Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[Zheng Liu]

On 06/28/2013 11:48 PM, Dave Hansen wrote:
> On 06/27/2013 10:47 PM, Zheng Liu wrote:
>>> I've been doing some testing involving large amounts of
>>> page cache.  It's quite painful to get hundreds of GB
>>> of page cache mapped in, especially when I am trying to
>>> do it in parallel threads.  This is true even when the
>>> page cache is already allocated and I only need to map
>>> it in.  The test:
>>>
>>> 1. take 160 16MB files
>>> 2. clone 160 threads, mmap the 16MB files, and either
>>>   a. walk through the file touching each page
>>
>> Why not change MAP_POPULATE flag in mmap(2)?  Now it is only for private
>> mappings.  But maybe we could let it support shared mapping.
> 
> Adding that support to mmap() will certainly _help_ some folks.  But,
> anything that mmap()s something is taking mmap_sem for write.  That
> means that threaded apps doing mmap()/munmap() frequently are _not_
> scalable.
> 
> IOW, a process needing to do a bunch of MAP_POPULATEs isn't
> parallelizable, but one using this mechanism would be.

I look at the code, and it seems that we will handle MAP_POPULATE flag
after we release mmap_sem locking in vm_mmap_pgoff():

                down_write(&mm->mmap_sem);
                ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
                                    &populate);
                up_write(&mm->mmap_sem);
                if (populate)
                        mm_populate(ret, populate);

Am I missing something?

Regards,
						- Zheng

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