That is, kfence_init_pool_early(memblock) has it and kfence_init_pool_late(page alloc) does not.
>
> The draft is like the following.

Looks reasonable - feel free to send v2.

Thanks,
-- Marco