> 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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