linux-kernel - Re: [PATCH] mlock: revert the optimization for dirtying pages and triggering writeback.

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Message-ID: <4D456E3E.1020800@tao.ma>
Date:	Sun, 30 Jan 2011 21:57:18 +0800
From:	Tao Ma <tm@....ma>
To:	Michel Lespinasse <walken@...gle.com>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mlock: revert the optimization for dirtying pages and
 triggering writeback.

On 01/30/2011 06:26 PM, Michel Lespinasse wrote:
> On Sat, Jan 29, 2011 at 11:15 PM, Tao Ma<tm@....ma>  wrote:
>>         buf = mmap(NULL, file_len, PROT_WRITE, MAP_SHARED, fd, 0);
>>         if (buf == MAP_FAILED) {
>>                 perror("mmap");
>>                 goto out;
>>         }
>>
>>         if (mlock(buf, file_len)<  0) {
>>                 perror("mlock");
>>                 goto out;
>>         }
> Thanks Tao for tracing this to an individual change. I can reproduce
> this on my system. The issue is that the file is mapped without the
> PROT_READ permission, so mlock can't fault in the pages. Up to 2.6.37
> this worked because mlock was using a write.
>
> The test case does show there was a behavior change; however it's not
> clear to me that the tested behavior is valid.
>
> I can see two possible resolutions:
>
> 1- do nothing, if we can agree that the test case is invalid
The test case does exist in the real world and used widespread. ;)
It is blktrace. 
git://git.kernel.org/pub/scm/linux/kernel/git/axboe/blktrace.git
I can paste codes here also.
In blktrace.c setup_mmap:
mip->fs_buf = my_mmap(NULL, mip->fs_buf_len, PROT_WRITE,
                                       MAP_SHARED, fd,
                                       mip->fs_size - mip->fs_off);
> 2- restore the previous behavior for writable, non-readable, shared
> mappings while preserving the optimization for read/write shared
> mappings. The test would then look like:
>          if ((vma->vm_flags&  VM_WRITE)&&  (vma->vm_flags&  (VM_READ |
> VM_SHARED)) != VM_SHARED)
>                  gup_flags |= FOLL_WRITE;
I am not sure whether it is proper or not. I guess a fat comment is 
needed here
to explain the corner case. So do you have some statistics that your change
improve the performance a lot? If yes, I agree with you. Otherwise, I would
prefer to revert it back to the original design.

Regards,
Tao
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