mm/swapfile: add cond_resched() in get_swap_pages()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: mm/swapfile: add cond_resched() in get_swap_pages() The softlockup still occurs in get_swap_pages() under memory pressure. 64 CPU cores, 64GB memory, and 28 zram devices, the disksize of each zram device is 50MB with same priority as si. Use the stress-ng tool to increase memory pressure, causing the system to oom frequently. The plist_for_each_entry_safe() loops in get_swap_pages() could reach tens of thousands of times to find available space (extreme case: cond_resched() is not called in scan_swap_map_slots()). Let's add cond_resched() into get_swap_pages() when failed to find available space to avoid softlockup.

References

• https://git.kernel.org/stable/c/29f0349c5c76b627fe06b87d4b13fa03a6ce8e64
• https://git.kernel.org/stable/c/387217b97e99699c34e6d95ce2b91b327fcd853e
• https://git.kernel.org/stable/c/d49c85a1913385eed46dd16a25ad0928253767f0
• https://git.kernel.org/stable/c/30187be29052bba9203b0ae2bdd815e0bc2faaab
• https://git.kernel.org/stable/c/5dbe1ebd56470d03b78fc31491a9e4d433106ef2
• https://git.kernel.org/stable/c/49178d4d61e78aed8c837dfeea8a450700f196e2
• https://git.kernel.org/stable/c/7717fc1a12f88701573f9ed897cc4f6699c661e3