On 7/30/20 3:17 PM, Daniel Vetter wrote:
On Thu, Jul 30, 2020 at 2:17 PM Thomas Hellström (Intel)
<thomas_os@xxxxxxxxxxxx> wrote:
On 7/28/20 3:58 PM, Daniel Vetter wrote:
GPU drivers need this in their shrinkers, to be able to throw out
mmap'ed buffers. Note that we also need dma_resv_lock in shrinkers,
but that loop is resolved by trylocking in shrinkers.
So full hierarchy is now (ignore some of the other branches we already
have primed):
mmap_read_lock -> dma_resv -> shrinkers -> i_mmap_lock_write
I hope that's not inconsistent with anything mm or fs does, adding
relevant people.
Looks OK to me. The mapping_dirty_helpers run under the i_mmap_lock, but
don't allocate any memory AFAICT.
Since huge page-table-entry splitting may happen under the i_mmap_lock
from unmap_mapping_range() it might be worth figuring out how new page
directory pages are allocated, though.
ofc I'm not an mm expert at all, but I did try to scroll through all
i_mmap_lock_write/read callers. Found the following:
- kernel/events/uprobes.c in build_map_info:
/*
* Needs GFP_NOWAIT to avoid i_mmap_rwsem recursion through
* reclaim. This is optimistic, no harm done if it fails.
*/
- I got lost in the hugetlb.c code and couldn't convince myself it's
not allocating page directories at various levels with something else
than GFP_KERNEL.
So looks like the recursion is clearly there and known, but the
hugepage code is too complex and flying over my head.
-Daniel
OK, so I inverted your annotation and ran a memory hog, and got the
below splat. So clearly your proposed reclaim->i_mmap_lock locking order
is an already established one.
So
Reviewed-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxx>
8<---------------------------------------------------------------------------------------------
[ 308.324654] WARNING: possible circular locking dependency detected
[ 308.324655] 5.8.0-rc2+ #16 Not tainted
[ 308.324656] ------------------------------------------------------
[ 308.324657] kswapd0/98 is trying to acquire lock:
[ 308.324658] ffff92a16f758428 (&mapping->i_mmap_rwsem){++++}-{3:3},
at: rmap_walk_file+0x1c0/0x2f0
[ 308.324663]
but task is already holding lock:
[ 308.324664] ffffffffb0960240 (fs_reclaim){+.+.}-{0:0}, at:
__fs_reclaim_acquire+0x5/0x30
[ 308.324666]
which lock already depends on the new lock.
[ 308.324667]
the existing dependency chain (in reverse order) is:
[ 308.324667]
-> #1 (fs_reclaim){+.+.}-{0:0}:
[ 308.324670] fs_reclaim_acquire+0x34/0x40
[ 308.324672] dma_resv_lockdep+0x186/0x224
[ 308.324675] do_one_initcall+0x5d/0x2c0
[ 308.324676] kernel_init_freeable+0x222/0x288
[ 308.324678] kernel_init+0xa/0x107
[ 308.324679] ret_from_fork+0x1f/0x30
[ 308.324680]
-> #0 (&mapping->i_mmap_rwsem){++++}-{3:3}:
[ 308.324682] __lock_acquire+0x119f/0x1fc0
[ 308.324683] lock_acquire+0xa4/0x3b0
[ 308.324685] down_read+0x2d/0x110
[ 308.324686] rmap_walk_file+0x1c0/0x2f0
[ 308.324687] page_referenced+0x133/0x150
[ 308.324689] shrink_active_list+0x142/0x610
[ 308.324690] balance_pgdat+0x229/0x620
[ 308.324691] kswapd+0x200/0x470
[ 308.324693] kthread+0x11f/0x140
[ 308.324694] ret_from_fork+0x1f/0x30
[ 308.324694]
other info that might help us debug this:
[ 308.324695] Possible unsafe locking scenario:
[ 308.324695] CPU0 CPU1
[ 308.324696] ---- ----
[ 308.324696] lock(fs_reclaim);
[ 308.324697] lock(&mapping->i_mmap_rwsem);
[ 308.324698] lock(fs_reclaim);
[ 308.324699] lock(&mapping->i_mmap_rwsem);
[ 308.324699]
*** DEADLOCK ***
[ 308.324700] 1 lock held by kswapd0/98:
[ 308.324701] #0: ffffffffb0960240 (fs_reclaim){+.+.}-{0:0}, at:
__fs_reclaim_acquire+0x5/0x30
[ 308.324702]
stack backtrace:
[ 308.324704] CPU: 1 PID: 98 Comm: kswapd0 Not tainted 5.8.0-rc2+ #16
[ 308.324705] Hardware name: VMware, Inc. VMware Virtual Platform/440BX
Desktop Reference Platform, BIOS 6.00 07/29/2019
[ 308.324706] Call Trace:
[ 308.324710] dump_stack+0x92/0xc8
[ 308.324711] check_noncircular+0x12d/0x150
[ 308.324713] __lock_acquire+0x119f/0x1fc0
[ 308.324715] lock_acquire+0xa4/0x3b0
[ 308.324716] ? rmap_walk_file+0x1c0/0x2f0
[ 308.324717] ? __lock_acquire+0x394/0x1fc0
[ 308.324719] down_read+0x2d/0x110
[ 308.324720] ? rmap_walk_file+0x1c0/0x2f0
[ 308.324721] rmap_walk_file+0x1c0/0x2f0
[ 308.324722] page_referenced+0x133/0x150
[ 308.324724] ? __page_set_anon_rmap+0x70/0x70
[ 308.324725] ? page_get_anon_vma+0x190/0x190
[ 308.324726] shrink_active_list+0x142/0x610
[ 308.324728] balance_pgdat+0x229/0x620
[ 308.324730] kswapd+0x200/0x470
[ 308.324731] ? lockdep_hardirqs_on_prepare+0xf5/0x170
[ 308.324733] ? finish_wait+0x80/0x80
[ 308.324734] ? balance_pgdat+0x620/0x620
[ 308.324736] kthread+0x11f/0x140
[ 308.324737] ? kthread_create_worker_on_cpu+0x40/0x40
[ 308.324739] ret_from_fork+0x1f/0x30
/Thomas
