Date:	Tue, 13 Apr 2010 21:19:02 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] mm: disallow direct reclaim page writeback

On Tue, Apr 13, 2010 at 10:58:15AM +0100, Mel Gorman wrote:
> On Tue, Apr 13, 2010 at 10:17:58AM +1000, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@...hat.com>
> > 
> > When we enter direct reclaim we may have used an arbitrary amount of stack
> > space, and hence enterring the filesystem to do writeback can then lead to
> > stack overruns. This problem was recently encountered x86_64 systems with
> > 8k stacks running XFS with simple storage configurations.
> > 
> > Writeback from direct reclaim also adversely affects background writeback. The
> > background flusher threads should already be taking care of cleaning dirty
> > pages, and direct reclaim will kick them if they aren't already doing work. If
> > direct reclaim is also calling ->writepage, it will cause the IO patterns from
> > the background flusher threads to be upset by LRU-order writeback from
> > pageout() which can be effectively random IO. Having competing sources of IO
> > trying to clean pages on the same backing device reduces throughput by
> > increasing the amount of seeks that the backing device has to do to write back
> > the pages.
> > 
> 
> It's already known that the VM requesting specific pages be cleaned and
> reclaimed is a bad IO pattern but unfortunately it is still required by
> lumpy reclaim. This change would appear to break that although I haven't
> tested it to be 100% sure.

How do you test it? I'd really like to be able to test this myself....

> Even without high-order considerations, this patch would appear to make
> fairly large changes to how direct reclaim behaves. It would no longer
> wait on page writeback for example so direct reclaim will return sooner

AFAICT it still waits for pages under writeback in exactly the same manner
it does now. shrink_page_list() does the following completely
separately to the sc->may_writepage flag:

 666                 may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
 667                         (PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
 668
 669                 if (PageWriteback(page)) {
 670                         /*
 671                          * Synchronous reclaim is performed in two passes,
 672                          * first an asynchronous pass over the list to
 673                          * start parallel writeback, and a second synchronous
 674                          * pass to wait for the IO to complete.  Wait here
 675                          * for any page for which writeback has already
 676                          * started.
 677                          */
 678                         if (sync_writeback == PAGEOUT_IO_SYNC && may_enter_fs)
 679                                 wait_on_page_writeback(page);
 680                         else
 681                                 goto keep_locked;
 682                 }

So if the page is under writeback, PAGEOUT_IO_SYNC is set and
we can enter the fs, it will still wait for writeback to complete
just like it does now.

However, the current code only uses PAGEOUT_IO_SYNC in lumpy
reclaim, so for most typical workloads direct reclaim does not wait
on page writeback, either. Hence, this patch doesn't appear to
change the actions taken on a page under writeback in direct
reclaim....

> than it did potentially going OOM if there were a lot of dirty pages and
> it made no progress during direct reclaim.

I did a fair bit of low/small memory testing. This is a subjective
observation, but I definitely seemed to get less severe OOM
situations and better overall responisveness with this patch than
compared to when direct reclaim was doing writeback.

> > Hence for direct reclaim we should not allow ->writepages to be entered at all.
> > Set up the relevant scan_control structures to enforce this, and prevent
> > sc->may_writepage from being set in other places in the direct reclaim path in
> > response to other events.
> > 
> 
> If an FS caller cannot re-enter the FS, it should be using GFP_NOFS
> instead of GFP_KERNEL.

This problem is not a filesystem recursion problem which is, as I
understand it, what GFP_NOFS is used to prevent. It's _any_ kernel
code that uses signficant stack before trying to allocate memory
that is the problem. e.g a select() system call:

       Depth    Size   Location    (47 entries)
       -----    ----   --------
 0)     7568      16   mempool_alloc_slab+0x16/0x20
 1)     7552     144   mempool_alloc+0x65/0x140
 2)     7408      96   get_request+0x124/0x370
 3)     7312     144   get_request_wait+0x29/0x1b0
 4)     7168      96   __make_request+0x9b/0x490
 5)     7072     208   generic_make_request+0x3df/0x4d0
 6)     6864      80   submit_bio+0x7c/0x100
 7)     6784      96   _xfs_buf_ioapply+0x128/0x2c0 [xfs]
....
32)     3184      64   xfs_vm_writepage+0xab/0x160 [xfs]
33)     3120     384   shrink_page_list+0x65e/0x840
34)     2736     528   shrink_zone+0x63f/0xe10
35)     2208     112   do_try_to_free_pages+0xc2/0x3c0
36)     2096     128   try_to_free_pages+0x77/0x80
37)     1968     240   __alloc_pages_nodemask+0x3e4/0x710
38)     1728      48   alloc_pages_current+0x8c/0xe0
39)     1680      16   __get_free_pages+0xe/0x50
40)     1664      48   __pollwait+0xca/0x110
41)     1616      32   unix_poll+0x28/0xc0
42)     1584      16   sock_poll+0x1d/0x20
43)     1568     912   do_select+0x3d6/0x700
44)      656     416   core_sys_select+0x18c/0x2c0
45)      240     112   sys_select+0x4f/0x110
46)      128     128   system_call_fastpath+0x16/0x1b

There's 1.6k of stack used before memory allocation is called, 3.1k
used there before ->writepage is entered, XFS used 3.5k, and
if the mempool needed to allocate a page it would have blown the
stack. If there was any significant storage subsystem (add dm, md
and/or scsi of some kind), it would have blown the stack.

Basically, there is not enough stack space available to allow direct
reclaim to enter ->writepage _anywhere_ according to the stack usage
profiles we are seeing here....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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