Subject: Re: sk_lock: inconsistent {RECLAIM_FS-ON-W} -> {IN-RECLAIM_FS-W} usage

From: Wu Fengguang <[email-masked]>
Date: Mon, 8 Jun 2009 13:56:59 +0800

On Mon, Jun 08, 2009 at 01:53:26PM +0800, Wu Fengguang wrote:
> On Mon, Jun 08, 2009 at 01:07:26PM +0800, KOSAKI Motohiro wrote:
> > > On Mon, Jun 08, 2009 at 12:55:18PM +0800, KOSAKI Motohiro wrote:
> > > > Hi
> > > > 
> > > > > Hi,
> > > > > 
> > > > > This lockdep warning appears when doing stress memory tests over NFS.
> > > > > 
> > > > > page reclaim => nfs_writepage => tcp_sendmsg => lock sk_lock
> > > > > 
> > > > > tcp_close => lock sk_lock => tcp_send_fin => alloc_skb_fclone => page reclaim
> > > > > 
> > > > > Any ideas?
> > > > 
> > > > AFAIK, btrfs has re-dirty hack. 
> > > > 
> > > > ------------------------------------------------------------------
> > > > static int btrfs_writepage(struct page *page, struct writeback_control *wbc)
> > > > {
> > > >         struct extent_io_tree *tree;
> > > > 
> > > > 
> > > >         if (current->flags & PF_MEMALLOC) {
> > > >                 redirty_page_for_writepage(wbc, page);
> > > >                 unlock_page(page);
> > > >                 return 0;
> > > >         }
> > > >         tree = &BTRFS_I(page->mapping->host)->io_tree;
> > > >         return extent_write_full_page(tree, page, btrfs_get_extent, wbc);
> > > > }
> > > > ---------------------------------------------------------------
> > > > 
> > > > PF_MEMALLOC mean caller is try_to_free_pages(). (not normal write nor kswapd)
> > > > Can't nfs does similar hack? 
> > > 
> > > But the trace shows that current is kswapd:
> > > 
> > > [ 1638.403414]  [<ffffffff811c9b69>] nfs_flush_one+0xb9/0x100
> > > [ 1638.419417]  [<ffffffff811c3f82>] nfs_pageio_doio+0x32/0x70
> > > [ 1638.419417]  [<ffffffff811c3fc9>] nfs_pageio_complete+0x9/0x10
> > > [ 1638.427413]  [<ffffffff811c7ee5>] nfs_writepage_locked+0x85/0xc0
> > > [ 1638.435414]  [<ffffffff811c8509>] nfs_writepage+0x19/0x40
> > > [ 1638.435414]  [<ffffffff810ce005>] shrink_page_list+0x675/0x810
> > > [ 1638.435414]  [<ffffffff810ce761>] shrink_list+0x301/0x650
> > > [ 1638.435414]  [<ffffffff810ced23>] shrink_zone+0x273/0x370
> > > [ 1638.435414]  [<ffffffff810cf9f9>] kswapd+0x729/0x7a0
> > > [ 1638.435414]  [<ffffffff810666de>] kthread+0x9e/0xb0
> > > [ 1638.435414]  [<ffffffff8100d0ca>] child_rip+0xa/0x20
> > 
> > kswapd can't hold sk-lock before calling reclaim. Thus, we don't need
> > care its bogus warning, I think.
> 
> Right. Although this path is possible:
>         tcp_sendmsg() => page reclaim => tcp_send_fin()
> But it won't happen for the same socket, so one sk_lock won't be
> grabbed twice and go deadlock.
> 
> So it's a harmful warning for both direct/background page reclaims?

btw, can anyone explain these NFS warnings? It happens in a very
memory tight and busy nfsroot system.

[  113.267340] NFS: Server wrote zero bytes, expected 3671.
[  423.202607] NFS: Server wrote zero bytes, expected 108.
[  723.588411] NFS: Server wrote zero bytes, expected 560.
[ 1060.246747] NFS: Server wrote zero bytes, expected 54.
[ 1397.841183] NFS: Server wrote zero bytes, expected 402.
[ 1779.545035] NFS: Server wrote zero bytes, expected 319.

Thanks,
Fengguang
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