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

From: Tao Ma
Date: Sun Jan 30 2011 - 08:57:41 EST

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) {
goto out;

if (mlock(buf, file_len)< 0) {
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://
I can paste codes here also.
In blktrace.c setup_mmap:
mip->fs_buf = my_mmap(NULL, mip->fs_buf_len, PROT_WRITE,
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 |
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.

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