linux-kernel - Re: open(O_DIRECT) on a tmpfs?

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <9a8748490701050658n39f82791yb3d2e3c80f237a7e@mail.gmail.com>
Date:	Fri, 5 Jan 2007 15:58:38 +0100
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	"Hua Zhong" <hzhong@...il.com>
Cc:	"Hugh Dickins" <hugh@...itas.com>,
	"Bill Davidsen" <davidsen@....com>,
	Linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: open(O_DIRECT) on a tmpfs?

On 04/01/07, Hua Zhong <hzhong@...il.com> wrote:
> > I see that as a good argument _not_ to allow O_DIRECT on
> > tmpfs, which inevitably impacts cache, even if O_DIRECT were
> > requested.
> >
> > But I'd also expect any app requesting O_DIRECT in that way,
> > as a caring citizen, to fall back to going without O_DIRECT
> > when it's not supported.
>
> According to "man 2 open" on my system:
>
>        O_DIRECT
>               Try to minimize cache effects of the I/O to and from this file.
>               In  general  this will degrade performance, but it is useful in
>               special situations, such as  when  applications  do  their  own
>               caching.  File I/O is done directly to/from user space buffers.
>               The I/O is synchronous, i.e., at the completion of the  read(2)
>               or write(2) system call, data is guaranteed to have been trans-
>               ferred.  Under Linux 2.4 transfer sizes, and the  alignment  of
>               user  buffer and file offset must all be multiples of the logi-
>               cal block size of the file system. Under Linux 2.6 alignment to
>               512-byte boundaries suffices.
>               A semantically similar interface for block devices is described
>               in raw(8).
>
> This says nothing about (probably disk based) persistent backing store. I don't see why tmpfs has to conflict with it.
>
> So I'd argue that it makes more sense to support O_DIRECT on tmpfs as the memory IS the backing store.
>

I'd agree.  O_DIRECT means data will go direct to backing store, so if
RAM *is* the backing store as in the tmpfs case, then I see why
O_DIRECT should fail for it...

I often use tmpfs when I want to test new setups - it's easy to get
rid of again and it's fast during testing. Why shouldn't I be able to
test apps that use O_DIRECT this way?

-- 
Jesper Juhl <jesper.juhl@...il.com>
Don't top-post  http://www.catb.org/~esr/jargon/html/T/top-post.html
Plain text mails only, please      http://www.expita.com/nomime.html
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/