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SubjectRe: [PATCH v4 4/4] mm/shmem: fix shmem_swapin() race with swapoff
Date
Miaohe Lin <linmiaohe@huawei.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
>>

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