Re: [v3 PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush

[Andrew Morton]

On Mon, 20 May 2019 11:17:32 +0800 Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> 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.