On Tue 09-10-12 19:19:09, Hugh Dickins wrote:
> On Tue, 9 Oct 2012, Jan Kara wrote:
<snip a lot>
> > > But here's where I think the problem is. You're assuming that all
> > > filesystems go the same mapping_cap_account_writeback_dirty() (yeah,
> > > there's no such function, just a confusing maze of three) route as XFS.
> > >
> > > But filesystems like tmpfs and ramfs (perhaps they're the only two
> > > that matter here) don't participate in that, and wait for an mmap'ed
> > > page to be seen modified by the user (usually via pte_dirty, but that's
> > > a no-op on s390) before page is marked dirty; and page reclaim throws
> > > away undirtied pages.
> > I admit I haven't thought of tmpfs and similar. After some discussion Mel
> > pointed me to the code in mmap which makes a difference. So if I get it
> > right, the difference which causes us problems is that on tmpfs we map the
> > page writeably even during read-only fault. OK, then if I make the above
> > code in page_remove_rmap():
> > if ((PageSwapCache(page) ||
> > (!anon && !mapping_cap_account_dirty(page->mapping))) &&
> > page_test_and_clear_dirty(page_to_pfn(page), 1))
> > set_page_dirty(page);
> > Things should be ok (modulo the ugliness of this condition), right?
> (Setting aside my reservations above...) That's almost exactly right, but
> I think the issue of a racing truncation (which could reset page->mapping
> to NULL at any moment) means we have to be a bit more careful. Usually
> we guard against that with page lock, but here we can rely on mapcount.
> page_mapping(page), with its built-in PageSwapCache check, actually ends
> up making the condition look less ugly; and so far as I could tell,
> the extra code does get optimized out on x86 (unless CONFIG_DEBUG_VM,
> when we are left with its VM_BUG_ON(PageSlab(page))).
> But please look this over very critically and test (and if you like it,
> please adopt it as your own): I'm not entirely convinced yet myself.
Just to followup on this. The new version of the patch runs fine for
several days on our s390 build machines. I was also running fsx-linux on
tmpfs while pushing the machine to swap. fsx ran fine but I hit
WARN_ON(delalloc) in xfs_vm_releasepage(). The exact stack trace is:
[<000003c008edb38e>] xfs_vm_releasepage+0xc6/0xd4 [xfs]
I don't think it is really related but I'll hold off the patch for a while
to investigate what's going on...
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR