linux-kernel - Re: [PATCH v2 1/4] mm/vmalloc: fix vmalloc_to

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Message-Id: <1586863573.ufpx8o7f0i.astroid@bobo.none>
Date:   Tue, 14 Apr 2020 21:31:49 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, x86@...nel.org
Subject: Re: [PATCH v2 1/4] mm/vmalloc: fix vmalloc_to_page for huge vmap
 mappings

Excerpts from Matthew Wilcox's message of April 13, 2020 11:34 pm:
> On Mon, Apr 13, 2020 at 10:53:00PM +1000, Nicholas Piggin 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.
>> 
>> This change teaches vmalloc_to_page about larger pages, and returns
>> the struct page that corresponds to the offset within the large page.
>> This makes the API agnostic to mapping implementation details.
> 
> I'm trying to get us away from returning tail pages from various
> functions.  How much of a pain would it be to return the head page
> instead of the tail page?

Well, this is a fix for the interface for HUGE_VMAP stuff so it
doesn't really make sense to change the implementation here. If you
want to change or make a different API that would be a later patch, no?

> Obviously the implementation gets simpler,
> but can the callers cope?  I've been focusing on the page cache, so I
> haven't been looking at the vmalloc side of things at all.

Well callers that operate on ioremap today (and vmalloc tomorrow) won't
cope, because they're expecting a base page. If you wanted to change it
I suspect the way to go would be introduce a new function and move
everyone over individually.

Thanks,
Nick