linux-kernel - Re: [PATCH 10/16] tmpfs: fcntl(fd, F_MEM

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Message-ID: <7077b94a-d894-4d97-4b3e-23f516d58591@google.com>
Date:   Wed, 4 Aug 2021 02:15:26 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
cc:     Hugh Dickins <hughd@...gle.com>,
        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>,
        "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 10/16] tmpfs: fcntl(fd, F_MEM_LOCK) to memlock a tmpfs
 file

On Tue, 3 Aug 2021, Matthew Wilcox wrote:
> On Fri, Jul 30, 2021 at 12:55:22AM -0700, Hugh Dickins wrote:
> > A new uapi to lock the files on tmpfs in memory, to protect against swap
> > without mapping the files. This commit introduces two new commands to
> > fcntl and shmem: F_MEM_LOCK and F_MEM_UNLOCK. The locking will be
> > charged against RLIMIT_MEMLOCK of uid in namespace of the caller.
> 
> It's not clear to me why this is limited to shmfs.  Would it not also
> make sense for traditional filesystems, eg to force chrome's text pages
> to stay in the page cache, no matter how much memory the tabs allocate?

Right: if VFS people would like this to be available for all filesystems,
that's fine by me - it's just that we have not given thought to other
filesystems, and the demand was for tmpfs, so that was where to start.
I'm more confident adding fields to shmem inode than to generic inode.

(Plus tmpfs does have a stronger claim on CAP_IPC_LOCK etc, but there's
no real reason why that cannot be extended to similar use by other FSs).

hugetlbfs and ramfs, where the files are already memlocked?  Not worth a
special case, I think: if someone uses up memlock quota on them, so be it.

It looks as if tmpfs would still want its own special case, just to
handle the FALLOC_FL_KEEP_SIZE issue (see 12/16): tmpfs has beyond-i_size
pages in memory, but accounts them evictable; whereas I doubt any storage
filesystems would be using memory for them.

To be clear: I'm not intending to extend this to other filesystems at
the moment; but happy to do so if that's the consensus.

Hugh