linux-kernel - Re: [PATCH v2 4/8] mm/gup: track FOLL

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20200130113126.5ftq4gd5k7o7tipj@box>
Date:   Thu, 30 Jan 2020 14:31:26 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v2 4/8] mm/gup: track FOLL_PIN pages

On Wed, Jan 29, 2020 at 10:44:50PM -0800, John Hubbard wrote:
> On 1/29/20 5:51 AM, Kirill A. Shutemov wrote:
> > > +/**
> > > + * page_dma_pinned() - report if a page is pinned for DMA.
> > > + *
> > > + * This function checks if a page has been pinned via a call to
> > > + * pin_user_pages*().
> > > + *
> > > + * For non-huge pages, the return value is partially fuzzy: false is not fuzzy,
> > > + * because it means "definitely not pinned for DMA", but true means "probably
> > > + * pinned for DMA, but possibly a false positive due to having at least
> > > + * GUP_PIN_COUNTING_BIAS worth of normal page references".
> > > + *
> > > + * False positives are OK, because: a) it's unlikely for a page to get that many
> > > + * refcounts, and b) all the callers of this routine are expected to be able to
> > > + * deal gracefully with a false positive.
> > 
> > I wounder if we should reverse the logic and name -- page_not_dma_pinned()
> > or something -- too emphasise that we can only know for sure when the page
> > is not pinned, but not necessary when it is.
> > 
> 
> This is an interesting point. I agree that it's worth maybe adding information
> into the function name, but I'd like to keep the bool "positive", because there
> will be a number of callers that ask "if it is possibly dma-pinned, then ...".
> So combining that, how about this function name:
> 
> 	page_maybe_dma_pinned()
> 
> , which I could live with and I think would be acceptable?

I would still prefer the negative version, but up to you.

> > I see opportunity to split the patch further.
> 
> 
> ah, OK. I wasn't sure how far to go before I get tagged for "excessive
> patch splitting"! haha. Anyway, are you suggesting to put the
> page_ref_sub_return() routine into it's own patch?
> 
> Another thing to split out would be adding the flags to the remaining
> functions, such as undo_dev_pagemap(). That burns quite a few lines of
> diff. Anything else to split out?

Nothing I see immediately.

> 
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 0a55dec68925..b1079aaa6f24 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -958,6 +958,11 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
> > >   	 */
> > >   	WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
> > > +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> > > +	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
> > > +			 (FOLL_PIN | FOLL_GET)))
> > 
> > Too many parentheses.
> 
> 
> OK, I'll remove at least one. :)

I see two.

-- 
 Kirill A. Shutemov