Re: [RFC PATCH 18/18] Documentation: add document for pte_ref

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 2022/4/30 9:19 PM, Bagas Sanjaya wrote:
> Hi Qi,
> 
> On Fri, Apr 29, 2022 at 09:35:52PM +0800, Qi Zheng wrote:
>> +Now in order to pursue high performance, applications mostly use some
>> +high-performance user-mode memory allocators, such as jemalloc or tcmalloc.
>> +These memory allocators use madvise(MADV_DONTNEED or MADV_FREE) to release
>> +physical memory for the following reasons::
>> +
>> + First of all, we should hold as few write locks of mmap_lock as possible,
>> + since the mmap_lock semaphore has long been a contention point in the
>> + memory management subsystem. The mmap()/munmap() hold the write lock, and
>> + the madvise(MADV_DONTNEED or MADV_FREE) hold the read lock, so using
>> + madvise() instead of munmap() to released physical memory can reduce the
>> + competition of the mmap_lock.
>> +
>> + Secondly, after using madvise() to release physical memory, there is no
>> + need to build vma and allocate page tables again when accessing the same
>> + virtual address again, which can also save some time.
>> +
> 
> I think we can use enumerated list, like below:

Thanks for your review, LGTM, will do.

> 
> -- >8 --
> 
> diff --git a/Documentation/vm/pte_ref.rst b/Documentation/vm/pte_ref.rst
> index 0ac1e5a408d7c6..67b18e74fcb367 100644
> --- a/Documentation/vm/pte_ref.rst
> +++ b/Documentation/vm/pte_ref.rst
> @@ -10,18 +10,18 @@ Preface
>   Now in order to pursue high performance, applications mostly use some
>   high-performance user-mode memory allocators, such as jemalloc or tcmalloc.
>   These memory allocators use madvise(MADV_DONTNEED or MADV_FREE) to release
> -physical memory for the following reasons::
> -
> - First of all, we should hold as few write locks of mmap_lock as possible,
> - since the mmap_lock semaphore has long been a contention point in the
> - memory management subsystem. The mmap()/munmap() hold the write lock, and
> - the madvise(MADV_DONTNEED or MADV_FREE) hold the read lock, so using
> - madvise() instead of munmap() to released physical memory can reduce the
> - competition of the mmap_lock.
> -
> - Secondly, after using madvise() to release physical memory, there is no
> - need to build vma and allocate page tables again when accessing the same
> - virtual address again, which can also save some time.
> +physical memory for the following reasons:
> +
> +1. We should hold as few write locks of mmap_lock as possible,
> +   since the mmap_lock semaphore has long been a contention point in the
> +   memory management subsystem. The mmap()/munmap() hold the write lock, and
> +   the madvise(MADV_DONTNEED or MADV_FREE) hold the read lock, so using
> +   madvise() instead of munmap() to released physical memory can reduce the
> +   competition of the mmap_lock.
> +
> +2. After using madvise() to release physical memory, there is no
> +   need to build vma and allocate page tables again when accessing the same
> +   virtual address again, which can also save some time.
>   
>   The following is the largest user PTE page table memory that can be
>   allocated by a single user process in a 32-bit and a 64-bit system.
> 
>> +The following is the largest user PTE page table memory that can be
>> +allocated by a single user process in a 32-bit and a 64-bit system.
>> +
> 
> We can say "assuming 4K page size" here,
> 
>> ++---------------------------+--------+---------+
>> +|                           | 32-bit | 64-bit  |
>> ++===========================+========+=========+
>> +| user PTE page table pages | 3 MiB  | 512 GiB |
>> ++---------------------------+--------+---------+
>> +| user PMD page table pages | 3 KiB  | 1 GiB   |
>> ++---------------------------+--------+---------+
>> +
>> +(for 32-bit, take 3G user address space, 4K page size as an example;
>> + for 64-bit, take 48-bit address width, 4K page size as an example.)
>> +
> 
> ... instead of here.

will do.

> 
>> +There is also a lock-less scenario(such as fast GUP). Fortunately, we don't need
>> +to do any additional operations to ensure that the system is in order. Take fast
>> +GUP as an example::
>> +
>> +	thread A		thread B
>> +	fast GUP		madvise(MADV_DONTNEED)
>> +	========		======================
>> +
>> +	get_user_pages_fast_only()
>> +	--> local_irq_save();
>> +				call_rcu(pte_free_rcu)
>> +	    gup_pgd_range();
>> +	    local_irq_restore();
>> +	    			/* do pte_free_rcu() */
>> +
> 
> I see whitespace warning circa do pte_free_rcu() line above when
> applying this series.

will fix.

Thanks,
Qi

> 
> Thanks.
> 

-- 
Thanks,
Qi