Re: [PATCH] mm: shmem: do not call PageHWPoison on a ERR-page

[Yang Shi <shy828301@gmail.com>]

On Thu, Nov 11, 2021 at 3:20 AM Muchun Song <songmuchun@bytedance.com> wrote:
>
>
>
> On 2021/11/11 16:46, Ajay Garg wrote:
> > commit b9d02f1bdd98
> > ("mm: shmem: don't truncate page if memory failure happens")
> >
> > introduced a PageHWPoison(page) call in "shmem_read_mapping_page_gfp"
> > in shmem.c.
> >
> > Now, if "shmem_getpage_gfp" returns an error, page is set to ERR-page.
> > Therafter, calling PageHWPoison() on this ERR-page, causes KASAN to OOP
> > the kernel :
> >
> >   #############################
> > BUG: unable to handle page fault for address: fffffffffffffff4
> > PF: supervisor read access in kernel mode
> > PF: error_code(0x0000) - not-present page
> > PGD 18e019067 P4D 18e019067 PUD 18e01b067 PMD 0
> > Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC KASAN PTI
> > CPU: 0 PID: 4836 Comm: MATLAB Not tainted 5.15.0+ #18
> > Hardware name: Dell Inc. Latitude E6320/0GJF11, BIOS A19 11/14/2013
> > RIP: 0010:shmem_read_mapping_page_gfp+0xd3/0x140
> > Code: 4c 89 ff e8 6f eb ff ff 5a 59 85 c0 74 64 48 63 d8 48 89 5d 98 be 08 00 00 00 48 89 df e8 e5 67 0c 00 48 89 df e8 6d 5c 0c 00 <48> 8b 13 48 c7 c0 fb ff ff ff f7 c2 00 00 80 00 74 30 48 ba 00 00
> > RSP: 0018:ffff88806b33f998 EFLAGS: 00010246
> > RAX: 0000000000000000 RBX: fffffffffffffff4 RCX: ffffffffb7a37ba3
> > RDX: 0000000000000003 RSI: dffffc0000000000 RDI: fffffffffffffff4
> > RBP: ffff88806b33fa20 R08: 1ffffffffffffffe R09: fffffffffffffffb
> > R10: fffffbffffffffff R11: 0000000000000001 R12: 1ffff1100d667f33
> > R13: 00000000001120d2 R14: 00000000000005db R15: ffff88814e64e2d8
> > FS:  00007f379a384640(0000) GS:ffff888161a00000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: fffffffffffffff4 CR3: 00000000269dc004 CR4: 00000000000606f0
> > Call Trace:
> > <TASK>
> > ? shmem_fault+0x480/0x480
> > ? __cond_resched+0x1c/0x30
> > ? __kasan_check_read+0x11/0x20
> > shmem_get_pages+0x3a4/0xa70 [i915]
> > ? shmem_writeback+0x3b0/0x3b0 [i915]
> > ? i915_gem_object_wait_reservation+0x330/0x560 [i915]
> > ...
> > ...
> >   ################################
> >
> > So, we proceed with PageHWPoison() call, only if the page is not a
> > ERR-page.
> >
> >
> > P.S. : Alternative (optimised) solution :
> > ===========================================
> >
> > We could save some CPU cycles, if we directly replace
> >
> >          if (error)
> >                  page = ERR_PTR(error);
> >          else
> >                  unlock_page(page);
> >
> > with
> >
> >          if (error)
> >                  return ERR_PTR(error);
> >
> >
> > Fixes: b9d02f1bdd98 ("mm: shmem: don't truncate page if memory failure happens")
> > Signed-off-by: Ajay Garg <ajaygargnsit@gmail.com>
> > ---
> >   mm/shmem.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 23c91a8beb78..427863cbf0dc 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -4222,7 +4222,7 @@ struct page *shmem_read_mapping_page_gfp(struct address_space *mapping,
> >       else
> >               unlock_page(page);
> >
> > -     if (PageHWPoison(page))
> > +     if (!IS_ERR(page) && PageHWPoison(page))
> >               page = ERR_PTR(-EIO);
>
> How about the following changes since the above if block
> already do the judgment?
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index f0eee4e221a7..0c84b6624026 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -4195,13 +4195,13 @@ struct page *shmem_read_mapping_page_gfp(struct
> address_space *mapping,
>          BUG_ON(!shmem_mapping(mapping));
>          error = shmem_getpage_gfp(inode, index, &page, SGP_CACHE,
>                                    gfp, NULL, NULL, NULL);
> -       if (error)
> +       if (error) {
>                  page = ERR_PTR(error);
> -       else
> +       } else {
>                  unlock_page(page);
> -
> -       if (PageHWPoison(page))
> -               page = ERR_PTR(-EIO);
> +               if (PageHWPoison(page))
> +                       page = ERR_PTR(-EIO);
> +       }

Thanks guys. My bad. It is an apparent bug. IMHO, Muchun's version
seems better to me.

Anyway, whatever Andrew picks up could have Reviewed-by: Yang Shi
<shy828301@gmail.com>

>
>          return page;
>   #else
>
> >
> >       return page;
> >
>