linux-kernel - Re: [PATCH 04/16] MIPS: use the generic get_user_pages

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Date:   Tue, 25 Jun 2019 09:46:49 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Christoph Hellwig <hch@....de>, Kamal Dasu <kdasu.kdev@...il.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Nicholas Piggin <npiggin@...il.com>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-mips@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mm@...ck.org, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/16] MIPS: use the generic get_user_pages_fast code

On Fri, Jun 21, 2019 at 11:05:42AM -0300, Jason Gunthorpe wrote:
> Today this check is only being done on the get_user_pages_fast() -
> after this patch it is also done for __get_user_pages_fast().
> 
> Which means __get_user_pages_fast is now non-functional on a range of
> MIPS CPUs, but that seems OK as far as I can tell, so:

> However, looks to me like this patch is also a bug fix for this:

Yes.

> > -	pgdp = pgd_offset(mm, addr);
> > -	do {
> > -		pgd_t pgd = *pgdp;
> > -
> > -		next = pgd_addr_end(addr, end);
> > -		if (pgd_none(pgd))
> > -			goto slow;
> > -		if (!gup_pud_range(pgd, addr, next, gup_flags & FOLL_WRITE,
> > -				   pages, &nr))
> 
> This is different too, the core code has a p4d layer, but I see that
> whole thing gets NOP'd by the compiler as mips uses pgtable-nop4d.h -
> right?

Exactly.