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

From: Prakash Sangappa
Date: Tue May 02 2017 - 19:34:58 EST

On 5/2/17 2:32 PM, Dave Hansen wrote:
On 05/01/2017 11:00 AM, Prakash Sangappa wrote:
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
I think the main argument against doing something like this is further
specializing hugetlbfs. I was really hoping that userfaultfd would be
usable for your needs here.

Could you elaborate on other options that you considered? Did you look
at userfaultfd? What about an madvise() option that disallows backing

Yes, we did consider userfaultfd and madvise(). The use case in mind is the database.

With a database, large number of single threaded processes are involved which
will map hugetlbfs file and use it for shared memory. The concern with using
userfaultfd is the overhead of setup and having an additional thread per process
to monitor the userfaultfd. Even if the additional thread can be avoided, by using
an external monitor process and each process sending the userfaultfd to this
monitor process, setup overhead exists.

Similarly, a madvise() option also requires additional system call by every
process mapping the file, this is considered a overhead for the database.

If we do consider a new madvise() option, will it be acceptable since this will be
specifically for hugetlbfs file mappings? If so, would a new flag to mmap()
call itself be acceptable, which would define the proposed behavior?. That way
no additional system calls need to be made. Again this mmap flag would be
applicable specifically to hugetlbfs file mappings

With the proposed mount option, it would enforce one consistent behavior
and the application using this filesystem would not have to take additional
steps as with userfaultfd or madvise().