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References: <20230929-hib_zero_bitmap_fix-v1-1-6cfdcb785250@quicinc.com>
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From:   Brian Geffon <bgeffon@google.com>
Date:   Mon, 2 Oct 2023 13:56:58 -0400
Message-ID: <CADyq12znHG_VPLVxSe+2ofX-WR1Uha2hu1MhoUAquMnoD_oP0w@mail.gmail.com>
Subject: Re: [PATCH] PM: hibernate: Fix a bug in copying the zero bitmap to
 safe pages
To:     Pavankumar Kondeti <quic_pkondeti@quicinc.com>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Pavel Machek <pavel@ucw.cz>, Len Brown <len.brown@intel.com>,
        kernel@quicinc.com,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org
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On Fri, Sep 29, 2023 at 1:31=E2=80=AFPM Pavankumar Kondeti
<quic_pkondeti@quicinc.com> wrote:
>
> The following crash is observed 100% of the time during resume from
> the hibernation on a x86 QEMU system.
>
> [   12.931887]  ? __die_body+0x1a/0x60
> [   12.932324]  ? page_fault_oops+0x156/0x420
> [   12.932824]  ? search_exception_tables+0x37/0x50
> [   12.933389]  ? fixup_exception+0x21/0x300
> [   12.933889]  ? exc_page_fault+0x69/0x150
> [   12.934371]  ? asm_exc_page_fault+0x26/0x30
> [   12.934869]  ? get_buffer.constprop.0+0xac/0x100
> [   12.935428]  snapshot_write_next+0x7c/0x9f0
> [   12.935929]  ? submit_bio_noacct_nocheck+0x2c2/0x370
> [   12.936530]  ? submit_bio_noacct+0x44/0x2c0
> [   12.937035]  ? hib_submit_io+0xa5/0x110
> [   12.937501]  load_image+0x83/0x1a0
> [   12.937919]  swsusp_read+0x17f/0x1d0
> [   12.938355]  ? create_basic_memory_bitmaps+0x1b7/0x240
> [   12.938967]  load_image_and_restore+0x45/0xc0
> [   12.939494]  software_resume+0x13c/0x180
> [   12.939994]  resume_store+0xa3/0x1d0
>
> The commit being fixed introduced a bug in copying the zero bitmap
> to safe pages. A temporary bitmap is allocated in prepare_image()
> to make a copy of zero bitmap after the unsafe pages are marked.
> Freeing this temporary bitmap later results in an inconsistent state
> of unsafe pages. Since free bit is left as is for this temporary bitmap
> after free, these pages are treated as unsafe pages when they are
> allocated again. This results in incorrect calculation of the number
> of pages pre-allocated for the image.
>
> nr_pages =3D (nr_zero_pages + nr_copy_pages) - nr_highmem - allocated_uns=
afe_pages;
>
> The allocate_unsafe_pages is estimated to be higher than the actual
> which results in running short of pages in safe_pages_list. Hence the
> crash is observed in get_buffer() due to NULL pointer access of
> safe_pages_list.

After reading through the code, perhaps I'm missing something, I'm not
sure that this is really fixing the problem.

It seems like the problem would be that memory_bm_create() results in
calls to get_image_page() w/ safe_needed =3D PG_ANY =3D=3D 0, meaning that
get_image_page() will not touch allocated_unsafe_pages and instead
will mark the page as in use by setting it in the forbidden_pages_map
and the free_pages_map simultaneously. The problem is that the
free_pages_map was already populated by the call to mark_unsafe_pages,
meaning that if we allocated a safe page in get_image_page() we just
set the free bit when it otherwise should not be set.

When the page is later free'd via the call to memory_bm_free(&tmp,
PG_UNSAFE_KEEP), it results in calls to free_image_page() w/
clear_page_nosave =3D PG_UNSAFE_KEEP =3D=3D 0. This means that we do not
touch the free_pages_map because we don't call
swsusp_unset_page_free().

With all that being said it seems like the correct way to deal with
that would be to do:
   error =3D memory_bm_create(&tmp, GFP_ATOMIC, PG_SAFE);
Here we know that the pages were not in the free_pages_map initially.

Followed by freeing it as:
    memory_bm_free(&tmp, PG_UNSAFE_CLEAR);
And here we know that swsusp_unset_page_free() will be called making
sure the page is not in the free_pages_map afterwards.

And that should result in an unchanged free_pages_map. Does that make
sense? Please correct me if I'm misunderstanding something.

>
> Fixes: 005e8dddd497 ("PM: hibernate: don't store zero pages in the image =
file")
> Signed-off-by: Pavankumar Kondeti <quic_pkondeti@quicinc.com>
> ---
>  kernel/power/snapshot.c | 23 ++++++++++++++---------
>  1 file changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
> index 87e9f7e2bdc0..cb7341a71a21 100644
> --- a/kernel/power/snapshot.c
> +++ b/kernel/power/snapshot.c
> @@ -2628,7 +2628,7 @@ static int prepare_image(struct memory_bitmap *new_=
bm, struct memory_bitmap *bm,
>                 struct memory_bitmap *zero_bm)
>  {
>         unsigned int nr_pages, nr_highmem;
> -       struct memory_bitmap tmp;
> +       struct memory_bitmap tmp_zero_bm;
>         struct linked_page *lp;
>         int error;
>
> @@ -2636,6 +2636,16 @@ static int prepare_image(struct memory_bitmap *new=
_bm, struct memory_bitmap *bm,
>         free_image_page(buffer, PG_UNSAFE_CLEAR);
>         buffer =3D NULL;
>
> +       /*
> +        * Allocate a temporary bitmap to create a copy of zero_bm in
> +        * safe pages. This allocation needs to be done before marking
> +        * unsafe pages below so that it can be freed without altering
> +        * the state of unsafe pages.
> +        */
> +       error =3D memory_bm_create(&tmp_zero_bm, GFP_ATOMIC, PG_ANY);
> +       if (error)
> +               goto Free;
> +
>         nr_highmem =3D count_highmem_image_pages(bm);
>         mark_unsafe_pages(bm);
>
> @@ -2646,12 +2656,7 @@ static int prepare_image(struct memory_bitmap *new=
_bm, struct memory_bitmap *bm,
>         duplicate_memory_bitmap(new_bm, bm);
>         memory_bm_free(bm, PG_UNSAFE_KEEP);
>
> -       /* Make a copy of zero_bm so it can be created in safe pages */
> -       error =3D memory_bm_create(&tmp, GFP_ATOMIC, PG_ANY);
> -       if (error)
> -               goto Free;
> -
> -       duplicate_memory_bitmap(&tmp, zero_bm);
> +       duplicate_memory_bitmap(&tmp_zero_bm, zero_bm);
>         memory_bm_free(zero_bm, PG_UNSAFE_KEEP);
>
>         /* Recreate zero_bm in safe pages */
> @@ -2659,8 +2664,8 @@ static int prepare_image(struct memory_bitmap *new_=
bm, struct memory_bitmap *bm,
>         if (error)
>                 goto Free;
>
> -       duplicate_memory_bitmap(zero_bm, &tmp);
> -       memory_bm_free(&tmp, PG_UNSAFE_KEEP);
> +       duplicate_memory_bitmap(zero_bm, &tmp_zero_bm);
> +       memory_bm_free(&tmp_zero_bm, PG_UNSAFE_KEEP);
>         /* At this point zero_bm is in safe pages and it can be used for =
restoring. */
>
>         if (nr_highmem > 0) {
>
> ---
> base-commit: 6465e260f48790807eef06b583b38ca9789b6072
> change-id: 20230929-hib_zero_bitmap_fix-bc5884eba0ae
>
> Best regards,
> --
> Pavankumar Kondeti <quic_pkondeti@quicinc.com>
>