Re: [PATCH] mlock: revert the optimization for dirtying pages andtriggering writeback.

[Tao Ma]

On 01/30/2011 06:26 PM, Michel Lespinasse wrote:
> On Sat, Jan 29, 2011 at 11:15 PM, Tao Ma<tm@xxxxxx>  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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