linux-kernel - Re: [PATCH -V5 RESEND 03/21] swap: Support PMD swap mapping in swap

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Date:   Tue, 25 Sep 2018 12:19:53 -0700
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Huang Ying <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Shaohua Li <shli@...nel.org>, Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Zi Yan <zi.yan@...rutgers.edu>,
        Daniel Jordan <daniel.m.jordan@...cle.com>
Subject: Re: [PATCH -V5 RESEND 03/21] swap: Support PMD swap mapping in
 swap_duplicate()

On Tue, Sep 25, 2018 at 03:13:30PM +0800, Huang Ying wrote:
> @@ -3487,35 +3521,66 @@ static int __swap_duplicate_locked(struct swap_info_struct *p,
>  }
>  
>  /*
> - * Verify that a swap entry is valid and increment its swap map count.
> + * Verify that the swap entries from *entry is valid and increment their
> + * PMD/PTE swap mapping count.
>   *
>   * Returns error code in following case.
>   * - success -> 0
>   * - swp_entry is invalid -> EINVAL
> - * - swp_entry is migration entry -> EINVAL

I'm assuming it wasn't possible to hit this error before this patch, and you're
just removing it now since you're in the area?

>   * - swap-cache reference is requested but there is already one. -> EEXIST
>   * - swap-cache reference is requested but the entry is not used. -> ENOENT
>   * - swap-mapped reference requested but needs continued swap count. -> ENOMEM
> + * - the huge swap cluster has been split. -> ENOTDIR

Strangely intuitive choice of error code :)

>  /*
>   * Increase reference count of swap entry by 1.
> - * Returns 0 for success, or -ENOMEM if a swap_count_continuation is required
> - * but could not be atomically allocated.  Returns 0, just as if it succeeded,
> - * if __swap_duplicate() fails for another reason (-EINVAL or -ENOENT), which
> - * might occur if a page table entry has got corrupted.
> + *
> + * Return error code in following case.
> + * - success -> 0
> + * - swap_count_continuation is required but could not be atomically allocated.
> + *   *entry is used to return swap entry to call add_swap_count_continuation().
> + *								      -> ENOMEM
> + * - otherwise same as __swap_duplicate()
>   */
> -int swap_duplicate(swp_entry_t entry)
> +int swap_duplicate(swp_entry_t *entry, int entry_size)
>  {
>  	int err = 0;
>  
> -	while (!err && __swap_duplicate(entry, 1) == -ENOMEM)
> -		err = add_swap_count_continuation(entry, GFP_ATOMIC);
> +	while (!err &&
> +	       (err = __swap_duplicate(entry, entry_size, 1)) == -ENOMEM)
> +		err = add_swap_count_continuation(*entry, GFP_ATOMIC);
>  	return err;

Now we're returning any error we get from __swap_duplicate, apparently to
accommodate ENOTDIR later in the series, which is a change from the behavior
introduced in 570a335b8e22 ("swap_info: swap count continuations").  This might
belong in a separate patch given its potential for side effects.

Although, I don't understand why 570a335b8e22 ignored errors other than -ENOMEM
when both swap_duplicate callers _seem_ from a quick read to be able to respond
gracefully to any error.