Re: [PATCH 2/3] proc: kpagecount: use snapshot_page()

[David Hildenbrand <david@redhat.com>]

On 26.06.25 20:16, Luiz Capitulino wrote:
> Currently, the call to folio_precise_page_mapcount() from kpage_read()
> can race with a folio split. When the race happens we trigger a
> VM_BUG_ON_FOLIO() in folio_entire_mapcount() (see splat below).
> 
> This commit fixes this race by using snapshot_page() so that we
> retreive the folio mapcount using a folio snapshot.
> 
> Splat:
> 
> [ 2356.558576] page: refcount:1 mapcount:1 mapping:0000000000000000 index:0xffff85200 pfn:0x6f7c00
> [ 2356.558748] memcg:ffff000651775780
> [ 2356.558763] anon flags: 0xafffff60020838(uptodate|dirty|lru|owner_2|swapbacked|node=1|zone=2|lastcpupid=0xfffff)
> [ 2356.558796] raw: 00afffff60020838 fffffdffdb5d0048 fffffdffdadf7fc8 ffff00064c1629c1
> [ 2356.558817] raw: 0000000ffff85200 0000000000000000 0000000100000000 ffff000651775780
> [ 2356.558839] page dumped because: VM_BUG_ON_FOLIO(!folio_test_large(folio))
> [ 2356.558882] ------------[ cut here ]------------
> [ 2356.558897] kernel BUG at ./include/linux/mm.h:1103!
> [ 2356.558982] Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
> [ 2356.564729] CPU: 8 UID: 0 PID: 1864 Comm: folio-split-rac Tainted: G S      W           6.15.0+ #3 PREEMPT(voluntary)
> [ 2356.566196] Tainted: [S]=CPU_OUT_OF_SPEC, [W]=WARN
> [ 2356.566814] Hardware name: Red Hat KVM, BIOS edk2-20241117-3.el9 11/17/2024
> [ 2356.567684] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> [ 2356.568563] pc : kpage_read.constprop.0+0x26c/0x290
> [ 2356.569605] lr : kpage_read.constprop.0+0x26c/0x290
> [ 2356.569992] sp : ffff80008fb739b0
> [ 2356.570263] x29: ffff80008fb739b0 x28: ffff00064aa69580 x27: 00000000ff000000
> [ 2356.570842] x26: 0000fffffffffff8 x25: ffff00064aa69580 x24: ffff80008fb73ae0
> [ 2356.571411] x23: 0000000000000001 x22: 0000ffff86c6e8b8 x21: 0000000000000008
> [ 2356.571978] x20: 00000000006f7c00 x19: 0000ffff86c6e8b8 x18: 0000000000000000
> [ 2356.572581] x17: 3630303066666666 x16: 0000000000000003 x15: 0000000000001000
> [ 2356.573217] x14: 00000000ffffffff x13: 0000000000000004 x12: 00aaaaaa00aaaaaa
> [ 2356.577674] x11: 0000000000000000 x10: 00aaaaaa00aaaaaa x9 : ffffbf3afca6c300
> [ 2356.578332] x8 : 0000000000000002 x7 : 0000000000000001 x6 : 0000000000000001
> [ 2356.578984] x5 : ffff000c79812408 x4 : 0000000000000000 x3 : 0000000000000000
> [ 2356.579635] x2 : 0000000000000000 x1 : ffff00064aa69580 x0 : 000000000000003e
> [ 2356.580286] Call trace:
> [ 2356.580524]  kpage_read.constprop.0+0x26c/0x290 (P)
> [ 2356.580982]  kpagecount_read+0x28/0x40
> [ 2356.581336]  proc_reg_read+0x38/0x100
> [ 2356.581681]  vfs_read+0xcc/0x320
> [ 2356.581992]  ksys_read+0x74/0x118
> [ 2356.582306]  __arm64_sys_read+0x24/0x38
> [ 2356.582668]  invoke_syscall+0x70/0x100
> [ 2356.583022]  el0_svc_common.constprop.0+0x48/0xf8
> [ 2356.583456]  do_el0_svc+0x28/0x40
> [ 2356.583930]  el0_svc+0x38/0x118
> [ 2356.584328]  el0t_64_sync_handler+0x144/0x168
> [ 2356.584883]  el0t_64_sync+0x19c/0x1a0
> [ 2356.585350] Code: aa0103e0 9003a541 91082021 97f813fc (d4210000)
> [ 2356.586130] ---[ end trace 0000000000000000 ]---
> [ 2356.587377] note: folio-split-rac[1864] exited with irqs disabled
> [ 2356.588050] note: folio-split-rac[1864] exited with preempt_count 1
> 
> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
> ---
>   fs/proc/page.c | 21 +++++++++++++++++----
>   1 file changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 999af26c7298..936f8bbe5a6f 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -43,6 +43,22 @@ static inline unsigned long get_max_dump_pfn(void)
>   #endif
>   }
>   
> +static u64 get_kpage_count(const struct page *page)
> +{
> +	struct page_snapshot ps;
> +	u64 ret;
> +
> +	snapshot_page(&ps, page);
> +
> +	if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
> +		ret = folio_precise_page_mapcount(&ps.folio_snapshot,
> +						  &ps.page_snapshot);
> +	else
> +		ret = folio_average_page_mapcount(&ps.folio_snapshot);
> +
> +	return ret;
> +}
> +
>   static ssize_t kpage_read(struct file *file, char __user *buf,
>   		size_t count, loff_t *ppos,
>   		enum kpage_operation op)
> @@ -75,10 +91,7 @@ static ssize_t kpage_read(struct file *file, char __user *buf,
>   				info = stable_page_flags(page);
>   				break;
>   			case KPAGE_COUNT:
> -				if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
> -					info = folio_precise_page_mapcount(page_folio(page), page);
> -				else
> -					info = folio_average_page_mapcount(page_folio(page));
> +				info = get_kpage_count(page);
>   				break;
>   			case KPAGE_CGROUP:
>   				info = page_cgroup_ino(page);

Yes, that should work

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb