> On Jul 25, 2022, at 10:19 AM, Olga Kornievskaia <aglo@xxxxxxxxx> wrote: > > Hi Chuck, > > A general question: what's the rule for deciding whether to allocate > statically or dynamically? I thought that "small" structures it's > better to preallocate (statically) for performance reasons. Or is the > idea here that copy_notify/copy are rare operations that instead they > should be allocated dynamically and so that other operations doesn't > consume more memory in nfsd4_op structure? tl;dr: the latter. But it's not an issue of memory consumption, it's that this whole struct is cleared with memset(0) for each incoming RPC. nfsd4_op is a union -- it takes the size of the largest args struct in that union. Before copy offload was introduced, that was about 250 bytes. After, it is ~10x larger. There are 8 nfsd4_op structs in an array in nfsd4_compoundargs. The nfsd4_compoundargs structure is cleared by svc_generic_init_request() before each NFSv4 RPC is executed. So, in this case, the problem is 8 x 2KB = ~17KB to clear on each RPC call for an operation that is quite infrequently requested. In NFSD, typically the execution context is amenable to GFP_KERNEL, so it's generally OK to allocate dynamically for infrequently requested operations. -- Chuck Lever
