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SubjectRe: [PATCHv2 04/28] mm: make remove_migration_ptes() beyond mm/migration.c
On Fri, Feb 12, 2016 at 08:54:58AM -0800, Dave Hansen wrote:
> On 02/11/2016 06:21 AM, Kirill A. Shutemov wrote
> > We also shouldn't try to mlock() pte-mapped huge pages: pte-mapeed THP
> > pages are never mlocked.
>
> That's kinda subtle. Can you explain more?
>
> If we did the following:
>
> ptr = mmap(NULL, 512*PAGE_SIZE, ...);
> mlock(ptr, 512*PAGE_SIZE);
> fork();
> munmap(ptr + 100 * PAGE_SIZE, PAGE_SIZE);
>
> I'd expect to get two processes, each mapping the same compound THP, one
> with a PMD and the other with 511 ptes and one hole. Is there something
> different that goes on?

I'm not sure what exactly you want to ask with this code, but it will have
the following result:

- After fork() process will split the pmd in munlock(). For file thp
split pmd, means clear it out. Mapping split_huge_pmd() would munlock
the page as we do for anon thp;

- In child process the page is never mapped as mlock() is not inherited
and we don't copy page tables for shared VMA as they can re-faulted
later;

The basic semantic for mlock()ed file THP would be the same as for anon
THP: we only keep the page mlocked as long as it's mapped only with PMDs.
This way it's relatively simple to make sure that we don't leak mlocked
pages.

--
Kirill A. Shutemov

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