linux-kernel - Re: [PATCH] mm, thp: Relax the VM_DENYWRITE constraint on file-backed THPs

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Date:   Mon, 22 Mar 2021 16:55:22 -0700
From:   Song Liu <song@...nel.org>
To:     Collin Fijalkovich <cfijalkovich@...gle.com>
Cc:     Song Liu <songliubraving@...com>, surenb@...gle.com,
        hridya@...gle.com, kaleshsingh@...gle.com,
        Hugh Dickins <hughd@...gle.com>, timmurray@...gle.com,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH] mm, thp: Relax the VM_DENYWRITE constraint on file-backed THPs

On Mon, Mar 22, 2021 at 3:00 PM Collin Fijalkovich
<cfijalkovich@...gle.com> wrote:
>
> Transparent huge pages are supported for read-only non-shmem filesystems,
> but are only used for vmas with VM_DENYWRITE. This condition ensures that
> file THPs are protected from writes while an application is running
> (ETXTBSY).  Any existing file THPs are then dropped from the page cache
> when a file is opened for write in do_dentry_open(). Since sys_mmap
> ignores MAP_DENYWRITE, this constrains the use of file THPs to vmas
> produced by execve().
>
> Systems that make heavy use of shared libraries (e.g. Android) are unable
> to apply VM_DENYWRITE through the dynamic linker, preventing them from
> benefiting from the resultant reduced contention on the TLB.
>
> This patch reduces the constraint on file THPs allowing use with any
> executable mapping from a file not opened for write (see
> inode_is_open_for_write()). It also introduces additional conditions to
> ensure that files opened for write will never be backed by file THPs.

Thanks for working on this. We could also use this in many data center
workloads.

Question: when we use this on shared library, the library is still
writable. When the
shared library is opened for write, these pages will refault in as 4kB
pages, right?

Thanks,
Song