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    SubjectRe: [PATCH v4 19/45] kmsan: unpoison @tlb in arch_tlb_gather_mmu()
    On Fri, 1 Jul 2022 at 16:24, Alexander Potapenko <glider@google.com> wrote:
    >
    > This is a hack to reduce stackdepot pressure.

    Will it cause false negatives or other issues? If not, I'd just call
    it an optimization and not a hack.

    > struct mmu_gather contains 7 1-bit fields packed into a 32-bit unsigned
    > int value. The remaining 25 bits remain uninitialized and are never used,
    > but KMSAN updates the origin for them in zap_pXX_range() in mm/memory.c,
    > thus creating very long origin chains. This is technically correct, but
    > consumes too much memory.
    >
    > Unpoisoning the whole structure will prevent creating such chains.
    >
    > Signed-off-by: Alexander Potapenko <glider@google.com>

    Acked-by: Marco Elver <elver@google.com>

    > ---
    > Link: https://linux-review.googlesource.com/id/I76abee411b8323acfdbc29bc3a60dca8cff2de77
    > ---
    > mm/mmu_gather.c | 10 ++++++++++
    > 1 file changed, 10 insertions(+)
    >
    > diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
    > index a71924bd38c0d..add4244e5790d 100644
    > --- a/mm/mmu_gather.c
    > +++ b/mm/mmu_gather.c
    > @@ -1,6 +1,7 @@
    > #include <linux/gfp.h>
    > #include <linux/highmem.h>
    > #include <linux/kernel.h>
    > +#include <linux/kmsan-checks.h>
    > #include <linux/mmdebug.h>
    > #include <linux/mm_types.h>
    > #include <linux/mm_inline.h>
    > @@ -265,6 +266,15 @@ void tlb_flush_mmu(struct mmu_gather *tlb)
    > static void __tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
    > bool fullmm)
    > {
    > + /*
    > + * struct mmu_gather contains 7 1-bit fields packed into a 32-bit
    > + * unsigned int value. The remaining 25 bits remain uninitialized
    > + * and are never used, but KMSAN updates the origin for them in
    > + * zap_pXX_range() in mm/memory.c, thus creating very long origin
    > + * chains. This is technically correct, but consumes too much memory.
    > + * Unpoisoning the whole structure will prevent creating such chains.
    > + */
    > + kmsan_unpoison_memory(tlb, sizeof(*tlb));
    > tlb->mm = mm;
    > tlb->fullmm = fullmm;
    >
    > --
    > 2.37.0.rc0.161.g10f37bed90-goog
    >

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