linux-kernel - Re: [PATCH v2] mm: Unsigned 'nr_pages' always larger than zero

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Message-ID: <20190905031252.GN29434@bombadil.infradead.org>
Date:   Wed, 4 Sep 2019 20:12:52 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     zhong jiang <zhongjiang@...wei.com>
Cc:     akpm@...ux-foundation.org, vbabka@...e.cz, mhocko@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: Unsigned 'nr_pages' always larger than zero

On Thu, Sep 05, 2019 at 10:17:51AM +0800, zhong jiang wrote:
> With the help of unsigned_lesser_than_zero.cocci. Unsigned 'nr_pages'
> compare with zero. And __gup_longterm_locked pass an long local variant
> 'rc' to check_and_migrate_cma_pages. Hence it is nicer to change the
> parameter to long to fix the issue.

I think this patch is right, but I have concerns about this cocci grep.

The code says:

                if ((nr_pages > 0) && migrate_allow) {

There's nothing wrong with this (... other than the fact that nr_pages might
happen to be a negative errno).  nr_pages might be 0, and this would be
exactly the right test for that situation.  I suppose some might argue
that this should be != 0 instead of > 0, but it depends on the situation
which one would read better.

So please don't blindly make these changes; you're right this time.