linux-kernel - Re: [v3 PATCH] mm: mmu_gather: remove __tlb_reset

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Date:   Wed, 22 May 2019 09:00:34 +0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     jstancek@...hat.com, peterz@...radead.org, will.deacon@....com,
        npiggin@...il.com, aneesh.kumar@...ux.ibm.com, namit@...are.com,
        minchan@...nel.org, mgorman@...e.de, stable@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [v3 PATCH] mm: mmu_gather: remove __tlb_reset_range() for force
 flush



On 5/22/19 7:18 AM, Andrew Morton wrote:
> On Mon, 20 May 2019 11:17:32 +0800 Yang Shi <yang.shi@...ux.alibaba.com> wrote:
>
>> A few new fields were added to mmu_gather to make TLB flush smarter for
>> huge page by telling what level of page table is changed.
>>
>> __tlb_reset_range() is used to reset all these page table state to
>> unchanged, which is called by TLB flush for parallel mapping changes for
>> the same range under non-exclusive lock (i.e. read mmap_sem).  Before
>> commit dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in
>> munmap"), the syscalls (e.g. MADV_DONTNEED, MADV_FREE) which may update
>> PTEs in parallel don't remove page tables.  But, the forementioned
>> commit may do munmap() under read mmap_sem and free page tables.  This
>> may result in program hang on aarch64 reported by Jan Stancek.  The
>> problem could be reproduced by his test program with slightly modified
>> below.
>>
>> ...
>>
>> Use fullmm flush since it yields much better performance on aarch64 and
>> non-fullmm doesn't yields significant difference on x86.
>>
>> The original proposed fix came from Jan Stancek who mainly debugged this
>> issue, I just wrapped up everything together.
> Thanks.  I'll add
>
> Fixes: dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap")
>
> to this.

Thanks, Andrew.