> -----Original Message----- > From: linus971@xxxxxxxxx [mailto:linus971@xxxxxxxxx] On Behalf Of Linus > Torvalds > Sent: Sunday, August 27, 2017 12:19 AM > To: Wangkai (Kevin,C) > Cc: Jan Kara; viro@xxxxxxxxxxxxxxxxxx; Waiman Long; walters@xxxxxxxxxx; > linux-fsdevel@xxxxxxxxxxxxxxx; Renjinyong (Renjinyong, Business Support Dept) > Subject: Re: [PATCH] fs/dcache: dentries should free after files unlinked or > directories removed > > On Fri, Aug 25, 2017 at 11:56 PM, Wangkai (Kevin,C) > <wangkai86@xxxxxxxxxx> wrote: > > > > but I am worried, if there are programs create,delete many temporary files > and unique, > > the negative dentries will keep growing. > > The thing is, this has nothing to do with unlink. > > The *easiest* way to generate negative dentries is in fact to never > create any files at all: just look up millions of non-existent names. > > IOW, just something like this > > #include <stdio.h> > #include <sys/types.h> > #include <sys/stat.h> > #include <unistd.h> > > int main() > { > int i; > for (i = 0; i < 10000000; i++) { > char name[20]; > struct stat st; > > snprintf(name, sizeof(name), "n:%d", i); > stat(name, &st); > } > return 0; > } > > is a much easier and faster way to create negative dentries. > > And yes, it's entirely possible that we could/should have some way to > balance negative dentries against positive ones, but on the whole this > has not really come up as a huge problem. > > For example, your module that does a lot of GFP_ATOMIC allocations - > if it wasn't for dentries, it would have been something else. > GFP_ATOMIC *will* fail after a while, because it just can't replenish > the free memory. That's fundamental. That's what GFP_ATOMIC _means_. > It's very much meant for "occasional critical allocations", and if you > do just GFP_ATOMIC, you will fail. > > Linus (1) yes, negative dentries can come up in two ways: i) look up different files ii) create and delete different files (I have met) (2) the module allocation with flag GFP_ATOMIC was called from irq, which do some special packets receiving, called dev_alloc_skb, in this function the memory allocation with flag GFP_ATOMIC. Regards, Kevin
