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

[Hugh Dickins <hughd@google.com>]

On Wed, 4 Aug 2021, Kirill A. Shutemov wrote:
> 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.

That I've just addressed in the MFD_HUGEPAGE 07/16 thread.

> 
> > 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.

As to ignoring the range, yes, I see now that some do; and I'm relieved
to see "Len == 0 means as much as possible", that's great, I was afraid
of compat bugs over 0xffy numbers for the len.  And we would want, not
to ignore the range, but insist on offset 0, len 0 for now, if there's
any intention (not mine) of extending it to ranges in the future.

As to ignoring the prescription, that's just a matter of how we describe
it in the manpage, no matter whether it's fadvise() or fcntl().

And in the 07/16 thread you also said:

> 
> If a tunable needed, I would rather go with fadvise(). It would operate on
> a couple of bits per struct file and they get translated into VM_HUGEPAGE
> and VM_NOHUGEPAGE on mmap().

Not so sure about that detail: the point here is to decide what kind
of allocations to try for, before the file is mmap()ed; and it is the
file (the underlying object) that I want to condition here, rather than
the struct file of who has it open at the time, or their mmap()s.

But adding the flags into the vm_flags on mmap(): that's an interesting
idea, I haven't played with that at all.  Offhand, I don't think it will
give different allocation results from what I'm already doing, but might
affect what is shown by default in /proc/<pid>/smaps.

> 
> Later if needed fadvise() implementation may be extended to track
> requested ranges. But initially it can be simple.

I still prefer fcntl() myself, but we can go with either: what I'd
like to hear is the preference of linux-fsdevel and linux-api people.

Aside from the unused offset+len, my main problem with fadvise()
is that... it doesn't exist.  It's posix_fadvise() or fadvise64() or
fadvise64_64(), and all its good advices are POSIX_MADV_whatever.

Are we comfortable now adding LINUX_MADV_HUGEPAGE, LINUX_MADV_NOHUGEPAGE?

I find myself singing 64 64 Zoo Lane.

Hugh