Hey, TL;DR: is it expected behavior that the "VmSwap" output in /proc/PID/status can be > 0, even without swap, and if so, why? While experimenting with swap, I've run into a behavior that I can't quite explain. My setup (tested on 5.2.4): I reboot a machine with a swap partition; then I produce memory pressure by allocating lots of memory and writing to each page of allocated memory. So far, so good: memory is being swapped out, and that information is reflected in several respective files in /proc: $ cat /proc/swaps Filename Type Size Used Priority /dev/dm-1 partition 4194300 5644 -2 $ grep Swap /proc/meminfo SwapCached: 600 kB SwapTotal: 4194300 kB SwapFree: 4188656 kB $ grep VmSwap /proc/[0-9]*/status /proc/1/status:VmSwap: 464 kB /proc/257/status:VmSwap: 0 kB /proc/272/status:VmSwap: 0 kB /proc/498/status:VmSwap: 176 kB /proc/499/status:VmSwap: 292 kB /proc/528/status:VmSwap: 96 kB /proc/531/status:VmSwap: 56 kB /proc/535/status:VmSwap: 24 kB /proc/537/status:VmSwap: 20 kB /proc/541/status:VmSwap: 8 kB /proc/542/status:VmSwap: 4 kB /proc/548/status:VmSwap: 264 kB /proc/561/status:VmSwap: 76 kB /proc/564/status:VmSwap: 0 kB /proc/565/status:VmSwap: 428 kB /proc/578/status:VmSwap: 156 kB /proc/579/status:VmSwap: 1272 kB /proc/587/status:VmSwap: 196 kB /proc/588/status:VmSwap: 452 kB /proc/589/status:VmSwap: 908 kB /proc/599/status:VmSwap: 0 kB /proc/606/status:VmSwap: 0 kB Now I disable swap (swapoff -a), which, as far as I understand, should force all swapped out pages to be swapped back in. The swap is indeed gone: $ grep Swap /proc/meminfo SwapCached: 0 kB SwapTotal: 0 kB SwapFree: 0 kB However, some processes claim to still have swapped out memory: $ grep VmSwap /proc/[0-9]*/status /proc/1/status:VmSwap: 0 kB /proc/257/status:VmSwap: 0 kB /proc/272/status:VmSwap: 0 kB /proc/498/status:VmSwap: 0 kB /proc/499/status:VmSwap: 0 kB /proc/528/status:VmSwap: 12 kB /proc/531/status:VmSwap: 8 kB /proc/535/status:VmSwap: 16 kB /proc/537/status:VmSwap: 0 kB /proc/541/status:VmSwap: 0 kB /proc/542/status:VmSwap: 0 kB /proc/548/status:VmSwap: 0 kB /proc/561/status:VmSwap: 0 kB /proc/564/status:VmSwap: 0 kB /proc/565/status:VmSwap: 0 kB /proc/578/status:VmSwap: 0 kB /proc/579/status:VmSwap: 0 kB /proc/587/status:VmSwap: 0 kB /proc/588/status:VmSwap: 0 kB /proc/589/status:VmSwap: 0 kB /proc/599/status:VmSwap: 0 kB /proc/606/status:VmSwap: 0 kB How can this be? I understand that VmSwap doesn't account for shmem, so the sum of all VmSwaps might be *smaller* than (SwapTotal - SwapFree), but the other way round? A quick look into the source code tells me VmSwap is read from each process's MM_SWAPENTS, and while that one is only updated in a few places, this leads far enough into the deep end of the mm subsystem that I'm not sure I can immediately understand all situations in which that value is updated. So before I invest the time to dive into this code, I thought I'd ask here: is that behavior (VmSwap output in proc can be > 0 even without swap) expected, and if so, why? Cheers, Florian
