linux-kernel - Re: [PATCH v8 23/70] mm/mmap: change do_brk_flags() to expand existing VMA and add do_brk

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Message-ID: <20220428201947.GA1912192@roeck-us.net>
Date:   Thu, 28 Apr 2022 13:19:47 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     "maple-tree@...ts.infradead.org" <maple-tree@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yu Zhao <yuzhao@...gle.com>
Subject: Re: [PATCH v8 23/70] mm/mmap: change do_brk_flags() to expand
 existing VMA and add do_brk_munmap()

On Tue, Apr 26, 2022 at 03:06:35PM +0000, Liam Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> 
> Avoid allocating a new VMA when it a vma modification can occur.  When a
> brk() can expand or contract a VMA, then the single store operation will
> only modify one index of the maple tree instead of causing a node to split
> or coalesce.  This avoids unnecessary allocations/frees of maple tree
> nodes and VMAs.
> 
> Move some limit & flag verifications out of the do_brk_flags() function to
> use only relevant checks in the code path of bkr() and vm_brk_flags().
> 
> Set the vma to check if it can expand in vm_brk_flags() if extra criteria
> are met.
> 
> Drop userfaultfd from do_brk_flags() path and only use it in
> vm_brk_flags() path since that is the only place a munmap will happen.
> 
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>

A build failure seen when building ppc64:corenet64_smp_defconfig also
biscects to this patch.

mm/mmap.c: In function 'do_brk_flags':
mm/mmap.c:2908:17: error: implicit declaration of function
	'khugepaged_enter_vma_merge'; did you mean 'khugepaged_enter_vma'?

It appears that this is later fixed, but it hurts bisectability
(and prevents me from finding the actual build failure in linux-next
when trying to build corenet64_smp_defconfig).

While looking into the patch, I noticed the following.

[ ... ]

> @@ -2773,30 +2888,52 @@ static int do_brk_flags(unsigned long addr, unsigned long len,
>  	if (security_vm_enough_memory_mm(mm, len >> PAGE_SHIFT))
>  		return -ENOMEM;
>  
>  out:
>  	perf_event_mmap(vma);
>  	mm->total_vm += len >> PAGE_SHIFT;
> @@ -2807,18 +2944,21 @@ static int do_brk_flags(unsigned long addr, unsigned long len,
>  	validate_mm_mt(mm);
>  	return 0;
>  
> -no_vma_link:
>  	vm_area_free(vma);

That really looks wrong. vm_area_free(vma) can not be reached
after this patch has been applied.

Guenter

> +vma_alloc_fail:
> +	vm_unacct_memory(len >> PAGE_SHIFT);
>  	return -ENOMEM;
>  }
>