linux-kernel - Re: [PATCH 08/16] huge tmpfs: fcntl(fd, F_HUGEPAGE) and fcntl(fd, F

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Date:   Wed, 4 Aug 2021 17:08:05 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Michal Hocko <mhocko@...e.com>,
        Rik van Riel <riel@...riel.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexey Gladkov <legion@...nel.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Matthew Auld <matthew.auld@...el.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-mm@...ck.org
Subject: 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