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

From: Minchan Kim
Date: Thu May 29 2014 - 02:05:48 EST

On Wed, May 28, 2014 at 12:06:58PM -0400, Johannes Weiner wrote:
> 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()?

Maybe it works but the problem I can think of is churn of LRU because
anon pages scanned in direct reclaim would live another round in LRU
and as Dave already pointed out, it couldn't prevent synchronous
unplugging caused by another shedule point in direct reclaim path
so I buy Dave's idea which pass plug list off to the kblockd.

>
> ---
>
> 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()
>
> --
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--
Kind regards,
Minchan Kim
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