linux-kernel - Re: [PATCH 2/2] mm: khugepaged: check MMF_DISABLE

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAHbLzkrSaODz+SeT+GY3tOS6Jm8NSznmknP83RUCrn+Sr9cAAg@mail.gmail.com>
Date:   Mon, 5 Apr 2021 11:20:47 -0700
From:   Yang Shi <shy828301@...il.com>
To:     yanfei.xu@...driver.com
Cc:     Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: khugepaged: check MMF_DISABLE_THP ahead of
 iterating over vmas

On Sun, Apr 4, 2021 at 8:33 AM <yanfei.xu@...driver.com> wrote:
>
> From: Yanfei Xu <yanfei.xu@...driver.com>
>
> We could check MMF_DISABLE_THP ahead of iterating over all of vma.
> Otherwise if some mm_struct contain a large number of vma, there will
> be amounts meaningless cpu cycles cost.
>
> BTW, drop an unnecessary cond_resched(), because there is a another
> cond_resched() followed it and no consumed invocation between them.
>
> Signed-off-by: Yanfei Xu <yanfei.xu@...driver.com>
> ---
>  mm/khugepaged.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 2efe1d0c92ed..c293ec4a94ea 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2094,6 +2094,8 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
>          */
>         if (unlikely(!mmap_read_trylock(mm)))
>                 goto breakouterloop_mmap_lock;
> +       if (test_bit(MMF_DISABLE_THP, &mm->flags))
> +               goto breakouterloop_mmap_lock;

It is fine to check this flag. But mmap_lock has been acquired so you
should jump to breakouterloop.

>         if (likely(!khugepaged_test_exit(mm)))
>                 vma = find_vma(mm, khugepaged_scan.address);
>
> @@ -2101,7 +2103,6 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
>         for (; vma; vma = vma->vm_next) {
>                 unsigned long hstart, hend;
>
> -               cond_resched();

I don't have a strong opinion for removing this cond_resched(). But
IIUC khugepaged is a best effort job there is no harm to keep it IMHO.

>                 if (unlikely(khugepaged_test_exit(mm))) {
>                         progress++;
>                         break;
> --
> 2.27.0
>
>