>>>>> "Mikulas" == Mikulas Patocka <mpatocka@xxxxxxxxxx> writes: Mikulas> On Mon, 14 Aug 2017, John Stoffel wrote: >> >>>>> "Mikulas" == Mikulas Patocka <mpatocka@xxxxxxxxxx> writes: >> Mikulas> dm-crypt consumes excessive amount memory when the user attempts to zero Mikulas> a dm-crypt device with "blkdiscard -z". The command "blkdiscard -z" calls Mikulas> the BLKZEROOUT ioctl, it goes to the function __blkdev_issue_zeroout, Mikulas> __blkdev_issue_zeroout sends large amount of write bios that contain the Mikulas> zero page as their payload. >> Mikulas> For each incoming page, dm-crypt allocates another page that holds the Mikulas> encrypted data, so when processing "blkdiscard -z", dm-crypt tries to Mikulas> allocate the amount of memory that is equal to the size of the device. Mikulas> This can trigger OOM killer or cause system crash. >> Mikulas> This patch fixes the bug by limiting the amount of memory that dm-crypt Mikulas> allocates to 2% of total system memory. >> >> Is this limit per-device or system wide? it's not clear to me if the >> minimum is just one page, or more so it might be nice to clarify >> that. Mikulas> The limit is system-wide. The limit is divided by the number Mikulas> of active dm-crypt devices and each device receives an equal Mikulas> share. Ok, thanks for the clarification. Can it be expanded dynamically? I could see that for large systems, it might not scale well. Mikulas> There is a lower bound of BIO_MAX_PAGES * 16. The per-device Mikulas> limit is never lower than this. Nice. >> Also, for large memory systems, that's still alot of pages. Maybe it >> should be more exponential in it's clamping as memory sizes go up? 2% >> of 2T is 4G, which is pretty darn big... Mikulas> The more we restrict it, the less parallelism is used in dm-crypt, so it Mikulas> makes sense to have a big value on machines with many cores. True... but it's still a concern that it's hardcoded. Thanks for your replies. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
