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

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Date:	Sat, 20 Jun 2015 22:46:12 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Toshi Kani <toshi.kani@...com>
Cc:	akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
	willy@...ux.intel.com, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-nvdimm@...ts.01.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Fix MAP_POPULATE and mlock() for DAX

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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