linux-kernel - Re: [PATCH -next 1/1] mm: hugetlb_vmemmap: Fix WARN_ON in vmemmap_remap

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Message-Id: <614E3E83-1EAB-4C39-AF9C-83C0CCF26218@linux.dev>
Date:   Tue, 25 Oct 2022 14:36:19 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Wupeng Ma <mawupeng1@...wei.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>, catalin.marinas@....com,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next 1/1] mm: hugetlb_vmemmap: Fix WARN_ON in
 vmemmap_remap_pte



> On Oct 25, 2022, at 09:42, Wupeng Ma <mawupeng1@...wei.com> wrote:
> 
> From: Ma Wupeng <mawupeng1@...wei.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@...wei.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
> 
>