On Mon, Aug 30, 2021 at 06:05:58PM +0200, Vlastimil Babka wrote: > On 8/30/21 16:41, Matthew Wilcox wrote: > > On Mon, Aug 30, 2021 at 07:34:38AM -0700, Randy Dunlap wrote: > >> [add linux-mm mailing list] > >> > >> On 8/30/21 12:44 AM, Mikko Rantalainen wrote: > >> > It's not immediately obvious from fs/proc/meminfo.c function > >> > meminfo_proc_show() but the output of Cached: field seems to always > >> > include all of Shmem: field, too. > >> > > >> > Is this intentional? Usually cache is something that can be discarded if > >> > needed but shared memory (e.g. used to contain files in tmpfs) cannot be > >> > discarded without a data-loss. As such, I'd argue that it shouldn't be > >> > included in the Cached: output. > > > > That's a reasonable position to take. > > > > Another point of view is that everything in tmpfs is part of the page > > cache and can be written out to swap, so keeping it as part of Cached > > is not misleading. > > Yeah, but with that view, anonymous memory can be also written out to swap. But > it's non-intuitive that something called "Cached" will contain something that > (if not dirty) can't be just dropped. That's equally true for normal filesystems & shmem though. Consider shmem written to swap, then brought back in by a read. Now it can be dropped without being swapped out. Or even a file on shmem ftruncated to a large size, then only read. The pages will be clean and full of zeroes. They can be dropped under memory pressure without being written out. > I've had to point this Shmem oddity out a > number of times to someone, so I would say that it would be much better if it > was not part of Cached. > However, if we change it now, we might create even larger confusion. People > looking at the output for the first time (and IIRC also the 'free' command uses > it) on a new kernel wouldn't be misled anymore. But people working with both old > and new kernels will now have to take in account that it changed at some > point... not good. Another good point.
