linux-kernel - Re: [PATCH -mm] mm, swap: Fix false error message in __swp

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Date:   Tue, 24 Oct 2017 23:34:11 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     "Huang\, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Minchan Kim <minchan@...nel.org>, <stable@...r.kernel.org>,
        Christian Kujau <lists@...dbynature.de>
Subject: Re: [PATCH -mm] mm, swap: Fix false error message in __swp_swapcount()

Michal Hocko <mhocko@...nel.org> writes:

> On Tue 24-10-17 23:15:32, Huang, Ying wrote:
>> Hi, Michal,
>> 
>> Michal Hocko <mhocko@...nel.org> writes:
>> 
>> > On Tue 24-10-17 10:47:00, Huang, Ying wrote:
>> >> From: Ying Huang <ying.huang@...el.com>
>> >> 
>> >> __swp_swapcount() is used in __read_swap_cache_async().  Where the
>> >> invalid swap entry (offset > max) may be supplied during swap
>> >> readahead.  But __swp_swapcount() will print error message for these
>> >> expected invalid swap entry as below, which will make the users
>> >> confusing.
>> >   ^^
>> > confused... And I have to admit this changelog has left me confused as
>> > well. What is an invalid swap entry in the readahead? Ohh, let me
>> > re-real Fixes: commit. It didn't really help "We can avoid needlessly
>> > allocating page for swap slots that are not used by anyone.  No pages
>> > have to be read in for these slots."
>> >
>> > Could you be more specific about when and how this happens please?
>> 
>> Sorry for confusing.
>> 
>> When page fault occurs for a swap entry, the original swap readahead
>> (not new 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 rely on the __swp_swapcount() and swapcache_prepare() to
>> check it.  Although __swp_swapcount() checks for the swap entry passed
>> in, it will complain with error message for the expected invalid swap
>> entry.  This makes the end user confusing.
>> 
>> Is this a little clearer.
>
> yes, this makes more sense (modulo the same typo ;)). Can you make this
> information into the changelog please? Thanks.

Oh, Yes!  I should fix it.  Sure, I will add this into the changelog.

Best Regards,
Huang, Ying