On 5/20/20 1:31 PM, Vlastimil Babka wrote: > On 4/22/20 10:46 PM, Roman Gushchin wrote: >> To implement per-object slab memory accounting, we need to >> convert slab vmstat counters to bytes. Actually, out of >> 4 levels of counters: global, per-node, per-memcg and per-lruvec >> only two last levels will require byte-sized counters. >> It's because global and per-node counters will be counting the >> number of slab pages, and per-memcg and per-lruvec will be >> counting the amount of memory taken by charged slab objects. >> >> Converting all vmstat counters to bytes or even all slab >> counters to bytes would introduce an additional overhead. >> So instead let's store global and per-node counters >> in pages, and memcg and lruvec counters in bytes. >> >> To make the API clean all access helpers (both on the read >> and write sides) are dealing with bytes. >> >> To avoid back-and-forth conversions a new flavor of helpers >> is introduced, which always returns values in pages: >> node_page_state_pages() and global_node_page_state_pages(). >> >> Actually new helpers are just reading raw values. Old helpers are >> simple wrappers, which perform a conversion if the vmstat items are >> in bytes. Because at the moment no one actually need bytes, >> there are WARN_ON_ONCE() macroses inside to warn about inappropriate >> use cases. >> >> Thanks to Johannes Weiner for the idea of having the byte-sized API >> on top of the page-sized internal storage. >> >> Signed-off-by: Roman Gushchin <guro@xxxxxx> > > Reviewed-By: Vlastimil Babka <vbabka@xxxxxxx> > > But it's somewhat complicated, so it would be great to document it in comments > of e.g. include/linux/vmstat.h that what the API returns as unsigned long, can > be either bytes or pages depending on vmstat_item_in_bytes(). Also forgot to add that if those WARN_ON_ONCEs are going to stay, they should rather become VM_WARN_ON_ONCEs
