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
