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SubjectRe: [PATCH v2] fs: Do not check if there is a fsnotify watcher on pseudo inodes
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@techsingularity.net>
> >>
> >> Reviewed-by: Amir Goldstein <amir73il@gmail.com>
> >
> > 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@suse.com>
SUSE Labs, CR

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