Subject: Re: [PATCH] mm: shmem: do not call PageHWPoison on a ERR-page
To:     Ajay Garg <ajaygargnsit@gmail.com>, hughd@google.com,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20211111084617.6746-1-ajaygargnsit@gmail.com>
From:   Muchun Song <songmuchun@bytedance.com>
Message-ID: <6864f744-ca3e-f328-8793-2adb0146db03@bytedance.com>
Date:   Thu, 11 Nov 2021 19:06:53 +0800
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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);
+       }

         return page;
  #else

>   
>   	return page;
>