On Sun, Apr 16, 2023 at 01:42:44AM +0800, Wen Yang wrote: > > ??? 2023/4/13 00:54, Wen Yang ??????: > > > > ??? 2023/4/12 04:48, Andrew Morton ??????: > > > On Wed, 12 Apr 2023 02:24:26 +0800 wenyang.linux@xxxxxxxxxxx wrote: > > > > > > > For the /proc/sys/vm/compact_memory file, the admin-guide states: > > > > When 1 is written to the file, all zones are compacted such that free > > > > memory is available in contiguous blocks where possible. This can be > > > > important for example in the allocation of huge pages although > > > > processes > > > > will also directly compact memory as required > > > > > > > > But it was not strictly followed, writing any value would cause all > > > > zones to be compacted. In some critical scenarios, some applications > > > > operating it, such as echo 0, have caused serious problems. > > > Really? You mean someone actually did this and didn't observe the > > > effect during their testing? > > > > Thanks for your reply. > > > > Since /proc/sys/vm/compact_memory has been well documented for over a > > decade: > > > > https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/Documentation/admin-guide/sysctl/vm.rst#n109 > > > > > > it is believed that only writing 1 will trigger trigger all zones to be > > compacted. > > > > Especially for those who write applications, they may only focus on > > documentation and generally do not read kernel code. Moreover, such > > problems are not easily detected through testing on low pressure > > machines. > > > > Writing any meaningful or meaningless values will trigger it and affect > > the entire server: > > > > # echo 1 > /proc/sys/vm/compact_memory > > # echo 0 > /proc/sys/vm/compact_memory > > # echo dead > /proc/sys/vm/compact_memory > > # echo "hello world" > /proc/sys/vm/compact_memory > > > > The implementation of this high-risk operation may require following the > > admin-guides. > > > > -- > > > > Best wishes, > > > > Wen > > > > > Hello, do you think it's better to optimize the sysctl_compaction_handler > code or update the admin-guide document? > Enforce the 1 on the unlikely chance that the sysctl handler is ever extended to do something different and expects a bitmask. The original intent intent of the sysctl was debugging -- demonstrating a contiguous allocation failure when aggressive compaction should have succeeded. Later some machines dedicated to batch jobs used the compaction sysctl to compact memory before a new job started to reduce startup latencies. Drop the justification "In some critical scenarios, some applications operating it, such as echo 0, have caused serious problems." from the changelog. I cannot imagine a sane "critical scenario" where an application running as root is writing expected garbage to proc or sysfs files and then surprised when something unexpected happens. -- Mel Gorman SUSE Labs
