linux-kernel - Re: [patch] mm, thp: fix defrag setting if newline is not used

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Date:   Fri, 17 Jan 2020 09:24:01 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch] mm, thp: fix defrag setting if newline is not used

On 1/17/20 4:16 AM, Andrew Morton wrote:
> On Tue, 14 Jan 2020 17:58:36 -0800 (PST) David Rientjes <rientjes@...gle.com> wrote:
> 
>> If thp defrag setting "defer" is used and a newline is *not* used when
>> writing to the sysfs file, this is interpreted as the "defer+madvise"
>> option.
>>
>> This is because we do prefix matching and if five characters are written
>> without a newline, the current code ends up comparing to the first five
>> bytes of the "defer+madvise" option and using that instead.
>>
>> Find the length of what the user is writing and use that to guide our
>> decision on which string comparison to do.
> 
> Gee, why is this code so complicated?  Can't we just do
> 
> 	if (sysfs_streq(buf, "always")) {
> 		...
> 	} else if sysfs_streq(buf, "defer+madvise")) {
> 		...
> 	}
> 	...

Yeah, if we knew this existed :)

We would lose the prefix matching but hopefully nobody will complain.