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

[Mike Kravetz]

On 09/06/22 11:05, Mike Kravetz wrote:
> On 09/06/22 09:48, Mike Kravetz wrote:
> > On 09/06/22 15:57, Sven Schnelle wrote:
> > > Hi Mike,
> > > 
> > > Mike Kravetz <mike.kravetz@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > ---
> > > >  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 for the report, I will take a look.
> > 
> > My first thought is that this fix may not be applied,
> > https://lore.kernel.org/linux-mm/Ywepr7C2X20ZvLdn@monkey/
> > However, I see that that is in next-20220831.
> > 
> > Hopefully, this will recreate on x86.
> 
> One additional thought ...
> 
> With this patch, we will take the hugetlb fault mutex for EVERY index in the
> range being truncated or hole punched.  In the case of a very large file, that
> is no different than code today where we take the mutex when removing pages
> from the file.  What is different is taking the mutex for indices that are
> part of holes in the file.  Consider a very large file with only one page at
> the very large offset.  We would then take the mutex for each index in that
> very large hole.  Depending on the size of the hole, this could appear as a
> hang.
> 
> For the above locking scheme to work, we need to take the mutex for indices
> in holes in case there would happen to be a racing page fault.  However, there
> are only a limited number of fault mutexes (it is a table).  So, we only really
> need to take at a maximum num_fault_mutexes mutexes.  We could keep track of
> these with a bitmap.
> 
> I am not sure this is the issue you are seeing, but a test named
> tst-malloc-too-large-malloc-hugetlb2 may be doing this.
> 
> In any case, I think this issue needs to be addressed before this series can
> move forward.

Well, even if we address the issue of taking the same mutex multiple times,
this new synchronization scheme requires a folio lookup for EVERY index in
the truncated or hole punched range.  This can easily 'stall' a CPU if there
is a really big hole in a file.  One can recreate this easily with fallocate
to add a single page to a file at a really big offset, and then remove the file.

I am trying to come up with another algorithm to make this work.

Andrew, I wanted to give you a heads up that this series may need to be
pulled if I can not come up with something quickly.
-- 
Mike Kravetz