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

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Date: Fri, 2 Feb 2024 17:02:00 +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 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,
>


View attachment "test.sh" of type "text/plain" (385 bytes)

View attachment "alloc_page.c" of type "text/plain" (419 bytes)