docs: fix style in FAQ.md and add new item about memory allocation

FAQ.md

@@ -3,45 +3,45 @@
 ## Troubleshooting
 
 `Q1:`
-*I tried using an initrd for boot but it doesn't seem to be used for the boot.
+*I tried using an initrd for boot but it doesn't seem to be used.
 Is initrd supported?*  
 `A1:`
-Right now initrd is not supported in Firecracker. You can track the
-[#228](https://github.com/aws/PRIVATE-firecracker/issues/208) issue.
+Right now initrd is not supported in Firecracker. You can track issue
+[#228](https://github.com/aws/PRIVATE-firecracker/issues/208).
 
 `Q2:`
 *Firecracker is not showing any output on the console.*  
 `A2:`
 In order to debug the issue, you will first have to check the response of the
 `InstanceStart` API request. You can find examples in
-[README.md](https://github.com/aws/PRIVATE-firecracker/blob/master/README.md) in
-the "Power-On the MicroVM" section. If the result is:  
+[README.md](https://github.com/aws/PRIVATE-firecracker/blob/master/README.md)
+in the "Power-On the MicroVM" section. If the result is:
 
 - **Error**: Submit a new issue with the label "Support: Failure".
-- **Success**: If the boot was successful, you should get a response with 204 as
-the status code.
+- **Success**: If the boot was successful, you should get a response with 204
+  as the status code.
 
-If you have no output in the console, most likely you will have to update the
-kernel command line. By default, Firecracker starts with the serial console
-disabled for boot time performance reasons. Example of a kernel valid command
-line that enables the serial console:
-`console=ttyS0 reboot=k panic=1 pci=off nomodules`, which goes in the
-`boot_args` field of the `/boot-source` Firecracker API resource.
+  If you have no output in the console, most likely you will have to update the
+  kernel command line. By default, Firecracker starts with the serial console
+  disabled for boot time performance reasons. Example of a kernel valid command
+  line that enables the serial console:
+  `console=ttyS0 reboot=k panic=1 pci=off nomodules`, which goes in the
+  `boot_args` field of the `/boot-source` Firecracker API resource.
 
 `Q3:`
-*How can we pass multiple Ethernet devices through kernel command line?*  
+*How can we configure multiple Ethernet devices through the kernel command
+line?*  
 `A3:`
-The "ip=" boot param in the linux kernel only actually supports configuring a
+The `ip=` boot param in the linux kernel only actually supports configuring a
 single interface. Multiple interfaces can be set up in Firecracker using the
 API, but guest IP configuration at boot time through boot arguments can only be
 done for a single interface.
 
 `Q4:`
 *Each Firecracker opens 20+ file descriptors. Is this an issue?*  
 `A4:`
-The relatively high FD usage is expected and correct. Firecracker heavily relies
-on event file descriptors to drive device emulation. There is nothing you as a
-Firecracker user can do to reduce the number of used file descriptors.
+The relatively high FD usage is expected and correct. Firecracker heavily
+relies on event file descriptors to drive device emulation.
 
 `Q5:`
 *We are trying to create two network interfaces, `eth0` and `eth1` by calling
@@ -50,13 +50,29 @@ bring up interface `eth0` before `eth1`. Then both interfaces would be created
 with the correct MAC addresses. But if we swap the ordering of the calls by
 first calling `/network-interfaces/1` and then `/network-interfaces/0`, the MAC
 addresses of `eth0` and `eth1` will be swapped. Is this expected behavior?
-Should we always setup the eth device in the same order the network-interface
+Should we always setup the `eth` device in the same order the network-interface
 API is called?*  
 `A5:`
-The `0` and `1` in the `/network-interface` path are the API identifiers of the
-network-interface HTTP resources. For now these are only used for resource
-management in the API and have nothing to do with `eth0` and `eth1`. The id that
-you pass through the API call path is used for example to separate an update of
-an existing network-interface from creating a new interface. In short, this is
-expected behavior and the ids from the path are not exactly tied to the ids of
-the actual network interfaces.
+The `0` and `1` in the `/network-interfaces` path are the API identifiers of
+the `network-interfaces` HTTP resources. For now these are only used for
+resource management in the API and have nothing to do with `eth0` and `eth1`.
+The id that you pass through the API call path is used for example to separate
+an update of an existing network interface from creating a new interface. In
+short, this is expected behavior and the ids from the path are not tied to the
+ids of the actual network interfaces.
+
+`Q6`:
+*We are seeing page allocation failures from Firecracker in the `dmesg` output.
+Example:*
+```
+[80427.988646] fc_vmm: page allocation failure: order:6, mode:0x140c0c0
+(GFP_KERNEL|__GFP_COMP|__GFP_ZERO), nodemask=(null)
+[80427.989567] fc_vmm cpuset=27d8fd00-a29a-4745-b518-e6a4e6cd69dd mems_allowed=0
+```
+`A6`:
+The host is running out of memory. KVM is attempting to do an allocation of
+2^`order` bytes (in this case, 6) and there aren't sufficient contiguous pages.
+Possible mitigations are:
+- Track the failing allocations in the `dmesg` output and rebuild the host
+  kernel so as to use `vmalloc` instead of `kmalloc` for them.
+- Reduce memory pressure on the host.