From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Mon, 5 Nov 2007 18:57:22 +0000 (GMT) From: Hugh Dickins Subject: Re: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland In-Reply-To: <1194280730.6271.145.camel@localhost> Message-ID: References: <200710312353.l9VNr67n013016@agora.fsl.cs.sunysb.edu> <1194280730.6271.145.camel@localhost> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: Dave Hansen Cc: Erez Zadok , Pekka Enberg , Ryan Finnie , Andrew Morton , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, cjwatson@ubuntu.com, linux-mm@kvack.org, Christoph Hellwig List-ID: On Mon, 5 Nov 2007, Dave Hansen wrote: > > Actually, I think your s/while/if/ change is probably a decent fix. Any resemblance to a decent fix is purely coincidental. > Barring any other races, that loop should always have made progress on > mnt->__mnt_writers the way it is written. If we get to: > > > lock_and_coalesce_cpu_mnt_writer_counts(); > ----------------->HERE > > mnt_unlock_cpus(); > > and don't have a positive mnt->__mnt_writers, we know something is going > badly. We WARN_ON() there, which should at least give an earlier > warning that the system is not doing well. But it doesn't fix the > inevitable. Could you try the attached patch and see if it at least > warns you earlier? Thanks, Dave, yes, that gives me a nice warning: leak detected on mount(c25ebd80) writers count: -65537 WARNING: at fs/namespace.c:249 handle_write_count_underflow() [] show_trace_log_lvl+0x1b/0x2e [] show_trace+0x16/0x1b [] dump_stack+0x19/0x1e [] handle_write_count_underflow+0x4c/0x60 [] mnt_drop_write+0x69/0x8e [] __fput+0xff/0x162 [] fput+0x2e/0x33 [] unionfs_file_release+0xc2/0x1c5 [] __fput+0x8f/0x162 [] fput+0x2e/0x33 [] filp_close+0x50/0x5d [] sys_close+0x74/0xb4 [] sysenter_past_esp+0x5f/0x85 and the test then goes quietly on its way instead of hanging. Though I imagine, with your patch or mine, that it's then making an unfortunate frequency of calls to lock_and_coalesce_longer_name_than_I_care_to_type thereafter. But it's hardly your responsibility to optimize for bugs elsewhere. The 2.6.23-mm1 tree has MNT_USER at 0x200, so I adjusted your flag to #define MNT_IMBALANCED_WRITE_COUNT 0x400 /* just for debugging */ > > I have a decent guess what the bug is, too. In the unionfs code: I'll let Erez take it from there... Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org