linux-kernel - Re: [GIT PULL] uprobes: tmpfs support

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 for Android: free password hash cracker in your pocket

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

Date:	Mon, 2 Jun 2014 12:09:46 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Christoph Hellwig <hch@...radead.org>
cc:	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Jim Keniston <jkenisto@...ibm.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] uprobes: tmpfs support

On Mon, 2 Jun 2014, Christoph Hellwig wrote:
> On Mon, Jun 02, 2014 at 04:14:06PM +0200, Oleg Nesterov wrote:
> > Ingo, please pull from
> > 
> >   git://git.kernel.org/pub/scm/linux/kernel/git/oleg/misc uprobes/core
> > 
> > Based on tip:perf/uprobes
> 
> Eww, adding tmpfs-specific code to uprobes screams layering violation.
> 
> Hugh, what is the problem with implementing ->readpage for tmpfs again?

The problem is that ->readpage invites the caller to allocate a page
of their choice for pagecache, and then pass it down to the filesystem
to fill and use thereafter.

There are several ways in which that does not suit tmpfs, 3 spring to mind:
1. the page may already be in memory, but currently in swapcache not in
filecache: tmpfs knows how to manage that, the ->readpage caller does not
2. there may be a NUMA mempolicy applied to that file, which would choose
to allocate the page differently: tmpfs knows about that, caller does not
3. (handy side-effect) it happens to disable use of tmpfs file as swapfile

It was a great relief when tmpfs could finally jettison its ->readpage
back in v3.1 (though if you press me, I could admit to some remaining
embarrassments).  I certainly do not want it back.

Just think of tmpfs as a layering violation itself (memory as backing!
no wonder it has peculiar demands on the allocation of its backing)
and we're all good - there's a variety of ways in which the generic
code already happens to accommodate it (many PageSwapBacked tests,
or the mapping_cap_account_dirty/writeback tests, for example).

IIRC, you were in on the discussion of shmem_read_mapping_page() when we
introduced it: Oleg is simply adding a call to it to fix a uprobes bug.  
That the name explicitly mentions shmem instead of concealing it,
is not necessarily a bad thing.

Hugh
--
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/