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

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Message-ID: <20170711071612.GG24852@dhcp22.suse.cz>
Date:   Tue, 11 Jul 2017 09:16:12 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        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 08:56:04, 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."

I though this referred to this check
	if (vma->vm_next && vma->vm_next->vm_start < end)

becuase get_unampped_area with MAP_FIXED doesn't really check
anything. It will simply return the given address. Btw. this also rules
out find_vma.
 
> > 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.

Because it doesn't do anything AFAICS.

-- 
Michal Hocko
SUSE Labs