From: Wu Fengguang <fengguang.wu@intel.com>
Subject: Re: sk_lock: inconsistent {RECLAIM_FS-ON-W} -> {IN-RECLAIM_FS-W}
	usage
Date: Mon, 8 Jun 2009 13:53:26 +0800
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To: KOSAKI Motohiro <kosaki.motohiro-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
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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?

Thanks,
Fengguang