Re: [PATCH] mm: Fix MAP_POPULATE and mlock() for DAX

From: Kirill A. Shutemov
Date: Sat Jun 20 2015 - 15:46:52 EST

On Tue, Jun 16, 2015 at 04:28:30PM -0600, Toshi Kani wrote:
> DAX has the following issues in a shared or read-only private
> mmap'd file.
> - mmap(MAP_POPULATE) does not pre-fault
> - mlock() fails with -ENOMEM
> DAX uses VM_MIXEDMAP for mmap'd files, which do not have struct
> page associated with the ranges. Both MAP_POPULATE and mlock()
> call __mm_populate(), which in turn calls __get_user_pages().
> Because __get_user_pages() requires a valid page returned from
> follow_page_mask(), MAP_POPULATE and mlock(), i.e. FOLL_POPULATE,
> fail in the first page.
> Change __get_user_pages() to proceed FOLL_POPULATE when the
> translation is set but its page does not exist (-EFAULT), and
> @pages is not requested. With that, MAP_POPULATE and mlock()
> set translations to the requested range and complete successfully.
> MAP_POPULATE still provides a major performance improvement to
> DAX as it will avoid page faults during initial access to the
> pages.
> mlock() continues to set VM_LOCKED to vma and populate the range.
> Since there is no struct page, the range is pinned without marking
> pages mlocked.
> Note, MAP_POPULATE and mlock() already work for a write-able
> private mmap'd file on DAX since populate_vma_page_range() breaks
> COW, which allocates page caches.

I don't think that's true in all cases.

We would fail to break COW for mlock() if the mapping is populated with
read-only entries by the mlock() time. In this case follow_page_mask()
would fail with -EFAULT and faultin_page() will never executed.

Kirill A. Shutemov
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