Re: [PATCH 2/2] mm/khugepaged: Convert release_pte_pages() to use folios

[Marek Szyprowski]

Hi,

On 14.01.2023 01:15, Vishal Moola (Oracle) wrote:
> Converts release_pte_pages() to use folios instead of pages.
>
> Signed-off-by: Vishal Moola (Oracle) <vishal.moola@xxxxxxxxx>

This patch has been merged some time ago to linux-next as commit 
9bdfeea46f49 ("mm/khugepaged: convert release_pte_pages() to use 
folios"). It took me a while to bisect this (mainly because I was busy 
with other things), but I finally found that this change is responsible 
for the following kernel panic:

Unable to handle kernel paging request at virtual address fffffc0000000008
Mem abort info:
   ESR = 0x0000000096000006
   EC = 0x25: DABT (current EL), IL = 32 bits
   SET = 0, FnV = 0
   EA = 0, S1PTW = 0
   FSC = 0x06: level 2 translation fault
Data abort info:
   ISV = 0, ISS = 0x00000006
   CM = 0, WnR = 0
swapper pgtable: 4k pages, 48-bit VAs, pgdp=0000000021efa000
[fffffc0000000008] pgd=10000000df05a003, p4d=10000000df05a003, 
pud=10000000df059003, pmd=0000000000000000
Internal error: Oops: 0000000096000006 [#1] PREEMPT SMP
Modules linked in: ip_tables x_tables ipv6
CPU: 7 PID: 61 Comm: khugepaged Not tainted 6.2.0-rc4+ #13307
Hardware name: Samsung TM2E board (DT)
pstate: 80000005 (Nzcv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : hpage_collapse_scan_pmd+0x12ec/0x1a20
lr : hpage_collapse_scan_pmd+0x14b0/0x1a20
sp : ffff80000be13c20
x29: ffff80000be13c20 x28: 0000000000000001 x27: fffffc0000d3f5c0
x26: fffffc0000d3f600 x25: 00000000000001f9 x24: 0000000000000007
x23: ffff0000296f9dd8 x22: ffff800009e5b490 x21: 0000000000000000
x20: 000000000000000f x19: ffff80000a9d0000 x18: ffff80000af52e58
x17: 0000000000000028 x16: 0000000000009249 x15: ffff80000af971f8
x14: 0000000000000000 x13: 00000000000443a0 x12: 0000000000040000
x11: 000000000fffffff x10: ffff000024928880 x9 : ffff80000b5c6e98
x8 : ffff000024928000 x7 : 00000000b35d04b9 x6 : 0000000000000000
x5 : fffffc0000000000 x4 : ffff8000cbf2e000 x3 : 0000000000000000
x2 : 0000000000000000 x1 : 0000000000000000 x0 : fffffc0000000000
Call trace:
  hpage_collapse_scan_pmd+0x12ec/0x1a20
  khugepaged+0x7e0/0x8dc
  kthread+0x118/0x11c
  ret_from_fork+0x10/0x20
Code: d34cbc43 cb813061 d37ae421 8b050020 (f9400404)
---[ end trace 0000000000000000 ]---
Kernel panic - not syncing: Oops: Fatal exception
SMP: stopping secondary CPUs
Kernel Offset: disabled
CPU features: 0x8c000,41c78100,0000421b
Memory Limit: none
---[ end Kernel panic - not syncing: Oops: Fatal exception ]---


Reverting it on top of recent linux-next fixes the issue, so it looks 
that some kind of a corner case is missing in this patch. I can 
reproduce it usually during the system shutdown, 1 of 20 times on the 
average.


> ---
>   mm/khugepaged.c | 14 +++++++-------
>   1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 4888e8688401..27d010431ece 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -509,20 +509,20 @@ static void release_pte_page(struct page *page)
>   static void release_pte_pages(pte_t *pte, pte_t *_pte,
>   		struct list_head *compound_pagelist)
>   {
> -	struct page *page, *tmp;
> +	struct folio *folio, *tmp;
>   
>   	while (--_pte >= pte) {
>   		pte_t pteval = *_pte;
>   
> -		page = pte_page(pteval);
> +		folio = pfn_folio(pte_pfn(pteval));
>   		if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)) &&
> -				!PageCompound(page))
> -			release_pte_page(page);
> +				!folio_test_large(folio))
> +			release_pte_folio(folio);
>   	}
>   
> -	list_for_each_entry_safe(page, tmp, compound_pagelist, lru) {
> -		list_del(&page->lru);
> -		release_pte_page(page);
> +	list_for_each_entry_safe(folio, tmp, compound_pagelist, lru) {
> +		list_del(&folio->lru);
> +		release_pte_folio(folio);
>   	}
>   }
>   

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland