linux-kernel - Re: [PATCH v6 01/12] mm/vmalloc: fix vmalloc_to

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Date:   Sat, 22 Aug 2020 08:42:40 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.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,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        Zefan Li <lizefan@...wei.com>
Subject: Re: [PATCH v6 01/12] mm/vmalloc: fix vmalloc_to_page for huge vmap
 mappings

Excerpts from Andrew Morton's message of August 22, 2020 6:07 am:
> On Sat, 22 Aug 2020 01:12:05 +1000 Nicholas Piggin <npiggin@...il.com> wrote:
> 
>> vmalloc_to_page returns NULL for addresses mapped by larger pages[*].
>> Whether or not a vmap is huge depends on the architecture details,
>> alignments, boot options, etc., which the caller can not be expected
>> to know. Therefore HUGE_VMAP is a regression for vmalloc_to_page.
> 
> I assume this doesn't matter in current mainline?
> If wrong, then what are the user-visible effects and why no cc:stable?

I haven't heard any reports, but in theory it could cause a prolem. The
commit 029c54b095995 from the changelog was made to paper over it. But
that was fixed properly afterward I think by 737326aa510b.

Not sure of the user visible problems currently. I think generally you
wouldn't do vmalloc_to_page() on ioremap() memory, so maybe callilng it
a regression is a bit strong. _Technically_ a regression, maybe.

Thanks,
Nick