Hi! There's something that looks like a bug in tmpfs' implementation of fallocate. If you try to fallocate more than the available DRAM (yet with plenty of swap space), it will evict everything swappable out then fail, undoing all the work done so far first. The returned error is ENOMEM rather than POSIX mandated ENOSPC (for posix_allocate(), but our documentation doesn't mention ENOMEM for Linux-specific fallocate() either). Doing the same allocation in multiple calls -- be it via non-overlapping calls or even with same offset but increasing len -- works as expected. An example: Machine has 32GB RAM, minus 4GB memmapped as fake pmem. No big tasks (X, some shells, browser, ...). Run ｢while :;do free -m;done｣ on another terminal, then: # mount -osize=64G -t tmpfs none /mnt/vol1 # chown you /mnt/vol1 $ cd /mnt/vol1 $ fallocate -l 32G foo fallocate: fallocate failed: Cannot allocate memory $ fallocate -l 28G foo fallocate: fallocate failed: Cannot allocate memory $ fallocate -l 27G foo fallocate: fallocate failed: Cannot allocate memory $ fallocate -l 26G foo $ fallocate -l 52G foo It takes a few seconds for the allocation to succeed, then a couple for it to be torn down if it fails. More if it has to writeout the zeroes it allocated in the previous call. This raises multiple questions: * why would fallocate bother to prefault the memory instead of just reserving it? We want to kill overcommit, but reserving swap is as good -- if there's memory pressure, our big allocation will be evicted anyway. * why does it insist on doing everything in one piece? Biggest chunk I see to be beneficial is 1G (for hugepages). * when it fails, why does it undo the work done so far? This can matter for other reasons, such as EINTR -- and fallocate isn't expected to be atomic anyway. * if I'm wrong and atomicity+prefaulting are desired, why does fallocate forces just the delta (pages not yet allocated) to reside in core, rather than the entire requested range? Thus, I believe fallocate on tmpfs should behave consistently with other filesystems and succeed unless we run into ENOSPC. Am I missing something? Meow! -- ⢀⣴⠾⠻⢶⣦⠀ ⣾⠁⢠⠒⠀⣿⡁ ⢿⡄⠘⠷⠚⠋⠀ Have you accepted Khorne as your lord and saviour? ⠈⠳⣄⠀⠀⠀⠀
