linux-kernel - Re: possible deadlock in shmem

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Message-ID: <20200714034111.GD1696@sol.localdomain>
Date:   Mon, 13 Jul 2020 20:41:11 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Hillf Danton <hdanton@...a.com>
Cc:     syzbot <syzbot+7a0d9d0b26efefe61780@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, arve@...roid.com, christian@...uner.io,
        devel@...verdev.osuosl.org, gregkh@...uxfoundation.org,
        hughd@...gle.com, joel@...lfernandes.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, maco@...roid.com,
        syzkaller-bugs@...glegroups.com, tkjos@...roid.com,
        Markus Elfring <Markus.Elfring@....de>
Subject: Re: possible deadlock in shmem_fallocate (4)

On Tue, Jul 14, 2020 at 11:32:52AM +0800, Hillf Danton wrote:
> 
> Add FALLOC_FL_NOBLOCK and on the shmem side try to lock inode upon the
> new flag. And the overall upside is to keep the current gfp either in
> the khugepaged context or not.
> 
> --- a/include/uapi/linux/falloc.h
> +++ b/include/uapi/linux/falloc.h
> @@ -77,4 +77,6 @@
>   */
>  #define FALLOC_FL_UNSHARE_RANGE		0x40
>  
> +#define FALLOC_FL_NOBLOCK		0x80
> +

You can't add a new UAPI flag to fix a kernel-internal problem like this.

- Eric