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

[KOSAKI Motohiro]

> 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:

Please don't ignore bug port anytime.


> 1- do nothing, if we can agree that the test case is invalid
> 
> 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;

Maybe two separate conditiions are cleaner more. Like this,

	/*
	 * We want to touch writable mappings with a write fault in order
	 * to break COW, except for shared mappings because these don't COW
	 * and we would not want to dirty them for nothing.
	 */
	if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE)
 		gup_flags |= FOLL_WRITE;

	/*
	* We don't have writable permission. Therefore we can't use read operation
	*  even though it's faster.
	*/
	if ((vma->vm_flags & (VM_READ|VM_WRITE)) == VM_WRITE)
 		gup_flags |= FOLL_WRITE;


Thanks.


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