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SubjectRe: [PATCH -mm -V2] mm, swap: Fix false error message in __swp_swapcount()
Hi Huang,

On Fri, Oct 27, 2017 at 01:53:27PM +0800, Huang, Ying wrote:
> From: Huang Ying <huang.ying.caritas@gmail.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@linux.intel.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: <stable@vger.kernel.org> # 4.11-4.13
> Reported-by: Christian Kujau <lists@nerdbynature.de>
> Fixes: e8c26ab60598 ("mm/swap: skip readahead for unreferenced swap slots")
> Signed-off-by: "Huang, Ying" <ying.huang@intel.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)];
+}
+
/*
* 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;
+
blk_start_plug(&plug);
for (offset = start_offset; offset <= end_offset ; offset++) {
/* Ok, do the async read-ahead now */
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