On Mon 29-06-20 08:17:02, Eric Dumazet wrote: > On 6/16/20 12:47 AM, Jan Kara wrote: > > On Mon 15-06-20 19:26:38, Amir Goldstein wrote: > >>> This patch changes alloc_file_pseudo() to always opt out of fsnotify by > >>> setting FMODE_NONOTIFY flag so that no check is made for fsnotify watchers > >>> on pseudo files. This should be safe as the underlying helper for the > >>> dentry is d_alloc_pseudo which explicitly states that no lookups are ever > >>> performed meaning that fanotify should have nothing useful to attach to. > >>> > >>> The test motivating this was "perf bench sched messaging --pipe". On > >>> a single-socket machine using threads the difference of the patch was > >>> as follows. > >>> > >>> 5.7.0 5.7.0 > >>> vanilla nofsnotify-v1r1 > >>> Amean 1 1.3837 ( 0.00%) 1.3547 ( 2.10%) > >>> Amean 3 3.7360 ( 0.00%) 3.6543 ( 2.19%) > >>> Amean 5 5.8130 ( 0.00%) 5.7233 * 1.54%* > >>> Amean 7 8.1490 ( 0.00%) 7.9730 * 2.16%* > >>> Amean 12 14.6843 ( 0.00%) 14.1820 ( 3.42%) > >>> Amean 18 21.8840 ( 0.00%) 21.7460 ( 0.63%) > >>> Amean 24 28.8697 ( 0.00%) 29.1680 ( -1.03%) > >>> Amean 30 36.0787 ( 0.00%) 35.2640 * 2.26%* > >>> Amean 32 38.0527 ( 0.00%) 38.1223 ( -0.18%) > >>> > >>> The difference is small but in some cases it's outside the noise so > >>> while marginal, there is still some small benefit to ignoring fsnotify > >>> for files allocated via alloc_file_pseudo in some cases. > >>> > >>> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > >> > >> Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx> > > > > Thanks for the patch Mel and for review Amir! I've added the patch to my > > tree with small amendments to the changelog. > > > > Honza > > > > Note this patch broke some user programs (one instance being packetdrill) > > Typical legacy packetdrill script has : > > // Create a socket and set it to non-blocking. > 0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3 > +0 fcntl(3, F_GETFL) = 0x2 (flags O_RDWR) > +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0 > > > But after this change, fcntl(3, F_GETFL) returns 0x4000002 > > FMODE_NONOTIFY was not meant to be visible to user space. (otherwise > there would be a O_NONOTIFY) ? Interesting. As Mel said the patch is reverted anyway (Linus already applied the revert) but the question about FMODE_NONOTIFY is interesting. Userspace certainly cannot set the flag (the kernel enforces this on open(2) and fcntl(F_SETFL)). But it is visible to userspace via fcntl(F_GETFL) which may have been an oversight... I'm just not sure whether some of the fanotify(7) users which legitimately get such file descriptors don't depend on this flag being visible. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
