Re: [PATCH -next 1/1] mm: hugetlb_vmemmap: Fix WARN_ON in vmemmap_remap_pte

Muchun Song
2022-10-25T06:36:50Z

> On Oct 25, 2022, at 09:42, Wupeng Ma <mawupeng1@huawei.com> wrote:
>
> From: Ma Wupeng <mawupeng1@huawei.com>
>
> Commit f41f2ed43ca5 ("mm: hugetlb: free the vmemmap pages associated with
> each HugeTLB page") add vmemmap_remap_pte to remap the tail pages as
> read-only to catch illegal write operation to the tail page.
>
> However this will lead to WARN_ON in arm64 in __check_racy_pte_update()

Thanks for your finding this issue.

> since this may lead to dirty state cleaned. This check is introduced by
> commit 2f4b829c625e ("arm64: Add support for hardware updates of the
> access and dirty pte bits") and the initial check is as follow:
>
> BUG_ON(pte_write(*ptep) && !pte_dirty(pte));
>
> Since we do need to mark this pte as read-only to catch illegal write
> operation to the tail pages, use set_pte to replace set_pte_at to bypass
> this check.

In theory, the waring does not affect anything since the tail vmemmap
pages are supposed to be read-only. So, skipping this check for vmemmap
pages seem feasible. But I am not sure whether it is general to use
set_pte here instead of set_pte_at, I didn’t see any users of set_pte
from the common code routines except the code from arch/xxx. And this
issue is specific for arm64, so I suggest fixing it in __check_racy_pte_update()
itself.

Something like (Just some thoughts from mine):

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index b5df82aa99e6..df7716965a93 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -292,7 +292,8 @@ extern void __sync_icache_dcache(pte_t pteval);
* PTE_DIRTY || (PTE_WRITE && !PTE_RDONLY)
*/

-static inline void __check_racy_pte_update(struct mm_struct *mm, pte_t *ptep,
+static inline void __check_racy_pte_update(struct mm_struct *mm,
+ unsigned long addr, pte_t *ptep,
pte_t pte)
{
pte_t old_pte;
@@ -307,6 +308,10 @@ static inline void __check_racy_pte_update(struct mm_struct *mm, pte_t *ptep,
if (mm != current->active_mm && atomic_read(&mm->mm_users) <= 1)
return;

+ if (IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP) &&
+ addr >= VMEMMAP_START && addr <= VMEMMAP_END)
+ return;
+
/*
* Check for potential race with hardware updates of the pte
* (ptep_set_access_flags safely changes valid ptes without going

>
> The following shell command can be used to reproduce this WARN_ON in
> 6.1.0-rc1:
>
> echo 1 > /proc/sys/vm/hugetlb_optimize_vmemmap
> cat /proc/sys/vm/hugetlb_optimize_vmemmap
>
> echo 1024 > /proc/sys/vm/nr_overcommit_hugepages
> mkdir -p /root/hugetlb
> mount none /root/hugetlb -t hugetlbfs
> fallocate -l 2g /root/hugetlb/xx &
>
> Here is the detail WARN_ON log:
>
> ------------[ cut here ]------------
> __check_racy_pte_update: racy dirty state clearing: 0x0068000416899f03 -> 0x0060000416898f83
> WARNING: CPU: 3 PID: 394 at arch/arm64/include/asm/pgtable.h:318 vmemmap_remap_pte+0x118/0x120
> Modules linked in:
> CPU: 3 PID: 394 Comm: fallocate Not tainted 6.1.0-rc1 #224
> Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015
> Call trace:
> vmemmap_remap_pte+0x118/0x120
> vmemmap_remap_range+0x30c/0x560
> hugetlb_vmemmap_optimize+0x158/0x408
> __prep_new_huge_page+0x24/0x150
> prep_new_huge_page+0x30/0x60
> alloc_fresh_huge_page+0x1c4/0x1e0
> alloc_surplus_huge_page+0xa0/0x168
> alloc_huge_page+0x264/0x5b8
> hugetlbfs_fallocate+0x294/0x680
> vfs_fallocate+0x12c/0x568
> ksys_fallocate+0x50/0xa0
> __arm64_sys_fallocate+0x28/0x38
> invoke_syscall+0x4c/0x110
> el0_svc_common.constprop.0+0x68/0x128
> do_el0_svc+0x34/0xd0
> el0_svc+0x48/0xb8
> el0t_64_sync_handler+0xb8/0xc0
> el0t_64_sync+0x18c/0x190
> ---[ end trace 0000000000000000 ]---
>
> Fixes: f41f2ed43ca5 ("mm: hugetlb: free the vmemmap pages associated with each HugeTLB page")

Actually, this commit does not pose the issue for arm64. I think the correct commit
which should be fixed is 1e63ac088f20f7a4425c430c31ecd3cf167fb3f2.

Thanks.

> Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
> ---
> mm/hugetlb_vmemmap.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index ba2a2596fb4e..cb056265c31e 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -249,7 +249,7 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
> struct page *page = pte_page(*pte);
>
> list_add_tail(&page->lru, walk->vmemmap_pages);
> - set_pte_at(&init_mm, addr, pte, entry);
> + set_pte(pte, entry);
> }
>
> /*
> --
> 2.25.1
>
>