Re: [PATCH RFC] hugetlbfs 'noautofill' mount option

From: Anshuman Khandual
Date: Tue May 02 2017 - 06:54:53 EST

On 05/01/2017 11:30 PM, Prakash Sangappa wrote:
> Some applications like a database use hugetblfs for performance
> reasons. Files on hugetlbfs filesystem are created and huge pages
> allocated using fallocate() API. Pages are deallocated/freed using
> fallocate() hole punching support that has been added to hugetlbfs.
> These files are mmapped and accessed by many processes as shared memory.
> Such applications keep track of which offsets in the hugetlbfs file have
> pages allocated.
> Any access to mapped address over holes in the file, which can occur due

s/mapped/unmapped/ ^ ?

> to bugs in the application, is considered invalid and expect the process
> to simply receive a SIGBUS. However, currently when a hole in the file is
> accessed via the mapped address, kernel/mm attempts to automatically
> allocate a page at page fault time, resulting in implicitly filling the
> hole

But this is expected when you try to control the file allocation from
a mapped address. Any changes while walking past or writing the range
in the memory mapped should reflect exactly in the file on the disk.
Why its not a valid behavior ?

> in the file. This may not be the desired behavior for applications like the
> database that want to explicitly manage page allocations of hugetlbfs
> files.
> This patch adds a new hugetlbfs mount option 'noautofill', to indicate that
> pages should not be allocated at page fault time when accessed thru mmapped
> address.

When the page should be allocated for mapping ?