linux-kernel - Re: [PATCH 2/2] mm/readahead: limit sync readahead while too many active refault

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Message-ID: <0ea62dea-94d1-a387-a527-5b0aa0a5fba6@huawei.com>
Date: Thu, 29 Feb 2024 17:01:50 +0800
From: Liu Shixin <liushixin2@...wei.com>
To: Jan Kara <jack@...e.cz>
CC: Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner
	<brauner@...nel.org>, Matthew Wilcox <willy@...radead.org>, Andrew Morton
	<akpm@...ux-foundation.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 2/2] mm/readahead: limit sync readahead while too many
 active refault



On 2024/2/2 17:02, Liu Shixin wrote:
>
> On 2024/2/2 1:31, Jan Kara wrote:
>> On Thu 01-02-24 18:41:30, Liu Shixin wrote:
>>> On 2024/2/1 17:37, Jan Kara wrote:
>>>> On Thu 01-02-24 18:08:35, Liu Shixin wrote:
>>>>> When the pagefault is not for write and the refault distance is close,
>>>>> the page will be activated directly. If there are too many such pages in
>>>>> a file, that means the pages may be reclaimed immediately.
>>>>> In such situation, there is no positive effect to read-ahead since it will
>>>>> only waste IO. So collect the number of such pages and when the number is
>>>>> too large, stop bothering with read-ahead for a while until it decreased
>>>>> automatically.
>>>>>
>>>>> Define 'too large' as 10000 experientially, which can solves the problem
>>>>> and does not affect by the occasional active refault.
>>>>>
>>>>> Signed-off-by: Liu Shixin <liushixin2@...wei.com>
>>>> So I'm not convinced this new logic is needed. We already have
>>>> ra->mmap_miss which gets incremented when a page fault has to read the page
>>>> (and decremented when a page fault found the page already in cache). This
>>>> should already work to detect trashing as well, shouldn't it? If it does
>>>> not, why?
>>>>
>>>> 								Honza
>>> ra->mmap_miss doesn't help, it increased only one in do_sync_mmap_readahead()
>>> and then decreased one for every page in filemap_map_pages(). So in this scenario,
>>> it can't exceed MMAP_LOTSAMISS.
>> I see, OK. But that's a (longstanding) bug in how mmap_miss is handled. Can
>> you please test whether attached patches fix the trashing for you? At least
>> now I can see mmap_miss properly increments when we are hitting uncached
>> pages...  Thanks!
>>
>> 								Honza
> The patch doesn't seem to have much effect. I will try to analyze why it doesn't work.
> The attached file is my testcase.
>
> Thanks,

I think I figured out why mmap_miss doesn't work. After do_sync_mmap_readahead(), there is a
__filemap_get_folio() to make sure the page is ready. Then, it is ready too in filemap_map_pages(),
so the mmap_miss will decreased once. mmap_miss goes back to 0, and can't stop read-ahead.
Overall, I don't think mmap_miss can solve this problem.

.