Re: [RFC 2/2] x86_64: expand kernel stack to 16K

Johannes Weiner
2014-05-28T16:07:16Z

On Wed, May 28, 2014 at 07:13:45PM +1000, Dave Chinner wrote:
> On Wed, May 28, 2014 at 06:37:38PM +1000, Dave Chinner wrote:
> > [ cc XFS list ]
>
> [and now there is a complete copy on the XFs list, I'll add my 2c]
>
> > On Wed, May 28, 2014 at 03:53:59PM +0900, Minchan Kim wrote:
> > > While I play inhouse patches with much memory pressure on qemu-kvm,
> > > 3.14 kernel was randomly crashed. The reason was kernel stack overflow.
> > >
> > > When I investigated the problem, the callstack was a little bit deeper
> > > by involve with reclaim functions but not direct reclaim path.
> > >
> > > I tried to diet stack size of some functions related with alloc/reclaim
> > > so did a hundred of byte but overflow was't disappeard so that I encounter
> > > overflow by another deeper callstack on reclaim/allocator path.
>
> That's a no win situation. The stack overruns through ->writepage
> we've been seeing with XFS over the past *4 years* are much larger
> than a few bytes. The worst case stack usage on a virtio block
> device was about 10.5KB of stack usage.
>
> And, like this one, it came from the flusher thread as well. The
> difference was that the allocation that triggered the reclaim path
> you've reported occurred when 5k of the stack had already been
> used...
>
> > > Of course, we might sweep every sites we have found for reducing
> > > stack usage but I'm not sure how long it saves the world(surely,
> > > lots of developer start to add nice features which will use stack
> > > agains) and if we consider another more complex feature in I/O layer
> > > and/or reclaim path, it might be better to increase stack size(
> > > meanwhile, stack usage on 64bit machine was doubled compared to 32bit
> > > while it have sticked to 8K. Hmm, it's not a fair to me and arm64
> > > already expaned to 16K. )
>
> Yup, that's all been pointed out previously. 8k stacks were never
> large enough to fit the linux IO architecture on x86-64, but nobody
> outside filesystem and IO developers has been willing to accept that
> argument as valid, despite regular stack overruns and filesystem
> having to add workaround after workaround to prevent stack overruns.
>
> That's why stuff like this appears in various filesystem's
> ->writepage:
>
> /*
> * Refuse to write the page out if we are called from reclaim context.
> *
> * This avoids stack overflows when called from deeply used stacks in
> * random callers for direct reclaim or memcg reclaim. We explicitly
> * allow reclaim from kswapd as the stack usage there is relatively low.
> *
> * This should never happen except in the case of a VM regression so
> * warn about it.
> */
> if (WARN_ON_ONCE((current->flags & (PF_MEMALLOC|PF_KSWAPD)) ==
> PF_MEMALLOC))
> goto redirty;
>
> That still doesn't guarantee us enough stack space to do writeback,
> though, because memory allocation can occur when reading in metadata
> needed to do delayed allocation, and so we could trigger GFP_NOFS
> memory allocation from the flusher thread with 4-5k of stack already
> consumed, so that would still overrun teh stack.
>
> So, a couple of years ago we started defering half the writeback
> stack usage to a worker thread (commit c999a22 "xfs: introduce an
> allocation workqueue"), under the assumption that the worst stack
> usage when we call memory allocation is around 3-3.5k of stack used.
> We thought that would be safe, but the stack trace you've posted
> shows that alloc_page(GFP_NOFS) can consume upwards of 5k of stack,
> which means we're still screwed despite all the workarounds we have
> in place.

The allocation and reclaim stack itself is only 2k per the stacktrace
below. What got us in this particular case is that we engaged a
complicated block layer setup from within the allocation context in
order to swap out a page.

In the past we disabled filesystem ->writepage from within the
allocation context and deferred it to kswapd for stack reasons (see
the WARN_ON_ONCE and the comment in your above quote), but I think we
have to go further and do the same for even swap_writepage():

> > > I guess this topic was discussed several time so there might be
> > > strong reason not to increase kernel stack size on x86_64, for me not
> > > knowing so Ccing x86_64 maintainers, other MM guys and virtio
> > > maintainers.
> > >
> > > Depth Size Location (51 entries)
> > >
> > > 0) 7696 16 lookup_address+0x28/0x30
> > > 1) 7680 16 _lookup_address_cpa.isra.3+0x3b/0x40
> > > 2) 7664 24 __change_page_attr_set_clr+0xe0/0xb50
> > > 3) 7640 392 kernel_map_pages+0x6c/0x120
> > > 4) 7248 256 get_page_from_freelist+0x489/0x920
> > > 5) 6992 352 __alloc_pages_nodemask+0x5e1/0xb20
> > > 6) 6640 8 alloc_pages_current+0x10f/0x1f0
> > > 7) 6632 168 new_slab+0x2c5/0x370
> > > 8) 6464 8 __slab_alloc+0x3a9/0x501
> > > 9) 6456 80 __kmalloc+0x1cb/0x200
> > > 10) 6376 376 vring_add_indirect+0x36/0x200
> > > 11) 6000 144 virtqueue_add_sgs+0x2e2/0x320
> > > 12) 5856 288 __virtblk_add_req+0xda/0x1b0
> > > 13) 5568 96 virtio_queue_rq+0xd3/0x1d0
> > > 14) 5472 128 __blk_mq_run_hw_queue+0x1ef/0x440
> > > 15) 5344 16 blk_mq_run_hw_queue+0x35/0x40
> > > 16) 5328 96 blk_mq_insert_requests+0xdb/0x160
> > > 17) 5232 112 blk_mq_flush_plug_list+0x12b/0x140
> > > 18) 5120 112 blk_flush_plug_list+0xc7/0x220
> > > 19) 5008 64 io_schedule_timeout+0x88/0x100
> > > 20) 4944 128 mempool_alloc+0x145/0x170
> > > 21) 4816 96 bio_alloc_bioset+0x10b/0x1d0
> > > 22) 4720 48 get_swap_bio+0x30/0x90
> > > 23) 4672 160 __swap_writepage+0x150/0x230
> > > 24) 4512 32 swap_writepage+0x42/0x90

Without swap IO from the allocation context, the stack would have
ended here, which would have been easily survivable. And left the
writeout work to kswapd, which has a much shallower stack than this:

> > > 25) 4480 320 shrink_page_list+0x676/0xa80
> > > 26) 4160 208 shrink_inactive_list+0x262/0x4e0
> > > 27) 3952 304 shrink_lruvec+0x3e1/0x6a0
> > > 28) 3648 80 shrink_zone+0x3f/0x110
> > > 29) 3568 128 do_try_to_free_pages+0x156/0x4c0
> > > 30) 3440 208 try_to_free_pages+0xf7/0x1e0
> > > 31) 3232 352 __alloc_pages_nodemask+0x783/0xb20
> > > 32) 2880 8 alloc_pages_current+0x10f/0x1f0
> > > 33) 2872 200 __page_cache_alloc+0x13f/0x160
> > > 34) 2672 80 find_or_create_page+0x4c/0xb0
> > > 35) 2592 80 ext4_mb_load_buddy+0x1e9/0x370
> > > 36) 2512 176 ext4_mb_regular_allocator+0x1b7/0x460
> > > 37) 2336 128 ext4_mb_new_blocks+0x458/0x5f0
> > > 38) 2208 256 ext4_ext_map_blocks+0x70b/0x1010
> > > 39) 1952 160 ext4_map_blocks+0x325/0x530
> > > 40) 1792 384 ext4_writepages+0x6d1/0xce0
> > > 41) 1408 16 do_writepages+0x23/0x40
> > > 42) 1392 96 __writeback_single_inode+0x45/0x2e0
> > > 43) 1296 176 writeback_sb_inodes+0x2ad/0x500
> > > 44) 1120 80 __writeback_inodes_wb+0x9e/0xd0
> > > 45) 1040 160 wb_writeback+0x29b/0x350
> > > 46) 880 208 bdi_writeback_workfn+0x11c/0x480
> > > 47) 672 144 process_one_work+0x1d2/0x570
> > > 48) 528 112 worker_thread+0x116/0x370
> > > 49) 416 240 kthread+0xf3/0x110
> > > 50) 176 176 ret_from_fork+0x7c/0xb0
>
> Impressive: 3 nested allocations - GFP_NOFS, GFP_NOIO and then
> GFP_ATOMIC before the stack goes boom. XFS usually only needs 2...

Do they also usually involve swap_writepage()?

---

diff --git a/mm/page_io.c b/mm/page_io.c
index 7c59ef681381..02e7e3c168cf 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -233,6 +233,22 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
{
int ret = 0;

+ /*
+ * Refuse to write the page out if we are called from reclaim context.
+ *
+ * This avoids stack overflows when called from deeply used stacks in
+ * random callers for direct reclaim or memcg reclaim. We explicitly
+ * allow reclaim from kswapd as the stack usage there is relatively low.
+ *
+ * This should never happen except in the case of a VM regression so
+ * warn about it.
+ */
+ if (WARN_ON_ONCE((current->flags & (PF_MEMALLOC|PF_KSWAPD)) ==
+ PF_MEMALLOC)) {
+ SetPageDirty(page);
+ goto out;
+ }
+
if (try_to_free_swap(page)) {
unlock_page(page);
goto out;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 61c576083c07..99cca6633e0d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -985,13 +985,12 @@ static unsigned long shrink_page_list(struct list_head *page_list,

if (PageDirty(page)) {
/*
- * Only kswapd can writeback filesystem pages to
- * avoid risk of stack overflow but only writeback
+ * Only kswapd can writeback pages to avoid
+ * risk of stack overflow but only writeback
* if many dirty pages have been encountered.
*/
- if (page_is_file_cache(page) &&
- (!current_is_kswapd() ||
- !zone_is_reclaim_dirty(zone))) {
+ if (!current_is_kswapd() ||
+ !zone_is_reclaim_dirty(zone))) {
/*
* Immediately reclaim when written back.
* Similar in principal to deactivate_page()