Re: [PATCH 4/8] hugetlb: handle truncate racing with page faults

Sven Schnelle
2022-09-06T15:24:05Z

Hi Mike,

Mike Kravetz <mike.kravetz@oracle.com> writes:

> When page fault code needs to allocate and instantiate a new hugetlb
> page (huegtlb_no_page), it checks early to determine if the fault is
> beyond i_size. When discovered early, it is easy to abort the fault and
> return an error. However, it becomes much more difficult to handle when
> discovered later after allocating the page and consuming reservations
> and adding to the page cache. Backing out changes in such instances
> becomes difficult and error prone.
>
> Instead of trying to catch and backout all such races, use the hugetlb
> fault mutex to handle truncate racing with page faults. The most
> significant change is modification of the routine remove_inode_hugepages
> such that it will take the fault mutex for EVERY index in the truncated
> range (or hole in the case of hole punch). Since remove_inode_hugepages
> is called in the truncate path after updating i_size, we can experience
> races as follows.
> - truncate code updates i_size and takes fault mutex before a racing
> fault. After fault code takes mutex, it will notice fault beyond
> i_size and abort early.
> - fault code obtains mutex, and truncate updates i_size after early
> checks in fault code. fault code will add page beyond i_size.
> When truncate code takes mutex for page/index, it will remove the
> page.
> - truncate updates i_size, but fault code obtains mutex first. If
> fault code sees updated i_size it will abort early. If fault code
> does not see updated i_size, it will add page beyond i_size and
> truncate code will remove page when it obtains fault mutex.
>
> Note, for performance reasons remove_inode_hugepages will still use
> filemap_get_folios for bulk folio lookups. For indicies not returned in
> the bulk lookup, it will need to lookup individual folios to check for
> races with page fault.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
> fs/hugetlbfs/inode.c | 184 +++++++++++++++++++++++++++++++------------
> mm/hugetlb.c | 41 +++++-----
> 2 files changed, 152 insertions(+), 73 deletions(-)

With linux next starting from next-20220831 i see hangs with this
patch applied while running the glibc test suite. The patch doesn't
revert cleanly on top, so i checked out one commit before that one and
with that revision everything works.

It looks like the malloc test suite in glibc triggers this. I cannot
identify a single test causing it, but instead the combination of
multiple tests. Running the test suite on a single CPU works. Given the
subject of the patch that's likely not a surprise.

This is on s390, and the warning i get from RCU is:

[ 1951.906997] rcu: INFO: rcu_sched self-detected stall on CPU
[ 1951.907009] rcu: 60-....: (6000 ticks this GP) idle=968c/1/0x4000000000000000 softirq=43971/43972 fqs=2765
[ 1951.907018] (t=6000 jiffies g=116125 q=1008072 ncpus=64)
[ 1951.907024] CPU: 60 PID: 1236661 Comm: ld64.so.1 Not tainted 6.0.0-rc3-next-20220901 #340
[ 1951.907027] Hardware name: IBM 3906 M04 704 (z/VM 7.1.0)
[ 1951.907029] Krnl PSW : 0704e00180000000 00000000003d9042 (hugetlb_fault_mutex_hash+0x2a/0xd8)
[ 1951.907044] R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
[ 1951.907095] Call Trace:
[ 1951.907098] [<00000000003d9042>] hugetlb_fault_mutex_hash+0x2a/0xd8
[ 1951.907101] ([<00000000005845a6>] fault_lock_inode_indicies+0x8e/0x128)
[ 1951.907107] [<0000000000584876>] remove_inode_hugepages+0x236/0x280
[ 1951.907109] [<0000000000584a7c>] hugetlbfs_evict_inode+0x3c/0x60
[ 1951.907111] [<000000000044fe96>] evict+0xe6/0x1c0
[ 1951.907116] [<000000000044a608>] __dentry_kill+0x108/0x1e0
[ 1951.907119] [<000000000044ac64>] dentry_kill+0x6c/0x290
[ 1951.907121] [<000000000044afec>] dput+0x164/0x1c0
[ 1951.907123] [<000000000042a4d6>] __fput+0xee/0x290
[ 1951.907127] [<00000000001794a8>] task_work_run+0x88/0xe0
[ 1951.907133] [<00000000001f77a0>] exit_to_user_mode_prepare+0x1a0/0x1a8
[ 1951.907137] [<0000000000d0e42e>] __do_syscall+0x11e/0x200
[ 1951.907142] [<0000000000d1d392>] system_call+0x82/0xb0
[ 1951.907145] Last Breaking-Event-Address:
[ 1951.907146] [<0000038001d839c0>] 0x38001d839c0

One of the hanging test cases is usually malloc/tst-malloc-too-large-malloc-hugetlb2.

Any thoughts?

Thanks,
Sven