Re: [PATCH] fs: xfs: xfs_log: Don't use KM_MAYFAIL at xfs_log_reserve().

[Luis Chamberlain <mcgrof@kernel.org>]

On Thu, Aug 01, 2019 at 01:46:14PM -0700, Darrick J. Wong wrote:
> On Thu, Aug 01, 2019 at 06:50:57PM +0000, Luis Chamberlain wrote:
> > On Thu, Aug 01, 2019 at 07:06:35PM +0900, Tetsuo Handa wrote:
> > > When the system is close-to-OOM, fsync() may fail due to -ENOMEM because
> > > xfs_log_reserve() is using KM_MAYFAIL. It is a bad thing to fail writeback
> > > operation due to user-triggerable OOM condition. Since we are not using
> > > KM_MAYFAIL at xfs_trans_alloc() before calling xfs_log_reserve(), let's
> > > use the same flags at xfs_log_reserve().
> > > 
> > >   oom-torture: page allocation failure: order:0, mode:0x46c40(GFP_NOFS|__GFP_NOWARN|__GFP_RETRY_MAYFAIL|__GFP_COMP), nodemask=(null)
> > >   CPU: 7 PID: 1662 Comm: oom-torture Kdump: loaded Not tainted 5.3.0-rc2+ #925
> > >   Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00
> > >   Call Trace:
> > >    dump_stack+0x67/0x95
> > >    warn_alloc+0xa9/0x140
> > >    __alloc_pages_slowpath+0x9a8/0xbce
> > >    __alloc_pages_nodemask+0x372/0x3b0
> > >    alloc_slab_page+0x3a/0x8d0
> > >    new_slab+0x330/0x420
> > >    ___slab_alloc.constprop.94+0x879/0xb00
> > >    __slab_alloc.isra.89.constprop.93+0x43/0x6f
> > >    kmem_cache_alloc+0x331/0x390
> > >    kmem_zone_alloc+0x9f/0x110 [xfs]
> > >    kmem_zone_alloc+0x9f/0x110 [xfs]
> > >    xlog_ticket_alloc+0x33/0xd0 [xfs]
> > >    xfs_log_reserve+0xb4/0x410 [xfs]
> > >    xfs_trans_reserve+0x1d1/0x2b0 [xfs]
> > >    xfs_trans_alloc+0xc9/0x250 [xfs]
> > >    xfs_setfilesize_trans_alloc.isra.27+0x44/0xc0 [xfs]
> > >    xfs_submit_ioend.isra.28+0xa5/0x180 [xfs]
> > >    xfs_vm_writepages+0x76/0xa0 [xfs]
> > >    do_writepages+0x17/0x80
> > >    __filemap_fdatawrite_range+0xc1/0xf0
> > >    file_write_and_wait_range+0x53/0xa0
> > >    xfs_file_fsync+0x87/0x290 [xfs]
> > >    vfs_fsync_range+0x37/0x80
> > >    do_fsync+0x38/0x60
> > >    __x64_sys_fsync+0xf/0x20
> > >    do_syscall_64+0x4a/0x1c0
> > >    entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > > 
> > > Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > 
> > That's quite an opaque commit log for what started off as a severe email
> > thread of potential leak of information. As such, can you expand on this
> > commit log considerably to explain the situation a bit better?
> 
> I'm pretty sure this didn't solve the underlying stale data exposure
> problem, which might be why you think this is "opaque".  It fixes a bug
> that causes data writeback failure (which was the exposure vector this
> time) but I think the ultimate fix for the exposure problem are the two
> patches I linked to quite a ways back in this discussion....
> 
> --D
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/commit/?id=bd012b434a56d9fac3cbc33062b8e2cd6e1ad0a0
> https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/commit/?id=adcf7c0c87191fd3616813c8ce9790f89a9a8eba

Got it, thanks! Even with this, I still think the current commit could
say a bit a more about the effects of not having this patch applied.
What are the effects of say having the above two patches applied but not
the one being submitted now?

  Luis