linux-kernel - Re: [RFC] mm/mremap: Remove redundant checks inside vma

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Date:   Tue, 11 Jul 2017 13:22:53 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        mike.kravetz@...cle.com
Subject: Re: [RFC] mm/mremap: Remove redundant checks inside vma_expandable()

On Tue 11-07-17 16:38:46, Anshuman Khandual wrote:
> On 07/11/2017 12:26 PM, Vlastimil Babka wrote:
> > On 07/11/2017 08:50 AM, Michal Hocko wrote:
> >> On Tue 11-07-17 08:26:40, Vlastimil Babka wrote:
> >>> On 07/11/2017 08:03 AM, Michal Hocko wrote:
> >>>>
> >>>> Are you telling me that two if conditions cause more than a second
> >>>> difference? That sounds suspicious.
> >>>
> >>> It's removing also a call to get_unmapped_area(), AFAICS. That means a
> >>> vma search?
> >>
> >> Ohh, right. I have somehow missed that. Is this removal intentional?
> > 
> > I think it is: "Checking for availability of virtual address range at
> > the end of the VMA for the incremental size is also reduntant at this
> > point."
> > 
> >> The
> >> changelog is silent about it.
> > 
> > It doesn't explain why it's redundant, indeed. Unfortunately, the commit
> > f106af4e90ea ("fix checks for expand-in-place mremap") which added this,
> > also doesn't explain why it's needed.
> 
> Its redundant because there are calls to get_unmapped_area() down the
> line in the function whose failure will anyway fail the expansion of
> the VMA.

mremap code is quite complex and I am not sure you are right here.
Anyway, please make sure you document why you believe those checks are
not needed when resubmitting your patch.

-- 
Michal Hocko
SUSE Labs