linux-kernel - Re: [PATCH v4 4/4] mm/shmem: fix shmem

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Date:   Sun, 25 Apr 2021 12:20:13 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     <akpm@...ux-foundation.org>, <dennis@...nel.org>,
        <tim.c.chen@...ux.intel.com>, <hughd@...gle.com>,
        <hannes@...xchg.org>, <mhocko@...e.com>, <iamjoonsoo.kim@....com>,
        <alexs@...nel.org>, <willy@...radead.org>, <minchan@...nel.org>,
        <richard.weiyang@...il.com>, <shy828301@...il.com>,
        <david@...hat.com>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>
Subject: Re: [PATCH v4 4/4] mm/shmem: fix shmem_swapin() race with swapoff

Miaohe Lin <linmiaohe@...wei.com> writes:

> On 2021/4/25 11:07, Huang, Ying wrote:
>> I think it's better to put_swap_device() just before returning from the
>> function.  It's not a big issue to slow down swapoff() a little.  And
>> this will make the logic easier to be understood.
>> 
>
> shmem_swapin_page() already has a methed, i.e. locked page, to prevent races. I was intended
> to not mix with that. But your suggestion is good as this will make the logic easier to be
> understood.
>
> Just to make sure, is this what you mean? Many thanks!

Yes.  Just a minor comment.

>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 26c76b13ad23..737e5b3200c3 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1696,6 +1696,7 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>         struct address_space *mapping = inode->i_mapping;
>         struct shmem_inode_info *info = SHMEM_I(inode);
>         struct mm_struct *charge_mm = vma ? vma->vm_mm : current->mm;
> +       struct swap_info_struct *si;
>         struct page *page;
>         swp_entry_t swap;
>         int error;
> @@ -1704,6 +1705,12 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>         swap = radix_to_swp_entry(*pagep);
>         *pagep = NULL;
>
> +       /* Prevent swapoff from happening to us. */
> +       si = get_swap_device(swap);
> +       if (unlikely(!si)) {

I don't think it's necessary to use unlikely() here.

Best Regards,
Huang, Ying

> +               error = EINVAL;
> +               goto failed;
> +       }
>         /* Look it up and read it in.. */
>         page = lookup_swap_cache(swap, NULL, 0);
>         if (!page) {
> @@ -1765,6 +1772,8 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>         swap_free(swap);
>
>         *pagep = page;
> +       if (si)
> +               put_swap_device(si);
>         return 0;
>  failed:
>         if (!shmem_confirm_swap(mapping, index, swap))
> @@ -1775,6 +1784,9 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>                 put_page(page);
>         }
>
> +       if (si)
> +               put_swap_device(si);
> +
>         return error;
>  }
>
>> Best Regards,
>> Huang, Ying
>>