linux-kernel - Re: [PATCH 1/3] mm/mincore: make mincore() more conservative

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Message-ID: <20190307004036.GA16785@nautica>
Date:   Thu, 7 Mar 2019 01:40:36 +0100
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-api@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jann Horn <jannh@...gle.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Chinner <david@...morbit.com>,
        Kevin Easton <kevin@...rana.org>,
        Matthew Wilcox <willy@...radead.org>,
        Cyril Hrubis <chrubis@...e.cz>, Tejun Heo <tj@...nel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Daniel Gruss <daniel@...ss.cc>
Subject: Re: [PATCH 1/3] mm/mincore: make mincore() more conservative

Jiri Kosina wrote on Thu, Mar 07, 2019:
> > I'm not sure this is correct in all cases.   If
> > 
> > 	addr = 4095
> > 	vma->vm_end = 4096
> > 	pages = 1000
> > 
> > then `end' is 4096 and `(end - addr) << PAGE_SHIFT' is zero, but it
> > should have been 1.
> 
> Good catch! It should rather be something like
> 
> 	unsigned long pages = (end >> PAGE_SHIFT) - (addr >> PAGE_SHIFT);

That would be 0 for addr = 0 and vma->vm_end = 1; I assume we would
still want to count that as one page.
I'm not too familiar with this area of the code, but I think there's a
handy macro we can use for this, perhaps
  DIV_ROUND_UP(end - addr, PAGE_SIZE) ?

kernel/kexec_core.c has defined PAGE_COUNT() which seems more
appropriate but I do not see a global equivalent
#define PAGE_COUNT(x) (((x) + PAGE_SIZE - 1) >> PAGE_SHIFT)

-- 
Dominique