Re: [PATCH 08/16] huge tmpfs: fcntl(fd, F_HUGEPAGE) and fcntl(fd, F_NOHUGEPAGE)

On Fri, Jul 30, 2021 at 12:48:33AM -0700, Hugh Dickins wrote:

Add support for fcntl(fd, F_HUGEPAGE) and fcntl(fd, F_NOHUGEPAGE), to
select hugeness per file: useful to override the default hugeness of the
shmem mount, when occasionally needing to store a hugepage file in a
smallpage mount or vice versa.

Hm. But why is the new MFD_* needed if the fcntl() can do the same.

These fcntls just specify whether or not to try for huge pages when
allocating to the object later: F_HUGEPAGE does not touch small pages
already allocated (though khugepaged may do so when the file is mapped
afterwards), F_NOHUGEPAGE does not split huge pages already allocated.

Why fcntl?  Because it's already in use (for sealing) on memfds; and I'm
anxious to keep this simple, just applying it to whole files: fallocate,
madvise and posix_fadvise each involve a range, which would need a new
kind of tree attached to the inode for proper support.

Most of fadvise() operations ignore the range. I like fadvise() because
it's less prescriptive: kernel is free to ignore it.

-- 
 Kirill A. Shutemov