linux-kernel - Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

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Date:   Thu, 04 Jan 2018 19:38:08 +0800
From:   "夷则(Caspar)" <jinli.zjl@...baba-inc.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     "Andrew Morton" <akpm@...ux-foundation.org>,
        <green@...uxhacker.ru>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        "杨勇(智彻)" <zhiche.yy@...baba-inc.com>,
        "十刀" <shidao.ytt@...baba-inc.com>
Subject: Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof



On 2018/1/4 19:34, Mel Gorman wrote:
> On Thu, Jan 04, 2018 at 02:13:43PM +0800, ??????(Caspar) wrote:
>>
>>
>> On 2018/1/3 18:48, Mel Gorman wrote:
>>> On Wed, Jan 03, 2018 at 02:53:43PM +0800, ??????(Caspar) wrote:
>>>>
>>>>
>>>>> ?? 2017??12??23????12:16?????? <shidao.ytt@...baba-inc.com> ??????
>>>>>
>>>>> From: "shidao.ytt" <shidao.ytt@...baba-inc.com>
>>>>>
>>>>> in commit 441c228f817f7 ("mm: fadvise: document the
>>>>> fadvise(FADV_DONTNEED) behaviour for partial pages") Mel Gorman
>>>>> explained why partial pages should be preserved instead of discarded
>>>>> when using fadvise(FADV_DONTNEED), however the actual codes to calcuate
>>>>> end_index was unexpectedly wrong, the code behavior didn't match to the
>>>>> statement in comments; Luckily in another commit 18aba41cbf
>>>>> ("mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED")
>>>>> Oleg Drokin fixed this behavior
>>>>>
>>>>> Here I come up with a new idea that actually we can still discard the
>>>>> last parital page iff the page-unaligned endbyte is also the end of
>>>>> file, since no one else will use the rest of the page and it should be
>>>>> safe enough to discard.
>>>>
>>>> +akpm...
>>>>
>>>> Hi Mel, Andrew:
>>>>
>>>> Would you please take a look at this patch, to see if this proposal
>>>> is reasonable enough, thanks in advance!
>>>>
>>>
>>> I'm backlogged after being out for the Christmas. Superficially the patch
>>> looks ok but I wondered how often it happened in practice as we already
>>> would discard files smaller than a page on DONTNEED. It also requires
>>
>> Actually, we would *not*. Let's look into the codes.
>>
> 
> You're right of course. I suggest updating the changelog with what you
> found and the test case. I think it's reasonable to special case the
> discarding of partial pages if it's the end of a file with the potential
> addendum of checking if the endbyte is past the end of the file. The man
> page should also be updated.

Sure, will do and send out v2.

Thanks,
Caspar
>