linux-kernel - Re: [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may get wrongly discarded

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]

Date:	Wed, 11 Mar 2009 23:36:15 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Howells <dhowells@...hat.com>, torvalds@...ux-foundation.org,
	peterz@...radead.org, Enrik.Berkhan@...com,
	uclinux-dev@...inux.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may get wrongly discarded

On Wed, Mar 11, 2009 at 03:03:02PM -0700, Andrew Morton wrote:
> On Wed, 11 Mar 2009 15:30:35 +0000
> David Howells <dhowells@...hat.com> wrote:
> 
> > From: Enrik Berkhan <Enrik.Berkhan@...com>
> > 
> > The pages attached to a ramfs inode's pagecache by truncation from nothing - as
> > done by SYSV SHM for example - may get discarded under memory pressure.
> 
> Something has gone wrong in core VM.
> 
> > The problem is that the pages are not marked dirty.  Anything that creates data
> > in an MMU-based ramfs will cause the pages holding that data will cause the
> > set_page_dirty() aop to be called.
> > 
> > For the NOMMU-based mmap, set_page_dirty() may be called by write(), but it
> > won't be called by page-writing faults on writable mmaps, and it isn't called
> > by ramfs_nommu_expand_for_mapping() when a file is being truncated from nothing
> > to allocate a contiguous run.
> > 
> > The solution is to mark the pages dirty at the point of allocation by
> > the truncation code.
> 
> Page reclaim shouldn't be even attempting to reclaim or write back
> ramfs pagecache pages - reclaim can't possibly do anything with these
> pages!
> 
> Arguably those pages shouldn't be on the LRU at all, but we haven't
> done that yet.
> 
> Now, my problem is that I can't 100% be sure that we _ever_ implemented
> this properly.  I _think_ we did, in which case we later broke it.  If
> we've always been (stupidly) trying to pageout these pages then OK, I
> guess your patch is a suitable 2.6.29 stopgap.
> 
> If, however, we broke it then we've probably broken other filesystems
> and we should fix the regression instead.
> 
> Running bdi_cap_writeback_dirty() in may_write_to_queue() might be the
> way to fix all this.

The pages are not dirty, so no pageout() which says PAGE_KEEP.  It
will just go through and reclaim the clean, unmapped pages.
--
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/