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

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Date:   Fri, 19 Feb 2021 17:45:52 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Ding Tianhong <dingtianhong@...wei.com>, 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

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?

> 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)

> 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.

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