On 2/18/20 1:36 PM, Mina Almasry wrote:
On Tue, Feb 18, 2020 at 11:25 AM Mina Almasry almasrymina@google.com wrote:
On Tue, Feb 18, 2020 at 11:14 AM Mike Kravetz mike.kravetz@oracle.com wrote:
On 2/18/20 10:35 AM, Mina Almasry wrote:
On Tue, Feb 18, 2020 at 6:21 AM Qian Cai cai@lca.pw wrote:
On Tue, 2020-02-11 at 15:19 -0800, Andrew Morton wrote:
On Tue, 11 Feb 2020 13:31:20 -0800 Mina Almasry almasrymina@google.com wrote:
[ 7933.806377][T14355] ------------[ cut here ]------------ [ 7933.806541][T14355] kernel BUG at mm/hugetlb.c:490! VM_BUG_ON(t - f <= 1); [ 7933.806562][T14355] Oops: Exception in kernel mode, sig: 5 [#1]
<snip> > Hi Qian, > > Yes this VM_BUG_ON was added by a patch in the series ("hugetlb: > disable region_add file_region coalescing") so it's definitely related > to the series. I'm taking a look at why this VM_BUG_ON fires. Can you > confirm you reproduce this by running hugemmap06 from the ltp on a > powerpc machine? Can I maybe have your config? > > Thanks!
Hi Mina,
Looking at the region_chg code again, we do a
resv->adds_in_progress += *out_regions_needed;and then potentially drop the lock to allocate the needed entries. Could anopther thread (only adding reservation for a single page) then come in and notice that there are not enough entries in the cache and hit the VM_BUG_ON()?
Maybe. Also I'm thinking the code thinks actual_regions_needed >= in_regions_needed, but that doesn't seem like a guarantee. I think this call sequence with the same t->f range would violate that:
region_chg (regions_needed=1) region_chg (regions_needed=1) region_add (fills in the range) region_add (in_regions_needed = 1, actual_regions_needed = 0, so assumptions in the code break).
Luckily it seems the ltp readily reproduces this, so I'm working on reproducing it. I should have a fix soon, at least if I can reproduce it as well.
I had a bit of trouble reproducing this but I got it just now.
Makes sense I've never run into this even though others can readily reproduce it. I happen to run my kernels on a pretty beefy 36 core machine and in that setup things seem to execute fast and there is never a queue of pending file_region inserts into the resv_map. Once I limited qemu to only use 2 cores I ran into the issue right away. Looking into a fix now.
This may not be optimal, but it resolves the issue for me. I just put it together to test the theory that the region_chg code was at fault.