On Fri, May 08, 2020 at 03:23:33PM +0300, Konstantin Khlebnikov wrote: > This patch implements heuristic which detects such scenarios and prevents > unbounded growth of completely unneeded negative dentries. It keeps up to > three latest negative dentry in each bucket unless they were referenced. > > At first dput of negative dentry when it swept to the tail of siblings > we'll also clear it's reference flag and look at next dentries in chain. > Then kill third in series of negative, unused and unreferenced denries. > > This way each hash bucket will preserve three negative dentry to let them > get reference and survive. Adding positive or used dentry into hash chain > also protects few recent negative dentries. In result total size of dcache > asymptotically limited by count of buckets and positive or used dentries. > > This heuristic isn't bulletproof and solves only most practical case. > It's easy to deceive: just touch same random name twice. I'm not sure if that's "easy to deceive" ... My concern with limiting negative dentries is something like a kernel compilation where there are many (11 for mm/mmap.c, 9 in general) and there will be a lot of places where <linux/fs.h> does not exist -isystem /usr/lib/gcc/x86_64-linux-gnu/9/include -I../arch/x86/include -I./arch/x86/include/generated -I../include -I./include -I../arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I../include/uapi -I./include/generated/uapi -I ../mm -I ./mm So it'd be good to know that kernel compilation times are unaffected by this patch.
