linux-kernel - Re: [PATCH v12 13/14] mm/vmalloc: Hugepage vmalloc mappings

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Date:   Fri, 19 Feb 2021 16:52:05 +0800
From:   Ding Tianhong <dingtianhong@...wei.com>
To:     Nicholas Piggin <npiggin@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>
CC:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Christoph Hellwig <hch@...radead.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        <linux-arch@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: Re: [PATCH v12 13/14] mm/vmalloc: Hugepage vmalloc mappings

On 2021/2/19 15:45, Nicholas Piggin wrote:
> Excerpts from Ding Tianhong's message of February 19, 2021 1:45 pm:
>> Hi Nicholas:
>>
>> I met some problem for this patch, like this:
>>
>> kva = vmalloc(3*1024k);
>>
>> remap_vmalloc_range(xxx, kva, xxx)
>>
>> It failed because that the check for page_count(page) is null so return, it break the some logic for current modules.
>> because the new huge page is not valid for composed page.
> 
> Hey Ding, that's a good catch. How are you testing this stuff, do you 
> have a particular driver that does this?
> 

yes, The driver would get a memory from the vmalloc in kernel space, and then the physical same memory will mmap to the user space. The drivers could not work when applying this patch.

>> I think some guys really don't get used to the changes for the vmalloc that the small pages was transparency to the hugepage
>> when the size is bigger than the PMD_SIZE.
> 
> I think in this case vmalloc could allocate the large page as a compound
> page which would solve this problem I think? (without having actually 
> tested it)
> 

yes, i think the __GFP_COMP flag could fix this.

>> can we think about give a new static huge page to fix it? just like use a a new vmalloc_huge_xxx function to disginguish the current function,
>> the user could choose to use the transparent hugepage or static hugepage for vmalloc.
> 
> Yeah that's a good question, there are a few things in the huge vmalloc 
> code that accounts things as small pages and you can't assume large or 
> small. If there is benefit from forcing large pages that could certainly
> be added.
> 

The vmalloc transparent is good, but not fit every user scenes, some guys like to use the deterministic function
for performance critical area.

Thanks
Ding

> Interestingly, remap_vmalloc_range in theory could map the pages as 
> large in userspace as well. That takes more work but if something
> really needs that for performance, it could be done.
> 
> Thanks,
> Nick
> .
>