We've had cases where we trigger an OOM despite having memory freely
available on the system. For example, here, we had about 21GB free:
kernel: Node 0 Normal: 2418758*4kB (UME) 1549533*8kB (UE) 0*16kB
0*32kB 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB =
22071296kB
The problem being, all the memory is in 4K and 8K contiguous regions,
but the allocation request was for a 16K contiguous region:
kernel: SafeExecutors-4 invoked oom-killer:
gfp_mask=0x42dc0(GFP_KERNEL|__GFP_NOWARN|__GFP_COMP|__GFP_ZERO),
order=2, oom_score_adj=0
The offending allocation came from this call trace:
kernel: Call Trace:
kernel: dump_stack+0x57/0x7a
kernel: dump_header+0x4f/0x1e1
kernel: oom_kill_process.cold.33+0xb/0x10
kernel: out_of_memory+0x1ad/0x490
kernel: __alloc_pages_slowpath+0xd55/0xe40
kernel: __alloc_pages_nodemask+0x2df/0x330
kernel: kmalloc_large_node+0x42/0x90
kernel: __kmalloc_node+0x25a/0x320
kernel: ? spl_kmem_free_impl+0x21/0x30 [spl]
kernel: spl_kmem_alloc_impl+0xa5/0x100 [spl]
kernel: spl_kmem_zalloc+0x19/0x20 [spl]
kernel: zfsdev_ioctl+0x2b/0xe0 [zfs]
kernel: do_vfs_ioctl+0xa9/0x640
kernel: ? __audit_syscall_entry+0xdd/0x130
kernel: ksys_ioctl+0x67/0x90
kernel: __x64_sys_ioctl+0x1a/0x20
kernel: do_syscall_64+0x5e/0x200
kernel: entry_SYSCALL_64_after_hwframe+0x44/0xa9
kernel: RIP: 0033:0x7fdca3674317
The problem is, for each ioctl that ZFS makes, it has to allocate a
zfs_cmd_t structure, which is 13744 bytes in size (on my system):
sdb> sizeof zfs_cmd
(size_t)13744
This size, coupled with the fact that we currently allocate it with
kmem_zalloc, means we need a 16K contiguous region of memory to satisfy
the request.
The solution taken by this change, is to use "vmem" instead of "kmem" to
do the allocation, such that we don't necessarily need a contiguous 16K
memory region to satisfy the allocation.
Arguably, a better solution would be not to require such a large
allocation to begin with (e.g. reduce the size of the zfs_cmd_t
structure), but that'd be a much larger change than this "one liner".
Thus, I've opted for this approach for now; we can always circle back
and attempt to reduce the size of the structure in the future.
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Reviewed-by: Don Brady <don.brady@delphix.com>
Signed-off-by: Prakash Surya <prakash.surya@delphix.com>
Closes#14474
13312e2fa1