Re: [PATCH RESEND v2 3/4] mm: sparsemem: use page table lock to protect kernel pmd operations

[Muchun Song]

On Sat, Sep 18, 2021 at 1:07 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Sat, Sep 18, 2021 at 12:09 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> >
> > The init_mm.page_table_lock is used to protect kernel page tables, we
> > can use it to serialize splitting vmemmap PMD mappings instead of mmap
> > write lock, which can increase the concurrency of vmemmap_remap_free().
> >
>
> Curious what is the actual benefit we get in user scenarios from this patch,
> 1. we set bootargs to reserve hugetlb statically
> 2. we "echo" some figures to sys or proc.
>
> In other words, Who is going to care about this concurrency?

Actually, It increase the concurrency between allocations of
HugeTLB pages. But it is not my first consideration. There are
a lot of users of mmap read lock of init_mm. The mmap write
lock is holding through vmemmap_remap_free(), I want to make
it does not affect other users of mmap read lock.

I suppose a lot of developers are trying to avoid using mmap write
lock. I am also one of them.

> Can we have some details on this to put in the commit log?

For sure. Those judgments above should be placed in the
commit log.

Thanks.