Last year I sent an RFC about using oom-reaper while killing a process: https://patchwork.kernel.org/cover/10894999. During LSFMM2019 discussion https://lwn.net/Articles/787217 a couple of alternative options were discussed with the most promising one (outlined in the last paragraph of https://lwn.net/Articles/787217) suggesting to use a remote version of madvise(MADV_DONTNEED) operation to force memory reclaim of a killed process. With process_madvise() making its way through reviews (https://patchwork.kernel.org/patch/11747133/), I would like to revive this discussion and get feedback on several possible options, their pros and cons. The need is similar to why oom-reaper was introduced - when a process is being killed to free memory we want to make sure memory is freed even if the victim is in uninterruptible sleep or is busy and reaction to SIGKILL is delayed by an unpredictable amount of time. I experimented with enabling process_madvise(MADV_DONTNEED) operation and using it to force memory reclaim of the target process after sending SIGKILL. Unfortunately this approach requires the caller to read proc/pid/maps to extract the list of VMAs to pass as an input to process_madvise(). This is a time consuming operation. I measured times similar to what Minchan indicated in https://lore.kernel.org/linux-mm/20190528032632.GF6879@xxxxxxxxxx/ and the reason reading proc/pid/maps consumes that much time is the number of read syscalls required to read this file. proc/pid/maps file, being a seq_file, can be read in chunks of up to 4096 bytes (1 page). Even if userspace provides bigger buffer, only up to 4096 bytes will be read with one syscall. Measured on Qualcomm® Snapdragon 855™ using its Big core of 2.84GHz a single read syscall takes between 50 and 200us (in case there was no contention on mmap_sem or some other lock during the syscall). Taking one typical example from my tests, a 219232 bytes long proc/pid/maps file describing 1623 VMAs required 55 read syscalls. With mmap_sem contention proc/pid/maps read can take even longer. In my tests I measured typical delays of 3-7ms with occasional delays of up to 20ms when a read syscall was blocked and the process got into uninterruptible sleep. While the objective is to guarantee forward progress even when the victim cannot terminate, we still want this mechanism to be efficient because we perform these operations to relieve memory pressure before it affects user experience. Alternative options I would like your feedback are: 1. Introduce a dedicated process_madvise(MADV_DONTNEED_MM) specifically for this case to indicate that the whole mm can be freed. 2. A new syscall to efficiently obtain a vector of VMAs (start, length, flags) of the process instead of reading /proc/pid/maps. The size of the vector is still limited by UIO_MAXIOV (1024), so several calls might be needed to query larger number of VMAs, however it will still be an order of magnitude more efficient than reading /proc/pid/maps file in 4K or smaller chunks. 3. Use process_madvise() flags parameter to indicate a bulk operation which ignores input vectors. Sample usage: process_madvise(pidfd, MADV_DONTNEED, vector=NULL, vlen=0, flags=PMADV_FLAG_FILE | PMADV_FLAG_ANON); 4. madvise()/process_madvise() handle gaps between VMAs, so we could provide one vector element spanning the entire address space. There are technical issues with this approach (process_madvise return value can't handle such a large number of bytes and there is MAX_RW_COUNT limit on max number of bytes one process_madvise call can handle) but I would still like to hear opinions about it. If this option is preferable maybe we can deal with these limitations. We can also go back to reclaiming victim's memory asynchronously but synchronous method has the following advantages: - reaping will be performed in the caller's context and therefore with caller's priority, CPU affinity, CPU bandwidth, reaping workload will be charged to the caller and accounted for. - reaping is a blocking/synchronous operation for the caller, so when it's finished, the caller can be sure mm is freed (or almost freed considering lazy freeing and batching mechanisms) and it can reassess the memory conditions right away. - for very large MMs (not really my case) caller could split the VMA vector and perform reaping from multiple threads to make it faster. This would not be possible with options (1) and (3). Would really appreciate your feedback on these options for future development. Thanks, Suren.
