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Message-ID: <87inevc26n.fsf@yhuang-dev.intel.com>
Date: Tue, 31 Oct 2017 13:32:32 +0800
From: "Huang\, Ying" <ying.huang@...el.com>
To: Minchan Kim <minchan@...nel.org>
Cc: "Huang\, Ying" <ying.huang@...el.com>,
Andrew Morton <akpm@...ux-foundation.org>,
<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
Huang Ying <huang.ying.caritas@...il.com>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Michal Hocko <mhocko@...e.com>, <stable@...r.kernel.org>,
Christian Kujau <lists@...dbynature.de>
Subject: Re: [PATCH -mm -V2] mm, swap: Fix false error message in __swp_swapcount()
Hi, Minchan,
Minchan Kim <minchan@...nel.org> writes:
> Hi Huang,
>
> On Fri, Oct 27, 2017 at 01:53:27PM +0800, Huang, Ying wrote:
>> From: Huang Ying <huang.ying.caritas@...il.com>
>>
>> When a page fault occurs for a swap entry, the physical swap readahead
>> (not the VMA base swap readahead) may readahead several swap entries
>> after the fault swap entry. The readahead algorithm calculates some
>> of the swap entries to readahead via increasing the offset of the
>> fault swap entry without checking whether they are beyond the end of
>> the swap device and it relys on the __swp_swapcount() and
>> swapcache_prepare() to check it. Although __swp_swapcount() checks
>> for the swap entry passed in, it will complain with the error message
>> as follow for the expected invalid swap entry. This may make the end
>> users confused.
>>
>> swap_info_get: Bad swap offset entry 0200f8a7
>>
>> To fix the false error message, the swap entry checking is added in
>> swap readahead to avoid to pass the out-bound swap entries and the
>> swap entry reserved for the swap header to __swp_swapcount() and
>> swapcache_prepare().
>>
>> Cc: Tim Chen <tim.c.chen@...ux.intel.com>
>> Cc: Minchan Kim <minchan@...nel.org>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: <stable@...r.kernel.org> # 4.11-4.13
>> Reported-by: Christian Kujau <lists@...dbynature.de>
>> Fixes: e8c26ab60598 ("mm/swap: skip readahead for unreferenced swap slots")
>> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
>> ---
>> include/linux/swap.h | 1 +
>> mm/swap_state.c | 6 ++++--
>> mm/swapfile.c | 21 +++++++++++++++++++++
>> 3 files changed, 26 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 84255b3da7c1..43b4b821c805 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -476,6 +476,7 @@ extern int page_swapcount(struct page *);
>> extern int __swap_count(struct swap_info_struct *si, swp_entry_t entry);
>> extern int __swp_swapcount(swp_entry_t entry);
>> extern int swp_swapcount(swp_entry_t entry);
>> +extern bool swap_entry_check(swp_entry_t entry);
>> extern struct swap_info_struct *page_swap_info(struct page *);
>> extern struct swap_info_struct *swp_swap_info(swp_entry_t entry);
>> extern bool reuse_swap_page(struct page *, int *);
>> diff --git a/mm/swap_state.c b/mm/swap_state.c
>> index 6c017ced11e6..7dd70e77058d 100644
>> --- a/mm/swap_state.c
>> +++ b/mm/swap_state.c
>> @@ -569,11 +569,13 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
>> /* Read a page_cluster sized and aligned cluster around offset. */
>> start_offset = offset & ~mask;
>> end_offset = offset | mask;
>> - if (!start_offset) /* First page is swap header. */
>> - start_offset++;
>>
>> blk_start_plug(&plug);
>> for (offset = start_offset; offset <= end_offset ; offset++) {
>> + swp_entry_t ent = swp_entry(swp_type(entry), offset);
>> +
>> + if (!swap_entry_check(ent))
>> + continue;
>> /* Ok, do the async read-ahead now */
>> page = __read_swap_cache_async(
>> swp_entry(swp_type(entry), offset),
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 3074b02eaa09..b04cec29c234 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -1107,6 +1107,27 @@ static struct swap_info_struct *swap_info_get_cont(swp_entry_t entry,
>> return p;
>> }
>>
>> +bool swap_entry_check(swp_entry_t entry)
>> +{
>> + struct swap_info_struct *p;
>> + unsigned long offset, type;
>> +
>> + type = swp_type(entry);
>> + if (type >= nr_swapfiles)
>> + goto bad_file;
>> + p = swap_info[type];
>> + offset = swp_offset(entry);
>> + if (unlikely(!offset || offset >= p->max))
>> + goto out;
>> +
>> + return true;
>> +
>> +bad_file:
>> + pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
>> +out:
>> + return false;
>> +}
>> +
>> static unsigned char __swap_entry_free(struct swap_info_struct *p,
>> swp_entry_t entry, unsigned char usage)
>> {
>> --
>> 2.14.2
>
> Although it's better than old, we can make it simple, still.
>
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 291c4b534658..f50d5a48f03a 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -41,6 +41,13 @@ static inline unsigned swp_type(swp_entry_t entry)
> return (entry.val >> SWP_TYPE_SHIFT(entry));
> }
>
> +extern struct swap_info_struct *swap_info[];
> +
> +static inline struct swap_info_struct *swp_si(swp_entry_t entry)
> +{
> + return swap_info[swp_type(entry)];
> +}
> +
Why not just use swp_swap_info()?
> /*
> * Extract the `offset' field from a swp_entry_t. The swp_entry_t is in
> * arch-independent format
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 378262d3a197..a0fe2d54ad09 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -554,6 +554,7 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> struct vm_area_struct *vma, unsigned long addr)
> {
> struct page *page;
> + struct swap_info_struct *si = swp_si(entry);
> unsigned long entry_offset = swp_offset(entry);
> unsigned long offset = entry_offset;
> unsigned long start_offset, end_offset;
> @@ -572,6 +573,9 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> if (!start_offset) /* First page is swap header. */
> start_offset++;
>
> + if (end_offset >= si->max)
> + end_offset = si->max - 1;
> +
I don't think two implementation has much difference. Both look OK for
me.
Best Regards,
Huang, Ying
> blk_start_plug(&plug);
> for (offset = start_offset; offset <= end_offset ; offset++) {
> /* Ok, do the async read-ahead now */
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