On Tue, Oct 11, 2022 at 11:57:53AM -0400, Trond Myklebust wrote: > On Tue, 2022-10-11 at 17:00 +0200, Guillaume Nault wrote: > > On Mon, Oct 10, 2022 at 11:18:19PM -0700, Christoph Hellwig wrote: > > > On Mon, Oct 10, 2022 at 06:56:50PM +0200, Guillaume Nault wrote: > > > > That's what my RFC patch did. It was rejected because reading > > > > current->flags may incur a cache miss thus slowing down TCP fast > > > > path. > > > > See the discussion in the Link tag: > > > > https://lore.kernel.org/netdev/b4d8cb09c913d3e34f853736f3f5628abfd7f4b6.1656699567.git.gnault@xxxxxxxxxx/ > > > > > > As GFP_NOFS/NOIO are on their way out the networking people will > > > have to > > > do this anyway. > > > > We can always think of a nicer solution in the future. But right now > > we > > have a real bug to fix. > > > > Commit a1231fda7e94 ("SUNRPC: Set memalloc_nofs_save() on all > > rpciod/xprtiod jobs") introduces a bug that crashes the kernel. I > > can't > > see anything wrong with a partial revert. > > > > How about instead just adding a dedicated flag to the socket that > switches between the two page_frag modes? > > That would remain future proofed, and it would give kernel users a > lever with which to do the right thing without unnecessarily > constraining the allocation modes. The problem is to find a hole in struct sock, in a cacheline that wouldn't incur a cache miss. So far the best option seems to add a special flag in sk_allocation, which would serve the same purpose as GFP_NOFS in sk_page_frag(). But that'd mean clearing this bit every time sk_allocation is used for memory allocation. Not really something I'd propose for stable trees. > -- > Trond Myklebust > Linux NFS client maintainer, Hammerspace > trond.myklebust@xxxxxxxxxxxxxxx > >
