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

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Date:   Thu, 7 Mar 2019 06:46:00 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Dominique Martinet <asmadeus@...ewreck.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

On Thu, 7 Mar 2019, Dominique Martinet wrote:

> > > 
> > > 	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.

Yeah, that was bogus as well, ETOOTIRED yesterday, sorry for the noise. 
Both the variants are off.

> 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)

I'll fix that up when doing the other changes requested by Andrew.

Thanks,

-- 
Jiri Kosina
SUSE Labs